FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/module/zfs/metaslab.c

     /*
      * CDDL HEADER START
      *
      * The contents of this file are subject to the terms of the
      * Common Development and Distribution License (the "License").
      * You may not use this file except in compliance with the License.
      *
      * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
      * or https://opensource.org/licenses/CDDL-1.0.
     * See the License for the specific language governing permissions
     * and limitations under the License.
     *
     * When distributing Covered Code, include this CDDL HEADER in each
     * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
     * If applicable, add the following below this CDDL HEADER, with the
     * fields enclosed by brackets "[]" replaced with your own identifying
     * information: Portions Copyright [yyyy] [name of copyright owner]
     *
     * CDDL HEADER END
     */
    /*
     * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
     * Copyright (c) 2011, 2019 by Delphix. All rights reserved.
     * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
     * Copyright (c) 2015, Nexenta Systems, Inc. All rights reserved.
     * Copyright (c) 2017, Intel Corporation.
     */
    
    #include <sys/zfs_context.h>
    #include <sys/dmu.h>
    #include <sys/dmu_tx.h>
    #include <sys/space_map.h>
    #include <sys/metaslab_impl.h>
    #include <sys/vdev_impl.h>
    #include <sys/vdev_draid.h>
    #include <sys/zio.h>
    #include <sys/spa_impl.h>
    #include <sys/zfeature.h>
    #include <sys/vdev_indirect_mapping.h>
    #include <sys/zap.h>
    #include <sys/btree.h>
    
    #define WITH_DF_BLOCK_ALLOCATOR
    
    #define GANG_ALLOCATION(flags) \
            ((flags) & (METASLAB_GANG_CHILD | METASLAB_GANG_HEADER))
    
    /*
     * Metaslab granularity, in bytes. This is roughly similar to what would be
     * referred to as the "stripe size" in traditional RAID arrays. In normal
     * operation, we will try to write this amount of data to each disk before
     * moving on to the next top-level vdev.
     */
    static uint64_t metaslab_aliquot = 1024 * 1024;
    
    /*
     * For testing, make some blocks above a certain size be gang blocks.
     */
    uint64_t metaslab_force_ganging = SPA_MAXBLOCKSIZE + 1;
    
    /*
     * In pools where the log space map feature is not enabled we touch
     * multiple metaslabs (and their respective space maps) with each
     * transaction group. Thus, we benefit from having a small space map
     * block size since it allows us to issue more I/O operations scattered
     * around the disk. So a sane default for the space map block size
     * is 8~16K.
     */
    int zfs_metaslab_sm_blksz_no_log = (1 << 14);
    
    /*
     * When the log space map feature is enabled, we accumulate a lot of
     * changes per metaslab that are flushed once in a while so we benefit
     * from a bigger block size like 128K for the metaslab space maps.
     */
    int zfs_metaslab_sm_blksz_with_log = (1 << 17);
    
    /*
     * The in-core space map representation is more compact than its on-disk form.
     * The zfs_condense_pct determines how much more compact the in-core
     * space map representation must be before we compact it on-disk.
     * Values should be greater than or equal to 100.
     */
    uint_t zfs_condense_pct = 200;
    
    /*
     * Condensing a metaslab is not guaranteed to actually reduce the amount of
     * space used on disk. In particular, a space map uses data in increments of
     * MAX(1 << ashift, space_map_blksz), so a metaslab might use the
     * same number of blocks after condensing. Since the goal of condensing is to
     * reduce the number of IOPs required to read the space map, we only want to
     * condense when we can be sure we will reduce the number of blocks used by the
     * space map. Unfortunately, we cannot precisely compute whether or not this is
     * the case in metaslab_should_condense since we are holding ms_lock. Instead,
     * we apply the following heuristic: do not condense a spacemap unless the
     * uncondensed size consumes greater than zfs_metaslab_condense_block_threshold
     * blocks.
     */
    static const int zfs_metaslab_condense_block_threshold = 4;
   
   /*
    * The zfs_mg_noalloc_threshold defines which metaslab groups should
    * be eligible for allocation. The value is defined as a percentage of
    * free space. Metaslab groups that have more free space than
    * zfs_mg_noalloc_threshold are always eligible for allocations. Once
    * a metaslab group's free space is less than or equal to the
    * zfs_mg_noalloc_threshold the allocator will avoid allocating to that
    * group unless all groups in the pool have reached zfs_mg_noalloc_threshold.
    * Once all groups in the pool reach zfs_mg_noalloc_threshold then all
    * groups are allowed to accept allocations. Gang blocks are always
    * eligible to allocate on any metaslab group. The default value of 0 means
    * no metaslab group will be excluded based on this criterion.
    */
   static uint_t zfs_mg_noalloc_threshold = 0;
   
   /*
    * Metaslab groups are considered eligible for allocations if their
    * fragmentation metric (measured as a percentage) is less than or
    * equal to zfs_mg_fragmentation_threshold. If a metaslab group
    * exceeds this threshold then it will be skipped unless all metaslab
    * groups within the metaslab class have also crossed this threshold.
    *
    * This tunable was introduced to avoid edge cases where we continue
    * allocating from very fragmented disks in our pool while other, less
    * fragmented disks, exists. On the other hand, if all disks in the
    * pool are uniformly approaching the threshold, the threshold can
    * be a speed bump in performance, where we keep switching the disks
    * that we allocate from (e.g. we allocate some segments from disk A
    * making it bypassing the threshold while freeing segments from disk
    * B getting its fragmentation below the threshold).
    *
    * Empirically, we've seen that our vdev selection for allocations is
    * good enough that fragmentation increases uniformly across all vdevs
    * the majority of the time. Thus we set the threshold percentage high
    * enough to avoid hitting the speed bump on pools that are being pushed
    * to the edge.
    */
   static uint_t zfs_mg_fragmentation_threshold = 95;
   
   /*
    * Allow metaslabs to keep their active state as long as their fragmentation
    * percentage is less than or equal to zfs_metaslab_fragmentation_threshold. An
    * active metaslab that exceeds this threshold will no longer keep its active
    * status allowing better metaslabs to be selected.
    */
   static uint_t zfs_metaslab_fragmentation_threshold = 70;
   
   /*
    * When set will load all metaslabs when pool is first opened.
    */
   int metaslab_debug_load = B_FALSE;
   
   /*
    * When set will prevent metaslabs from being unloaded.
    */
   static int metaslab_debug_unload = B_FALSE;
   
   /*
    * Minimum size which forces the dynamic allocator to change
    * it's allocation strategy.  Once the space map cannot satisfy
    * an allocation of this size then it switches to using more
    * aggressive strategy (i.e search by size rather than offset).
    */
   uint64_t metaslab_df_alloc_threshold = SPA_OLD_MAXBLOCKSIZE;
   
   /*
    * The minimum free space, in percent, which must be available
    * in a space map to continue allocations in a first-fit fashion.
    * Once the space map's free space drops below this level we dynamically
    * switch to using best-fit allocations.
    */
   uint_t metaslab_df_free_pct = 4;
   
   /*
    * Maximum distance to search forward from the last offset. Without this
    * limit, fragmented pools can see >100,000 iterations and
    * metaslab_block_picker() becomes the performance limiting factor on
    * high-performance storage.
    *
    * With the default setting of 16MB, we typically see less than 500
    * iterations, even with very fragmented, ashift=9 pools. The maximum number
    * of iterations possible is:
    *     metaslab_df_max_search / (2 * (1<<ashift))
    * With the default setting of 16MB this is 16*1024 (with ashift=9) or
    * 2048 (with ashift=12).
    */
   static uint_t metaslab_df_max_search = 16 * 1024 * 1024;
   
   /*
    * Forces the metaslab_block_picker function to search for at least this many
    * segments forwards until giving up on finding a segment that the allocation
    * will fit into.
    */
   static const uint32_t metaslab_min_search_count = 100;
   
   /*
    * If we are not searching forward (due to metaslab_df_max_search,
    * metaslab_df_free_pct, or metaslab_df_alloc_threshold), this tunable
    * controls what segment is used.  If it is set, we will use the largest free
    * segment.  If it is not set, we will use a segment of exactly the requested
    * size (or larger).
    */
   static int metaslab_df_use_largest_segment = B_FALSE;
   
   /*
    * Percentage of all cpus that can be used by the metaslab taskq.
    */
   int metaslab_load_pct = 50;
   
   /*
    * These tunables control how long a metaslab will remain loaded after the
    * last allocation from it.  A metaslab can't be unloaded until at least
    * metaslab_unload_delay TXG's and metaslab_unload_delay_ms milliseconds
    * have elapsed.  However, zfs_metaslab_mem_limit may cause it to be
    * unloaded sooner.  These settings are intended to be generous -- to keep
    * metaslabs loaded for a long time, reducing the rate of metaslab loading.
    */
   static uint_t metaslab_unload_delay = 32;
   static uint_t metaslab_unload_delay_ms = 10 * 60 * 1000; /* ten minutes */
   
   /*
    * Max number of metaslabs per group to preload.
    */
   uint_t metaslab_preload_limit = 10;
   
   /*
    * Enable/disable preloading of metaslab.
    */
   static int metaslab_preload_enabled = B_TRUE;
   
   /*
    * Enable/disable fragmentation weighting on metaslabs.
    */
   static int metaslab_fragmentation_factor_enabled = B_TRUE;
   
   /*
    * Enable/disable lba weighting (i.e. outer tracks are given preference).
    */
   static int metaslab_lba_weighting_enabled = B_TRUE;
   
   /*
    * Enable/disable metaslab group biasing.
    */
   static int metaslab_bias_enabled = B_TRUE;
   
   /*
    * Enable/disable remapping of indirect DVAs to their concrete vdevs.
    */
   static const boolean_t zfs_remap_blkptr_enable = B_TRUE;
   
   /*
    * Enable/disable segment-based metaslab selection.
    */
   static int zfs_metaslab_segment_weight_enabled = B_TRUE;
   
   /*
    * When using segment-based metaslab selection, we will continue
    * allocating from the active metaslab until we have exhausted
    * zfs_metaslab_switch_threshold of its buckets.
    */
   static int zfs_metaslab_switch_threshold = 2;
   
   /*
    * Internal switch to enable/disable the metaslab allocation tracing
    * facility.
    */
   static const boolean_t metaslab_trace_enabled = B_FALSE;
   
   /*
    * Maximum entries that the metaslab allocation tracing facility will keep
    * in a given list when running in non-debug mode. We limit the number
    * of entries in non-debug mode to prevent us from using up too much memory.
    * The limit should be sufficiently large that we don't expect any allocation
    * to every exceed this value. In debug mode, the system will panic if this
    * limit is ever reached allowing for further investigation.
    */
   static const uint64_t metaslab_trace_max_entries = 5000;
   
   /*
    * Maximum number of metaslabs per group that can be disabled
    * simultaneously.
    */
   static const int max_disabled_ms = 3;
   
   /*
    * Time (in seconds) to respect ms_max_size when the metaslab is not loaded.
    * To avoid 64-bit overflow, don't set above UINT32_MAX.
    */
   static uint64_t zfs_metaslab_max_size_cache_sec = 1 * 60 * 60; /* 1 hour */
   
   /*
    * Maximum percentage of memory to use on storing loaded metaslabs. If loading
    * a metaslab would take it over this percentage, the oldest selected metaslab
    * is automatically unloaded.
    */
   static uint_t zfs_metaslab_mem_limit = 25;
   
   /*
    * Force the per-metaslab range trees to use 64-bit integers to store
    * segments. Used for debugging purposes.
    */
   static const boolean_t zfs_metaslab_force_large_segs = B_FALSE;
   
   /*
    * By default we only store segments over a certain size in the size-sorted
    * metaslab trees (ms_allocatable_by_size and
    * ms_unflushed_frees_by_size). This dramatically reduces memory usage and
    * improves load and unload times at the cost of causing us to use slightly
    * larger segments than we would otherwise in some cases.
    */
   static const uint32_t metaslab_by_size_min_shift = 14;
   
   /*
    * If not set, we will first try normal allocation.  If that fails then
    * we will do a gang allocation.  If that fails then we will do a "try hard"
    * gang allocation.  If that fails then we will have a multi-layer gang
    * block.
    *
    * If set, we will first try normal allocation.  If that fails then
    * we will do a "try hard" allocation.  If that fails we will do a gang
    * allocation.  If that fails we will do a "try hard" gang allocation.  If
    * that fails then we will have a multi-layer gang block.
    */
   static int zfs_metaslab_try_hard_before_gang = B_FALSE;
   
   /*
    * When not trying hard, we only consider the best zfs_metaslab_find_max_tries
    * metaslabs.  This improves performance, especially when there are many
    * metaslabs per vdev and the allocation can't actually be satisfied (so we
    * would otherwise iterate all the metaslabs).  If there is a metaslab with a
    * worse weight but it can actually satisfy the allocation, we won't find it
    * until trying hard.  This may happen if the worse metaslab is not loaded
    * (and the true weight is better than we have calculated), or due to weight
    * bucketization.  E.g. we are looking for a 60K segment, and the best
    * metaslabs all have free segments in the 32-63K bucket, but the best
    * zfs_metaslab_find_max_tries metaslabs have ms_max_size <60KB, and a
    * subsequent metaslab has ms_max_size >60KB (but fewer segments in this
    * bucket, and therefore a lower weight).
    */
   static uint_t zfs_metaslab_find_max_tries = 100;
   
   static uint64_t metaslab_weight(metaslab_t *, boolean_t);
   static void metaslab_set_fragmentation(metaslab_t *, boolean_t);
   static void metaslab_free_impl(vdev_t *, uint64_t, uint64_t, boolean_t);
   static void metaslab_check_free_impl(vdev_t *, uint64_t, uint64_t);
   
   static void metaslab_passivate(metaslab_t *msp, uint64_t weight);
   static uint64_t metaslab_weight_from_range_tree(metaslab_t *msp);
   static void metaslab_flush_update(metaslab_t *, dmu_tx_t *);
   static unsigned int metaslab_idx_func(multilist_t *, void *);
   static void metaslab_evict(metaslab_t *, uint64_t);
   static void metaslab_rt_add(range_tree_t *rt, range_seg_t *rs, void *arg);
   kmem_cache_t *metaslab_alloc_trace_cache;
   
   typedef struct metaslab_stats {
           kstat_named_t metaslabstat_trace_over_limit;
           kstat_named_t metaslabstat_reload_tree;
           kstat_named_t metaslabstat_too_many_tries;
           kstat_named_t metaslabstat_try_hard;
   } metaslab_stats_t;
   
   static metaslab_stats_t metaslab_stats = {
           { "trace_over_limit",           KSTAT_DATA_UINT64 },
           { "reload_tree",                KSTAT_DATA_UINT64 },
           { "too_many_tries",             KSTAT_DATA_UINT64 },
           { "try_hard",                   KSTAT_DATA_UINT64 },
   };
   
   #define METASLABSTAT_BUMP(stat) \
           atomic_inc_64(&metaslab_stats.stat.value.ui64);
   
   
   static kstat_t *metaslab_ksp;
   
   void
   metaslab_stat_init(void)
   {
           ASSERT(metaslab_alloc_trace_cache == NULL);
           metaslab_alloc_trace_cache = kmem_cache_create(
               "metaslab_alloc_trace_cache", sizeof (metaslab_alloc_trace_t),
               0, NULL, NULL, NULL, NULL, NULL, 0);
           metaslab_ksp = kstat_create("zfs", 0, "metaslab_stats",
               "misc", KSTAT_TYPE_NAMED, sizeof (metaslab_stats) /
               sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
           if (metaslab_ksp != NULL) {
                   metaslab_ksp->ks_data = &metaslab_stats;
                   kstat_install(metaslab_ksp);
           }
   }
   
   void
   metaslab_stat_fini(void)
   {
           if (metaslab_ksp != NULL) {
                   kstat_delete(metaslab_ksp);
                   metaslab_ksp = NULL;
           }
   
           kmem_cache_destroy(metaslab_alloc_trace_cache);
           metaslab_alloc_trace_cache = NULL;
   }
   
   /*
    * ==========================================================================
    * Metaslab classes
    * ==========================================================================
    */
   metaslab_class_t *
   metaslab_class_create(spa_t *spa, const metaslab_ops_t *ops)
   {
           metaslab_class_t *mc;
   
           mc = kmem_zalloc(offsetof(metaslab_class_t,
               mc_allocator[spa->spa_alloc_count]), KM_SLEEP);
   
           mc->mc_spa = spa;
           mc->mc_ops = ops;
           mutex_init(&mc->mc_lock, NULL, MUTEX_DEFAULT, NULL);
           multilist_create(&mc->mc_metaslab_txg_list, sizeof (metaslab_t),
               offsetof(metaslab_t, ms_class_txg_node), metaslab_idx_func);
           for (int i = 0; i < spa->spa_alloc_count; i++) {
                   metaslab_class_allocator_t *mca = &mc->mc_allocator[i];
                   mca->mca_rotor = NULL;
                   zfs_refcount_create_tracked(&mca->mca_alloc_slots);
           }
   
           return (mc);
   }
   
   void
   metaslab_class_destroy(metaslab_class_t *mc)
   {
           spa_t *spa = mc->mc_spa;
   
           ASSERT(mc->mc_alloc == 0);
           ASSERT(mc->mc_deferred == 0);
           ASSERT(mc->mc_space == 0);
           ASSERT(mc->mc_dspace == 0);
   
           for (int i = 0; i < spa->spa_alloc_count; i++) {
                   metaslab_class_allocator_t *mca = &mc->mc_allocator[i];
                   ASSERT(mca->mca_rotor == NULL);
                   zfs_refcount_destroy(&mca->mca_alloc_slots);
           }
           mutex_destroy(&mc->mc_lock);
           multilist_destroy(&mc->mc_metaslab_txg_list);
           kmem_free(mc, offsetof(metaslab_class_t,
               mc_allocator[spa->spa_alloc_count]));
   }
   
   int
   metaslab_class_validate(metaslab_class_t *mc)
   {
           metaslab_group_t *mg;
           vdev_t *vd;
   
           /*
            * Must hold one of the spa_config locks.
            */
           ASSERT(spa_config_held(mc->mc_spa, SCL_ALL, RW_READER) ||
               spa_config_held(mc->mc_spa, SCL_ALL, RW_WRITER));
   
           if ((mg = mc->mc_allocator[0].mca_rotor) == NULL)
                   return (0);
   
           do {
                   vd = mg->mg_vd;
                   ASSERT(vd->vdev_mg != NULL);
                   ASSERT3P(vd->vdev_top, ==, vd);
                   ASSERT3P(mg->mg_class, ==, mc);
                   ASSERT3P(vd->vdev_ops, !=, &vdev_hole_ops);
           } while ((mg = mg->mg_next) != mc->mc_allocator[0].mca_rotor);
   
           return (0);
   }
   
   static void
   metaslab_class_space_update(metaslab_class_t *mc, int64_t alloc_delta,
       int64_t defer_delta, int64_t space_delta, int64_t dspace_delta)
   {
           atomic_add_64(&mc->mc_alloc, alloc_delta);
           atomic_add_64(&mc->mc_deferred, defer_delta);
           atomic_add_64(&mc->mc_space, space_delta);
           atomic_add_64(&mc->mc_dspace, dspace_delta);
   }
   
   uint64_t
   metaslab_class_get_alloc(metaslab_class_t *mc)
   {
           return (mc->mc_alloc);
   }
   
   uint64_t
   metaslab_class_get_deferred(metaslab_class_t *mc)
   {
           return (mc->mc_deferred);
   }
   
   uint64_t
   metaslab_class_get_space(metaslab_class_t *mc)
   {
           return (mc->mc_space);
   }
   
   uint64_t
   metaslab_class_get_dspace(metaslab_class_t *mc)
   {
           return (spa_deflate(mc->mc_spa) ? mc->mc_dspace : mc->mc_space);
   }
   
   void
   metaslab_class_histogram_verify(metaslab_class_t *mc)
   {
           spa_t *spa = mc->mc_spa;
           vdev_t *rvd = spa->spa_root_vdev;
           uint64_t *mc_hist;
           int i;
   
           if ((zfs_flags & ZFS_DEBUG_HISTOGRAM_VERIFY) == 0)
                   return;
   
           mc_hist = kmem_zalloc(sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE,
               KM_SLEEP);
   
           mutex_enter(&mc->mc_lock);
           for (int c = 0; c < rvd->vdev_children; c++) {
                   vdev_t *tvd = rvd->vdev_child[c];
                   metaslab_group_t *mg = vdev_get_mg(tvd, mc);
   
                   /*
                    * Skip any holes, uninitialized top-levels, or
                    * vdevs that are not in this metalab class.
                    */
                   if (!vdev_is_concrete(tvd) || tvd->vdev_ms_shift == 0 ||
                       mg->mg_class != mc) {
                           continue;
                   }
   
                   IMPLY(mg == mg->mg_vd->vdev_log_mg,
                       mc == spa_embedded_log_class(mg->mg_vd->vdev_spa));
   
                   for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
                           mc_hist[i] += mg->mg_histogram[i];
           }
   
           for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++) {
                   VERIFY3U(mc_hist[i], ==, mc->mc_histogram[i]);
           }
   
           mutex_exit(&mc->mc_lock);
           kmem_free(mc_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
   }
   
   /*
    * Calculate the metaslab class's fragmentation metric. The metric
    * is weighted based on the space contribution of each metaslab group.
    * The return value will be a number between 0 and 100 (inclusive), or
    * ZFS_FRAG_INVALID if the metric has not been set. See comment above the
    * zfs_frag_table for more information about the metric.
    */
   uint64_t
   metaslab_class_fragmentation(metaslab_class_t *mc)
   {
           vdev_t *rvd = mc->mc_spa->spa_root_vdev;
           uint64_t fragmentation = 0;
   
           spa_config_enter(mc->mc_spa, SCL_VDEV, FTAG, RW_READER);
   
           for (int c = 0; c < rvd->vdev_children; c++) {
                   vdev_t *tvd = rvd->vdev_child[c];
                   metaslab_group_t *mg = tvd->vdev_mg;
   
                   /*
                    * Skip any holes, uninitialized top-levels,
                    * or vdevs that are not in this metalab class.
                    */
                   if (!vdev_is_concrete(tvd) || tvd->vdev_ms_shift == 0 ||
                       mg->mg_class != mc) {
                           continue;
                   }
   
                   /*
                    * If a metaslab group does not contain a fragmentation
                    * metric then just bail out.
                    */
                   if (mg->mg_fragmentation == ZFS_FRAG_INVALID) {
                           spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
                           return (ZFS_FRAG_INVALID);
                   }
   
                   /*
                    * Determine how much this metaslab_group is contributing
                    * to the overall pool fragmentation metric.
                    */
                   fragmentation += mg->mg_fragmentation *
                       metaslab_group_get_space(mg);
           }
           fragmentation /= metaslab_class_get_space(mc);
   
           ASSERT3U(fragmentation, <=, 100);
           spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
           return (fragmentation);
   }
   
   /*
    * Calculate the amount of expandable space that is available in
    * this metaslab class. If a device is expanded then its expandable
    * space will be the amount of allocatable space that is currently not
    * part of this metaslab class.
    */
   uint64_t
   metaslab_class_expandable_space(metaslab_class_t *mc)
   {
           vdev_t *rvd = mc->mc_spa->spa_root_vdev;
           uint64_t space = 0;
   
           spa_config_enter(mc->mc_spa, SCL_VDEV, FTAG, RW_READER);
           for (int c = 0; c < rvd->vdev_children; c++) {
                   vdev_t *tvd = rvd->vdev_child[c];
                   metaslab_group_t *mg = tvd->vdev_mg;
   
                   if (!vdev_is_concrete(tvd) || tvd->vdev_ms_shift == 0 ||
                       mg->mg_class != mc) {
                           continue;
                   }
   
                   /*
                    * Calculate if we have enough space to add additional
                    * metaslabs. We report the expandable space in terms
                    * of the metaslab size since that's the unit of expansion.
                    */
                   space += P2ALIGN(tvd->vdev_max_asize - tvd->vdev_asize,
                       1ULL << tvd->vdev_ms_shift);
           }
           spa_config_exit(mc->mc_spa, SCL_VDEV, FTAG);
           return (space);
   }
   
   void
   metaslab_class_evict_old(metaslab_class_t *mc, uint64_t txg)
   {
           multilist_t *ml = &mc->mc_metaslab_txg_list;
           for (int i = 0; i < multilist_get_num_sublists(ml); i++) {
                   multilist_sublist_t *mls = multilist_sublist_lock(ml, i);
                   metaslab_t *msp = multilist_sublist_head(mls);
                   multilist_sublist_unlock(mls);
                   while (msp != NULL) {
                           mutex_enter(&msp->ms_lock);
   
                           /*
                            * If the metaslab has been removed from the list
                            * (which could happen if we were at the memory limit
                            * and it was evicted during this loop), then we can't
                            * proceed and we should restart the sublist.
                            */
                           if (!multilist_link_active(&msp->ms_class_txg_node)) {
                                   mutex_exit(&msp->ms_lock);
                                   i--;
                                   break;
                           }
                           mls = multilist_sublist_lock(ml, i);
                           metaslab_t *next_msp = multilist_sublist_next(mls, msp);
                           multilist_sublist_unlock(mls);
                           if (txg >
                               msp->ms_selected_txg + metaslab_unload_delay &&
                               gethrtime() > msp->ms_selected_time +
                               (uint64_t)MSEC2NSEC(metaslab_unload_delay_ms)) {
                                   metaslab_evict(msp, txg);
                           } else {
                                   /*
                                    * Once we've hit a metaslab selected too
                                    * recently to evict, we're done evicting for
                                    * now.
                                    */
                                   mutex_exit(&msp->ms_lock);
                                   break;
                           }
                           mutex_exit(&msp->ms_lock);
                           msp = next_msp;
                   }
           }
   }
   
   static int
   metaslab_compare(const void *x1, const void *x2)
   {
           const metaslab_t *m1 = (const metaslab_t *)x1;
           const metaslab_t *m2 = (const metaslab_t *)x2;
   
           int sort1 = 0;
           int sort2 = 0;
           if (m1->ms_allocator != -1 && m1->ms_primary)
                   sort1 = 1;
           else if (m1->ms_allocator != -1 && !m1->ms_primary)
                   sort1 = 2;
           if (m2->ms_allocator != -1 && m2->ms_primary)
                   sort2 = 1;
           else if (m2->ms_allocator != -1 && !m2->ms_primary)
                   sort2 = 2;
   
           /*
            * Sort inactive metaslabs first, then primaries, then secondaries. When
            * selecting a metaslab to allocate from, an allocator first tries its
            * primary, then secondary active metaslab. If it doesn't have active
            * metaslabs, or can't allocate from them, it searches for an inactive
            * metaslab to activate. If it can't find a suitable one, it will steal
            * a primary or secondary metaslab from another allocator.
            */
           if (sort1 < sort2)
                   return (-1);
           if (sort1 > sort2)
                   return (1);
   
           int cmp = TREE_CMP(m2->ms_weight, m1->ms_weight);
           if (likely(cmp))
                   return (cmp);
   
           IMPLY(TREE_CMP(m1->ms_start, m2->ms_start) == 0, m1 == m2);
   
           return (TREE_CMP(m1->ms_start, m2->ms_start));
   }
   
   /*
    * ==========================================================================
    * Metaslab groups
    * ==========================================================================
    */
   /*
    * Update the allocatable flag and the metaslab group's capacity.
    * The allocatable flag is set to true if the capacity is below
    * the zfs_mg_noalloc_threshold or has a fragmentation value that is
    * greater than zfs_mg_fragmentation_threshold. If a metaslab group
    * transitions from allocatable to non-allocatable or vice versa then the
    * metaslab group's class is updated to reflect the transition.
    */
   static void
   metaslab_group_alloc_update(metaslab_group_t *mg)
   {
           vdev_t *vd = mg->mg_vd;
           metaslab_class_t *mc = mg->mg_class;
           vdev_stat_t *vs = &vd->vdev_stat;
           boolean_t was_allocatable;
           boolean_t was_initialized;
   
           ASSERT(vd == vd->vdev_top);
           ASSERT3U(spa_config_held(mc->mc_spa, SCL_ALLOC, RW_READER), ==,
               SCL_ALLOC);
   
           mutex_enter(&mg->mg_lock);
           was_allocatable = mg->mg_allocatable;
           was_initialized = mg->mg_initialized;
   
           mg->mg_free_capacity = ((vs->vs_space - vs->vs_alloc) * 100) /
               (vs->vs_space + 1);
   
           mutex_enter(&mc->mc_lock);
   
           /*
            * If the metaslab group was just added then it won't
            * have any space until we finish syncing out this txg.
            * At that point we will consider it initialized and available
            * for allocations.  We also don't consider non-activated
            * metaslab groups (e.g. vdevs that are in the middle of being removed)
            * to be initialized, because they can't be used for allocation.
            */
           mg->mg_initialized = metaslab_group_initialized(mg);
           if (!was_initialized && mg->mg_initialized) {
                   mc->mc_groups++;
           } else if (was_initialized && !mg->mg_initialized) {
                   ASSERT3U(mc->mc_groups, >, 0);
                   mc->mc_groups--;
           }
           if (mg->mg_initialized)
                   mg->mg_no_free_space = B_FALSE;
   
           /*
            * A metaslab group is considered allocatable if it has plenty
            * of free space or is not heavily fragmented. We only take
            * fragmentation into account if the metaslab group has a valid
            * fragmentation metric (i.e. a value between 0 and 100).
            */
           mg->mg_allocatable = (mg->mg_activation_count > 0 &&
               mg->mg_free_capacity > zfs_mg_noalloc_threshold &&
               (mg->mg_fragmentation == ZFS_FRAG_INVALID ||
               mg->mg_fragmentation <= zfs_mg_fragmentation_threshold));
   
           /*
            * The mc_alloc_groups maintains a count of the number of
            * groups in this metaslab class that are still above the
            * zfs_mg_noalloc_threshold. This is used by the allocating
            * threads to determine if they should avoid allocations to
            * a given group. The allocator will avoid allocations to a group
            * if that group has reached or is below the zfs_mg_noalloc_threshold
            * and there are still other groups that are above the threshold.
            * When a group transitions from allocatable to non-allocatable or
            * vice versa we update the metaslab class to reflect that change.
            * When the mc_alloc_groups value drops to 0 that means that all
            * groups have reached the zfs_mg_noalloc_threshold making all groups
            * eligible for allocations. This effectively means that all devices
            * are balanced again.
            */
           if (was_allocatable && !mg->mg_allocatable)
                   mc->mc_alloc_groups--;
           else if (!was_allocatable && mg->mg_allocatable)
                   mc->mc_alloc_groups++;
           mutex_exit(&mc->mc_lock);
   
           mutex_exit(&mg->mg_lock);
   }
   
   int
   metaslab_sort_by_flushed(const void *va, const void *vb)
   {
           const metaslab_t *a = va;
           const metaslab_t *b = vb;
   
           int cmp = TREE_CMP(a->ms_unflushed_txg, b->ms_unflushed_txg);
           if (likely(cmp))
                   return (cmp);
   
           uint64_t a_vdev_id = a->ms_group->mg_vd->vdev_id;
           uint64_t b_vdev_id = b->ms_group->mg_vd->vdev_id;
           cmp = TREE_CMP(a_vdev_id, b_vdev_id);
           if (cmp)
                   return (cmp);
   
           return (TREE_CMP(a->ms_id, b->ms_id));
   }
   
   metaslab_group_t *
   metaslab_group_create(metaslab_class_t *mc, vdev_t *vd, int allocators)
   {
           metaslab_group_t *mg;
   
           mg = kmem_zalloc(offsetof(metaslab_group_t,
               mg_allocator[allocators]), KM_SLEEP);
           mutex_init(&mg->mg_lock, NULL, MUTEX_DEFAULT, NULL);
           mutex_init(&mg->mg_ms_disabled_lock, NULL, MUTEX_DEFAULT, NULL);
           cv_init(&mg->mg_ms_disabled_cv, NULL, CV_DEFAULT, NULL);
           avl_create(&mg->mg_metaslab_tree, metaslab_compare,
               sizeof (metaslab_t), offsetof(metaslab_t, ms_group_node));
           mg->mg_vd = vd;
           mg->mg_class = mc;
           mg->mg_activation_count = 0;
           mg->mg_initialized = B_FALSE;
           mg->mg_no_free_space = B_TRUE;
           mg->mg_allocators = allocators;
   
           for (int i = 0; i < allocators; i++) {
                   metaslab_group_allocator_t *mga = &mg->mg_allocator[i];
                   zfs_refcount_create_tracked(&mga->mga_alloc_queue_depth);
           }
   
           mg->mg_taskq = taskq_create("metaslab_group_taskq", metaslab_load_pct,
               maxclsyspri, 10, INT_MAX, TASKQ_THREADS_CPU_PCT | TASKQ_DYNAMIC);
   
           return (mg);
   }
   
   void
   metaslab_group_destroy(metaslab_group_t *mg)
   {
           ASSERT(mg->mg_prev == NULL);
           ASSERT(mg->mg_next == NULL);
           /*
            * We may have gone below zero with the activation count
            * either because we never activated in the first place or
            * because we're done, and possibly removing the vdev.
            */
           ASSERT(mg->mg_activation_count <= 0);
   
           taskq_destroy(mg->mg_taskq);
           avl_destroy(&mg->mg_metaslab_tree);
           mutex_destroy(&mg->mg_lock);
           mutex_destroy(&mg->mg_ms_disabled_lock);
           cv_destroy(&mg->mg_ms_disabled_cv);
   
           for (int i = 0; i < mg->mg_allocators; i++) {
                   metaslab_group_allocator_t *mga = &mg->mg_allocator[i];
                   zfs_refcount_destroy(&mga->mga_alloc_queue_depth);
           }
           kmem_free(mg, offsetof(metaslab_group_t,
               mg_allocator[mg->mg_allocators]));
   }
   
   void
   metaslab_group_activate(metaslab_group_t *mg)
   {
           metaslab_class_t *mc = mg->mg_class;
           spa_t *spa = mc->mc_spa;
           metaslab_group_t *mgprev, *mgnext;
   
           ASSERT3U(spa_config_held(spa, SCL_ALLOC, RW_WRITER), !=, 0);
   
           ASSERT(mg->mg_prev == NULL);
           ASSERT(mg->mg_next == NULL);
           ASSERT(mg->mg_activation_count <= 0);
   
           if (++mg->mg_activation_count <= 0)
                   return;
   
           mg->mg_aliquot = metaslab_aliquot * MAX(1,
               vdev_get_ndisks(mg->mg_vd) - vdev_get_nparity(mg->mg_vd));
           metaslab_group_alloc_update(mg);
   
           if ((mgprev = mc->mc_allocator[0].mca_rotor) == NULL) {
                   mg->mg_prev = mg;
                   mg->mg_next = mg;
           } else {
                   mgnext = mgprev->mg_next;
                   mg->mg_prev = mgprev;
                   mg->mg_next = mgnext;
                   mgprev->mg_next = mg;
                   mgnext->mg_prev = mg;
           }
           for (int i = 0; i < spa->spa_alloc_count; i++) {
                   mc->mc_allocator[i].mca_rotor = mg;
                   mg = mg->mg_next;
           }
   }
   
   /*
    * Passivate a metaslab group and remove it from the allocation rotor.
    * Callers must hold both the SCL_ALLOC and SCL_ZIO lock prior to passivating
    * a metaslab group. This function will momentarily drop spa_config_locks
    * that are lower than the SCL_ALLOC lock (see comment below).
    */
   void
   metaslab_group_passivate(metaslab_group_t *mg)
   {
           metaslab_class_t *mc = mg->mg_class;
           spa_t *spa = mc->mc_spa;
           metaslab_group_t *mgprev, *mgnext;
           int locks = spa_config_held(spa, SCL_ALL, RW_WRITER);
   
           ASSERT3U(spa_config_held(spa, SCL_ALLOC | SCL_ZIO, RW_WRITER), ==,
               (SCL_ALLOC | SCL_ZIO));
   
           if (--mg->mg_activation_count != 0) {
                   for (int i = 0; i < spa->spa_alloc_count; i++)
                           ASSERT(mc->mc_allocator[i].mca_rotor != mg);
                   ASSERT(mg->mg_prev == NULL);
                   ASSERT(mg->mg_next == NULL);
                   ASSERT(mg->mg_activation_count < 0);
                   return;
           }
   
           /*
            * The spa_config_lock is an array of rwlocks, ordered as
            * follows (from highest to lowest):
            *      SCL_CONFIG > SCL_STATE > SCL_L2ARC > SCL_ALLOC >
            *      SCL_ZIO > SCL_FREE > SCL_VDEV
            * (For more information about the spa_config_lock see spa_misc.c)
            * The higher the lock, the broader its coverage. When we passivate
            * a metaslab group, we must hold both the SCL_ALLOC and the SCL_ZIO
            * config locks. However, the metaslab group's taskq might be trying
            * to preload metaslabs so we must drop the SCL_ZIO lock and any
            * lower locks to allow the I/O to complete. At a minimum,
            * we continue to hold the SCL_ALLOC lock, which prevents any future
            * allocations from taking place and any changes to the vdev tree.
            */
           spa_config_exit(spa, locks & ~(SCL_ZIO - 1), spa);
           taskq_wait_outstanding(mg->mg_taskq, 0);
           spa_config_enter(spa, locks & ~(SCL_ZIO - 1), spa, RW_WRITER);
           metaslab_group_alloc_update(mg);
           for (int i = 0; i < mg->mg_allocators; i++) {
                   metaslab_group_allocator_t *mga = &mg->mg_allocator[i];
                   metaslab_t *msp = mga->mga_primary;
                   if (msp != NULL) {
                           mutex_enter(&msp->ms_lock);
                           metaslab_passivate(msp,
                               metaslab_weight_from_range_tree(msp));
                           mutex_exit(&msp->ms_lock);
                   }
                   msp = mga->mga_secondary;
                   if (msp != NULL) {
                           mutex_enter(&msp->ms_lock);
                           metaslab_passivate(msp,
                               metaslab_weight_from_range_tree(msp));
                           mutex_exit(&msp->ms_lock);
                   }
           }
   
           mgprev = mg->mg_prev;
           mgnext = mg->mg_next;
   
           if (mg == mgnext) {
                   mgnext = NULL;
           } else {
                   mgprev->mg_next = mgnext;
                   mgnext->mg_prev = mgprev;
           }
           for (int i = 0; i < spa->spa_alloc_count; i++) {
                   if (mc->mc_allocator[i].mca_rotor == mg)
                           mc->mc_allocator[i].mca_rotor = mgnext;
           }
   
           mg->mg_prev = NULL;
           mg->mg_next = NULL;
  }
  
  boolean_t
  metaslab_group_initialized(metaslab_group_t *mg)
  {
          vdev_t *vd = mg->mg_vd;
          vdev_stat_t *vs = &vd->vdev_stat;
  
          return (vs->vs_space != 0 && mg->mg_activation_count > 0);
  }
  
  uint64_t
  metaslab_group_get_space(metaslab_group_t *mg)
  {
          /*
           * Note that the number of nodes in mg_metaslab_tree may be one less
           * than vdev_ms_count, due to the embedded log metaslab.
           */
          mutex_enter(&mg->mg_lock);
          uint64_t ms_count = avl_numnodes(&mg->mg_metaslab_tree);
          mutex_exit(&mg->mg_lock);
          return ((1ULL << mg->mg_vd->vdev_ms_shift) * ms_count);
  }
  
  void
  metaslab_group_histogram_verify(metaslab_group_t *mg)
  {
          uint64_t *mg_hist;
          avl_tree_t *t = &mg->mg_metaslab_tree;
          uint64_t ashift = mg->mg_vd->vdev_ashift;
  
          if ((zfs_flags & ZFS_DEBUG_HISTOGRAM_VERIFY) == 0)
                  return;
  
          mg_hist = kmem_zalloc(sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE,
              KM_SLEEP);
  
          ASSERT3U(RANGE_TREE_HISTOGRAM_SIZE, >=,
              SPACE_MAP_HISTOGRAM_SIZE + ashift);
  
          mutex_enter(&mg->mg_lock);
          for (metaslab_t *msp = avl_first(t);
              msp != NULL; msp = AVL_NEXT(t, msp)) {
                  VERIFY3P(msp->ms_group, ==, mg);
                  /* skip if not active */
                  if (msp->ms_sm == NULL)
                          continue;
  
                  for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
                          mg_hist[i + ashift] +=
                              msp->ms_sm->sm_phys->smp_histogram[i];
                  }
          }
  
          for (int i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i ++)
                  VERIFY3U(mg_hist[i], ==, mg->mg_histogram[i]);
  
          mutex_exit(&mg->mg_lock);
  
          kmem_free(mg_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
  }
  
  static void
  metaslab_group_histogram_add(metaslab_group_t *mg, metaslab_t *msp)
  {
          metaslab_class_t *mc = mg->mg_class;
          uint64_t ashift = mg->mg_vd->vdev_ashift;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          if (msp->ms_sm == NULL)
                  return;
  
          mutex_enter(&mg->mg_lock);
          mutex_enter(&mc->mc_lock);
          for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
                  IMPLY(mg == mg->mg_vd->vdev_log_mg,
                      mc == spa_embedded_log_class(mg->mg_vd->vdev_spa));
                  mg->mg_histogram[i + ashift] +=
                      msp->ms_sm->sm_phys->smp_histogram[i];
                  mc->mc_histogram[i + ashift] +=
                      msp->ms_sm->sm_phys->smp_histogram[i];
          }
          mutex_exit(&mc->mc_lock);
          mutex_exit(&mg->mg_lock);
  }
  
  void
  metaslab_group_histogram_remove(metaslab_group_t *mg, metaslab_t *msp)
  {
          metaslab_class_t *mc = mg->mg_class;
          uint64_t ashift = mg->mg_vd->vdev_ashift;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          if (msp->ms_sm == NULL)
                  return;
  
          mutex_enter(&mg->mg_lock);
          mutex_enter(&mc->mc_lock);
          for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
                  ASSERT3U(mg->mg_histogram[i + ashift], >=,
                      msp->ms_sm->sm_phys->smp_histogram[i]);
                  ASSERT3U(mc->mc_histogram[i + ashift], >=,
                      msp->ms_sm->sm_phys->smp_histogram[i]);
                  IMPLY(mg == mg->mg_vd->vdev_log_mg,
                      mc == spa_embedded_log_class(mg->mg_vd->vdev_spa));
  
                  mg->mg_histogram[i + ashift] -=
                      msp->ms_sm->sm_phys->smp_histogram[i];
                  mc->mc_histogram[i + ashift] -=
                      msp->ms_sm->sm_phys->smp_histogram[i];
          }
          mutex_exit(&mc->mc_lock);
          mutex_exit(&mg->mg_lock);
  }
  
  static void
  metaslab_group_add(metaslab_group_t *mg, metaslab_t *msp)
  {
          ASSERT(msp->ms_group == NULL);
          mutex_enter(&mg->mg_lock);
          msp->ms_group = mg;
          msp->ms_weight = 0;
          avl_add(&mg->mg_metaslab_tree, msp);
          mutex_exit(&mg->mg_lock);
  
          mutex_enter(&msp->ms_lock);
          metaslab_group_histogram_add(mg, msp);
          mutex_exit(&msp->ms_lock);
  }
  
  static void
  metaslab_group_remove(metaslab_group_t *mg, metaslab_t *msp)
  {
          mutex_enter(&msp->ms_lock);
          metaslab_group_histogram_remove(mg, msp);
          mutex_exit(&msp->ms_lock);
  
          mutex_enter(&mg->mg_lock);
          ASSERT(msp->ms_group == mg);
          avl_remove(&mg->mg_metaslab_tree, msp);
  
          metaslab_class_t *mc = msp->ms_group->mg_class;
          multilist_sublist_t *mls =
              multilist_sublist_lock_obj(&mc->mc_metaslab_txg_list, msp);
          if (multilist_link_active(&msp->ms_class_txg_node))
                  multilist_sublist_remove(mls, msp);
          multilist_sublist_unlock(mls);
  
          msp->ms_group = NULL;
          mutex_exit(&mg->mg_lock);
  }
  
  static void
  metaslab_group_sort_impl(metaslab_group_t *mg, metaslab_t *msp, uint64_t weight)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          ASSERT(MUTEX_HELD(&mg->mg_lock));
          ASSERT(msp->ms_group == mg);
  
          avl_remove(&mg->mg_metaslab_tree, msp);
          msp->ms_weight = weight;
          avl_add(&mg->mg_metaslab_tree, msp);
  
  }
  
  static void
  metaslab_group_sort(metaslab_group_t *mg, metaslab_t *msp, uint64_t weight)
  {
          /*
           * Although in principle the weight can be any value, in
           * practice we do not use values in the range [1, 511].
           */
          ASSERT(weight >= SPA_MINBLOCKSIZE || weight == 0);
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          mutex_enter(&mg->mg_lock);
          metaslab_group_sort_impl(mg, msp, weight);
          mutex_exit(&mg->mg_lock);
  }
  
  /*
   * Calculate the fragmentation for a given metaslab group. We can use
   * a simple average here since all metaslabs within the group must have
   * the same size. The return value will be a value between 0 and 100
   * (inclusive), or ZFS_FRAG_INVALID if less than half of the metaslab in this
   * group have a fragmentation metric.
   */
  uint64_t
  metaslab_group_fragmentation(metaslab_group_t *mg)
  {
          vdev_t *vd = mg->mg_vd;
          uint64_t fragmentation = 0;
          uint64_t valid_ms = 0;
  
          for (int m = 0; m < vd->vdev_ms_count; m++) {
                  metaslab_t *msp = vd->vdev_ms[m];
  
                  if (msp->ms_fragmentation == ZFS_FRAG_INVALID)
                          continue;
                  if (msp->ms_group != mg)
                          continue;
  
                  valid_ms++;
                  fragmentation += msp->ms_fragmentation;
          }
  
          if (valid_ms <= mg->mg_vd->vdev_ms_count / 2)
                  return (ZFS_FRAG_INVALID);
  
          fragmentation /= valid_ms;
          ASSERT3U(fragmentation, <=, 100);
          return (fragmentation);
  }
  
  /*
   * Determine if a given metaslab group should skip allocations. A metaslab
   * group should avoid allocations if its free capacity is less than the
   * zfs_mg_noalloc_threshold or its fragmentation metric is greater than
   * zfs_mg_fragmentation_threshold and there is at least one metaslab group
   * that can still handle allocations. If the allocation throttle is enabled
   * then we skip allocations to devices that have reached their maximum
   * allocation queue depth unless the selected metaslab group is the only
   * eligible group remaining.
   */
  static boolean_t
  metaslab_group_allocatable(metaslab_group_t *mg, metaslab_group_t *rotor,
      int flags, uint64_t psize, int allocator, int d)
  {
          spa_t *spa = mg->mg_vd->vdev_spa;
          metaslab_class_t *mc = mg->mg_class;
  
          /*
           * We can only consider skipping this metaslab group if it's
           * in the normal metaslab class and there are other metaslab
           * groups to select from. Otherwise, we always consider it eligible
           * for allocations.
           */
          if ((mc != spa_normal_class(spa) &&
              mc != spa_special_class(spa) &&
              mc != spa_dedup_class(spa)) ||
              mc->mc_groups <= 1)
                  return (B_TRUE);
  
          /*
           * If the metaslab group's mg_allocatable flag is set (see comments
           * in metaslab_group_alloc_update() for more information) and
           * the allocation throttle is disabled then allow allocations to this
           * device. However, if the allocation throttle is enabled then
           * check if we have reached our allocation limit (mga_alloc_queue_depth)
           * to determine if we should allow allocations to this metaslab group.
           * If all metaslab groups are no longer considered allocatable
           * (mc_alloc_groups == 0) or we're trying to allocate the smallest
           * gang block size then we allow allocations on this metaslab group
           * regardless of the mg_allocatable or throttle settings.
           */
          if (mg->mg_allocatable) {
                  metaslab_group_allocator_t *mga = &mg->mg_allocator[allocator];
                  int64_t qdepth;
                  uint64_t qmax = mga->mga_cur_max_alloc_queue_depth;
  
                  if (!mc->mc_alloc_throttle_enabled)
                          return (B_TRUE);
  
                  /*
                   * If this metaslab group does not have any free space, then
                   * there is no point in looking further.
                   */
                  if (mg->mg_no_free_space)
                          return (B_FALSE);
  
                  /*
                   * Some allocations (e.g., those coming from device removal
                   * where the * allocations are not even counted in the
                   * metaslab * allocation queues) are allowed to bypass
                   * the throttle.
                   */
                  if (flags & METASLAB_DONT_THROTTLE)
                          return (B_TRUE);
  
                  /*
                   * Relax allocation throttling for ditto blocks.  Due to
                   * random imbalances in allocation it tends to push copies
                   * to one vdev, that looks a bit better at the moment.
                   */
                  qmax = qmax * (4 + d) / 4;
  
                  qdepth = zfs_refcount_count(&mga->mga_alloc_queue_depth);
  
                  /*
                   * If this metaslab group is below its qmax or it's
                   * the only allocatable metasable group, then attempt
                   * to allocate from it.
                   */
                  if (qdepth < qmax || mc->mc_alloc_groups == 1)
                          return (B_TRUE);
                  ASSERT3U(mc->mc_alloc_groups, >, 1);
  
                  /*
                   * Since this metaslab group is at or over its qmax, we
                   * need to determine if there are metaslab groups after this
                   * one that might be able to handle this allocation. This is
                   * racy since we can't hold the locks for all metaslab
                   * groups at the same time when we make this check.
                   */
                  for (metaslab_group_t *mgp = mg->mg_next;
                      mgp != rotor; mgp = mgp->mg_next) {
                          metaslab_group_allocator_t *mgap =
                              &mgp->mg_allocator[allocator];
                          qmax = mgap->mga_cur_max_alloc_queue_depth;
                          qmax = qmax * (4 + d) / 4;
                          qdepth =
                              zfs_refcount_count(&mgap->mga_alloc_queue_depth);
  
                          /*
                           * If there is another metaslab group that
                           * might be able to handle the allocation, then
                           * we return false so that we skip this group.
                           */
                          if (qdepth < qmax && !mgp->mg_no_free_space)
                                  return (B_FALSE);
                  }
  
                  /*
                   * We didn't find another group to handle the allocation
                   * so we can't skip this metaslab group even though
                   * we are at or over our qmax.
                   */
                  return (B_TRUE);
  
          } else if (mc->mc_alloc_groups == 0 || psize == SPA_MINBLOCKSIZE) {
                  return (B_TRUE);
          }
          return (B_FALSE);
  }
  
  /*
   * ==========================================================================
   * Range tree callbacks
   * ==========================================================================
   */
  
  /*
   * Comparison function for the private size-ordered tree using 32-bit
   * ranges. Tree is sorted by size, larger sizes at the end of the tree.
   */
  static int
  metaslab_rangesize32_compare(const void *x1, const void *x2)
  {
          const range_seg32_t *r1 = x1;
          const range_seg32_t *r2 = x2;
  
          uint64_t rs_size1 = r1->rs_end - r1->rs_start;
          uint64_t rs_size2 = r2->rs_end - r2->rs_start;
  
          int cmp = TREE_CMP(rs_size1, rs_size2);
          if (likely(cmp))
                  return (cmp);
  
          return (TREE_CMP(r1->rs_start, r2->rs_start));
  }
  
  /*
   * Comparison function for the private size-ordered tree using 64-bit
   * ranges. Tree is sorted by size, larger sizes at the end of the tree.
   */
  static int
  metaslab_rangesize64_compare(const void *x1, const void *x2)
  {
          const range_seg64_t *r1 = x1;
          const range_seg64_t *r2 = x2;
  
          uint64_t rs_size1 = r1->rs_end - r1->rs_start;
          uint64_t rs_size2 = r2->rs_end - r2->rs_start;
  
          int cmp = TREE_CMP(rs_size1, rs_size2);
          if (likely(cmp))
                  return (cmp);
  
          return (TREE_CMP(r1->rs_start, r2->rs_start));
  }
  typedef struct metaslab_rt_arg {
          zfs_btree_t *mra_bt;
          uint32_t mra_floor_shift;
  } metaslab_rt_arg_t;
  
  struct mssa_arg {
          range_tree_t *rt;
          metaslab_rt_arg_t *mra;
  };
  
  static void
  metaslab_size_sorted_add(void *arg, uint64_t start, uint64_t size)
  {
          struct mssa_arg *mssap = arg;
          range_tree_t *rt = mssap->rt;
          metaslab_rt_arg_t *mrap = mssap->mra;
          range_seg_max_t seg = {0};
          rs_set_start(&seg, rt, start);
          rs_set_end(&seg, rt, start + size);
          metaslab_rt_add(rt, &seg, mrap);
  }
  
  static void
  metaslab_size_tree_full_load(range_tree_t *rt)
  {
          metaslab_rt_arg_t *mrap = rt->rt_arg;
          METASLABSTAT_BUMP(metaslabstat_reload_tree);
          ASSERT0(zfs_btree_numnodes(mrap->mra_bt));
          mrap->mra_floor_shift = 0;
          struct mssa_arg arg = {0};
          arg.rt = rt;
          arg.mra = mrap;
          range_tree_walk(rt, metaslab_size_sorted_add, &arg);
  }
  
  /*
   * Create any block allocator specific components. The current allocators
   * rely on using both a size-ordered range_tree_t and an array of uint64_t's.
   */
  static void
  metaslab_rt_create(range_tree_t *rt, void *arg)
  {
          metaslab_rt_arg_t *mrap = arg;
          zfs_btree_t *size_tree = mrap->mra_bt;
  
          size_t size;
          int (*compare) (const void *, const void *);
          switch (rt->rt_type) {
          case RANGE_SEG32:
                  size = sizeof (range_seg32_t);
                  compare = metaslab_rangesize32_compare;
                  break;
          case RANGE_SEG64:
                  size = sizeof (range_seg64_t);
                  compare = metaslab_rangesize64_compare;
                  break;
          default:
                  panic("Invalid range seg type %d", rt->rt_type);
          }
          zfs_btree_create(size_tree, compare, size);
          mrap->mra_floor_shift = metaslab_by_size_min_shift;
  }
  
  static void
  metaslab_rt_destroy(range_tree_t *rt, void *arg)
  {
          (void) rt;
          metaslab_rt_arg_t *mrap = arg;
          zfs_btree_t *size_tree = mrap->mra_bt;
  
          zfs_btree_destroy(size_tree);
          kmem_free(mrap, sizeof (*mrap));
  }
  
  static void
  metaslab_rt_add(range_tree_t *rt, range_seg_t *rs, void *arg)
  {
          metaslab_rt_arg_t *mrap = arg;
          zfs_btree_t *size_tree = mrap->mra_bt;
  
          if (rs_get_end(rs, rt) - rs_get_start(rs, rt) <
              (1ULL << mrap->mra_floor_shift))
                  return;
  
          zfs_btree_add(size_tree, rs);
  }
  
  static void
  metaslab_rt_remove(range_tree_t *rt, range_seg_t *rs, void *arg)
  {
          metaslab_rt_arg_t *mrap = arg;
          zfs_btree_t *size_tree = mrap->mra_bt;
  
          if (rs_get_end(rs, rt) - rs_get_start(rs, rt) < (1ULL <<
              mrap->mra_floor_shift))
                  return;
  
          zfs_btree_remove(size_tree, rs);
  }
  
  static void
  metaslab_rt_vacate(range_tree_t *rt, void *arg)
  {
          metaslab_rt_arg_t *mrap = arg;
          zfs_btree_t *size_tree = mrap->mra_bt;
          zfs_btree_clear(size_tree);
          zfs_btree_destroy(size_tree);
  
          metaslab_rt_create(rt, arg);
  }
  
  static const range_tree_ops_t metaslab_rt_ops = {
          .rtop_create = metaslab_rt_create,
          .rtop_destroy = metaslab_rt_destroy,
          .rtop_add = metaslab_rt_add,
          .rtop_remove = metaslab_rt_remove,
          .rtop_vacate = metaslab_rt_vacate
  };
  
  /*
   * ==========================================================================
   * Common allocator routines
   * ==========================================================================
   */
  
  /*
   * Return the maximum contiguous segment within the metaslab.
   */
  uint64_t
  metaslab_largest_allocatable(metaslab_t *msp)
  {
          zfs_btree_t *t = &msp->ms_allocatable_by_size;
          range_seg_t *rs;
  
          if (t == NULL)
                  return (0);
          if (zfs_btree_numnodes(t) == 0)
                  metaslab_size_tree_full_load(msp->ms_allocatable);
  
          rs = zfs_btree_last(t, NULL);
          if (rs == NULL)
                  return (0);
  
          return (rs_get_end(rs, msp->ms_allocatable) - rs_get_start(rs,
              msp->ms_allocatable));
  }
  
  /*
   * Return the maximum contiguous segment within the unflushed frees of this
   * metaslab.
   */
  static uint64_t
  metaslab_largest_unflushed_free(metaslab_t *msp)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          if (msp->ms_unflushed_frees == NULL)
                  return (0);
  
          if (zfs_btree_numnodes(&msp->ms_unflushed_frees_by_size) == 0)
                  metaslab_size_tree_full_load(msp->ms_unflushed_frees);
          range_seg_t *rs = zfs_btree_last(&msp->ms_unflushed_frees_by_size,
              NULL);
          if (rs == NULL)
                  return (0);
  
          /*
           * When a range is freed from the metaslab, that range is added to
           * both the unflushed frees and the deferred frees. While the block
           * will eventually be usable, if the metaslab were loaded the range
           * would not be added to the ms_allocatable tree until TXG_DEFER_SIZE
           * txgs had passed.  As a result, when attempting to estimate an upper
           * bound for the largest currently-usable free segment in the
           * metaslab, we need to not consider any ranges currently in the defer
           * trees. This algorithm approximates the largest available chunk in
           * the largest range in the unflushed_frees tree by taking the first
           * chunk.  While this may be a poor estimate, it should only remain so
           * briefly and should eventually self-correct as frees are no longer
           * deferred. Similar logic applies to the ms_freed tree. See
           * metaslab_load() for more details.
           *
           * There are two primary sources of inaccuracy in this estimate. Both
           * are tolerated for performance reasons. The first source is that we
           * only check the largest segment for overlaps. Smaller segments may
           * have more favorable overlaps with the other trees, resulting in
           * larger usable chunks.  Second, we only look at the first chunk in
           * the largest segment; there may be other usable chunks in the
           * largest segment, but we ignore them.
           */
          uint64_t rstart = rs_get_start(rs, msp->ms_unflushed_frees);
          uint64_t rsize = rs_get_end(rs, msp->ms_unflushed_frees) - rstart;
          for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                  uint64_t start = 0;
                  uint64_t size = 0;
                  boolean_t found = range_tree_find_in(msp->ms_defer[t], rstart,
                      rsize, &start, &size);
                  if (found) {
                          if (rstart == start)
                                  return (0);
                          rsize = start - rstart;
                  }
          }
  
          uint64_t start = 0;
          uint64_t size = 0;
          boolean_t found = range_tree_find_in(msp->ms_freed, rstart,
              rsize, &start, &size);
          if (found)
                  rsize = start - rstart;
  
          return (rsize);
  }
  
  static range_seg_t *
  metaslab_block_find(zfs_btree_t *t, range_tree_t *rt, uint64_t start,
      uint64_t size, zfs_btree_index_t *where)
  {
          range_seg_t *rs;
          range_seg_max_t rsearch;
  
          rs_set_start(&rsearch, rt, start);
          rs_set_end(&rsearch, rt, start + size);
  
          rs = zfs_btree_find(t, &rsearch, where);
          if (rs == NULL) {
                  rs = zfs_btree_next(t, where, where);
          }
  
          return (rs);
  }
  
  #if defined(WITH_DF_BLOCK_ALLOCATOR) || \
      defined(WITH_CF_BLOCK_ALLOCATOR)
  
  /*
   * This is a helper function that can be used by the allocator to find a
   * suitable block to allocate. This will search the specified B-tree looking
   * for a block that matches the specified criteria.
   */
  static uint64_t
  metaslab_block_picker(range_tree_t *rt, uint64_t *cursor, uint64_t size,
      uint64_t max_search)
  {
          if (*cursor == 0)
                  *cursor = rt->rt_start;
          zfs_btree_t *bt = &rt->rt_root;
          zfs_btree_index_t where;
          range_seg_t *rs = metaslab_block_find(bt, rt, *cursor, size, &where);
          uint64_t first_found;
          int count_searched = 0;
  
          if (rs != NULL)
                  first_found = rs_get_start(rs, rt);
  
          while (rs != NULL && (rs_get_start(rs, rt) - first_found <=
              max_search || count_searched < metaslab_min_search_count)) {
                  uint64_t offset = rs_get_start(rs, rt);
                  if (offset + size <= rs_get_end(rs, rt)) {
                          *cursor = offset + size;
                          return (offset);
                  }
                  rs = zfs_btree_next(bt, &where, &where);
                  count_searched++;
          }
  
          *cursor = 0;
          return (-1ULL);
  }
  #endif /* WITH_DF/CF_BLOCK_ALLOCATOR */
  
  #if defined(WITH_DF_BLOCK_ALLOCATOR)
  /*
   * ==========================================================================
   * Dynamic Fit (df) block allocator
   *
   * Search for a free chunk of at least this size, starting from the last
   * offset (for this alignment of block) looking for up to
   * metaslab_df_max_search bytes (16MB).  If a large enough free chunk is not
   * found within 16MB, then return a free chunk of exactly the requested size (or
   * larger).
   *
   * If it seems like searching from the last offset will be unproductive, skip
   * that and just return a free chunk of exactly the requested size (or larger).
   * This is based on metaslab_df_alloc_threshold and metaslab_df_free_pct.  This
   * mechanism is probably not very useful and may be removed in the future.
   *
   * The behavior when not searching can be changed to return the largest free
   * chunk, instead of a free chunk of exactly the requested size, by setting
   * metaslab_df_use_largest_segment.
   * ==========================================================================
   */
  static uint64_t
  metaslab_df_alloc(metaslab_t *msp, uint64_t size)
  {
          /*
           * Find the largest power of 2 block size that evenly divides the
           * requested size. This is used to try to allocate blocks with similar
           * alignment from the same area of the metaslab (i.e. same cursor
           * bucket) but it does not guarantee that other allocations sizes
           * may exist in the same region.
           */
          uint64_t align = size & -size;
          uint64_t *cursor = &msp->ms_lbas[highbit64(align) - 1];
          range_tree_t *rt = msp->ms_allocatable;
          uint_t free_pct = range_tree_space(rt) * 100 / msp->ms_size;
          uint64_t offset;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          /*
           * If we're running low on space, find a segment based on size,
           * rather than iterating based on offset.
           */
          if (metaslab_largest_allocatable(msp) < metaslab_df_alloc_threshold ||
              free_pct < metaslab_df_free_pct) {
                  offset = -1;
          } else {
                  offset = metaslab_block_picker(rt,
                      cursor, size, metaslab_df_max_search);
          }
  
          if (offset == -1) {
                  range_seg_t *rs;
                  if (zfs_btree_numnodes(&msp->ms_allocatable_by_size) == 0)
                          metaslab_size_tree_full_load(msp->ms_allocatable);
  
                  if (metaslab_df_use_largest_segment) {
                          /* use largest free segment */
                          rs = zfs_btree_last(&msp->ms_allocatable_by_size, NULL);
                  } else {
                          zfs_btree_index_t where;
                          /* use segment of this size, or next largest */
                          rs = metaslab_block_find(&msp->ms_allocatable_by_size,
                              rt, msp->ms_start, size, &where);
                  }
                  if (rs != NULL && rs_get_start(rs, rt) + size <= rs_get_end(rs,
                      rt)) {
                          offset = rs_get_start(rs, rt);
                          *cursor = offset + size;
                  }
          }
  
          return (offset);
  }
  
  const metaslab_ops_t zfs_metaslab_ops = {
          metaslab_df_alloc
  };
  #endif /* WITH_DF_BLOCK_ALLOCATOR */
  
  #if defined(WITH_CF_BLOCK_ALLOCATOR)
  /*
   * ==========================================================================
   * Cursor fit block allocator -
   * Select the largest region in the metaslab, set the cursor to the beginning
   * of the range and the cursor_end to the end of the range. As allocations
   * are made advance the cursor. Continue allocating from the cursor until
   * the range is exhausted and then find a new range.
   * ==========================================================================
   */
  static uint64_t
  metaslab_cf_alloc(metaslab_t *msp, uint64_t size)
  {
          range_tree_t *rt = msp->ms_allocatable;
          zfs_btree_t *t = &msp->ms_allocatable_by_size;
          uint64_t *cursor = &msp->ms_lbas[0];
          uint64_t *cursor_end = &msp->ms_lbas[1];
          uint64_t offset = 0;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          ASSERT3U(*cursor_end, >=, *cursor);
  
          if ((*cursor + size) > *cursor_end) {
                  range_seg_t *rs;
  
                  if (zfs_btree_numnodes(t) == 0)
                          metaslab_size_tree_full_load(msp->ms_allocatable);
                  rs = zfs_btree_last(t, NULL);
                  if (rs == NULL || (rs_get_end(rs, rt) - rs_get_start(rs, rt)) <
                      size)
                          return (-1ULL);
  
                  *cursor = rs_get_start(rs, rt);
                  *cursor_end = rs_get_end(rs, rt);
          }
  
          offset = *cursor;
          *cursor += size;
  
          return (offset);
  }
  
  const metaslab_ops_t zfs_metaslab_ops = {
          metaslab_cf_alloc
  };
  #endif /* WITH_CF_BLOCK_ALLOCATOR */
  
  #if defined(WITH_NDF_BLOCK_ALLOCATOR)
  /*
   * ==========================================================================
   * New dynamic fit allocator -
   * Select a region that is large enough to allocate 2^metaslab_ndf_clump_shift
   * contiguous blocks. If no region is found then just use the largest segment
   * that remains.
   * ==========================================================================
   */
  
  /*
   * Determines desired number of contiguous blocks (2^metaslab_ndf_clump_shift)
   * to request from the allocator.
   */
  uint64_t metaslab_ndf_clump_shift = 4;
  
  static uint64_t
  metaslab_ndf_alloc(metaslab_t *msp, uint64_t size)
  {
          zfs_btree_t *t = &msp->ms_allocatable->rt_root;
          range_tree_t *rt = msp->ms_allocatable;
          zfs_btree_index_t where;
          range_seg_t *rs;
          range_seg_max_t rsearch;
          uint64_t hbit = highbit64(size);
          uint64_t *cursor = &msp->ms_lbas[hbit - 1];
          uint64_t max_size = metaslab_largest_allocatable(msp);
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          if (max_size < size)
                  return (-1ULL);
  
          rs_set_start(&rsearch, rt, *cursor);
          rs_set_end(&rsearch, rt, *cursor + size);
  
          rs = zfs_btree_find(t, &rsearch, &where);
          if (rs == NULL || (rs_get_end(rs, rt) - rs_get_start(rs, rt)) < size) {
                  t = &msp->ms_allocatable_by_size;
  
                  rs_set_start(&rsearch, rt, 0);
                  rs_set_end(&rsearch, rt, MIN(max_size, 1ULL << (hbit +
                      metaslab_ndf_clump_shift)));
  
                  rs = zfs_btree_find(t, &rsearch, &where);
                  if (rs == NULL)
                          rs = zfs_btree_next(t, &where, &where);
                  ASSERT(rs != NULL);
          }
  
          if ((rs_get_end(rs, rt) - rs_get_start(rs, rt)) >= size) {
                  *cursor = rs_get_start(rs, rt) + size;
                  return (rs_get_start(rs, rt));
          }
          return (-1ULL);
  }
  
  const metaslab_ops_t zfs_metaslab_ops = {
          metaslab_ndf_alloc
  };
  #endif /* WITH_NDF_BLOCK_ALLOCATOR */
  
  
  /*
   * ==========================================================================
   * Metaslabs
   * ==========================================================================
   */
  
  /*
   * Wait for any in-progress metaslab loads to complete.
   */
  static void
  metaslab_load_wait(metaslab_t *msp)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          while (msp->ms_loading) {
                  ASSERT(!msp->ms_loaded);
                  cv_wait(&msp->ms_load_cv, &msp->ms_lock);
          }
  }
  
  /*
   * Wait for any in-progress flushing to complete.
   */
  static void
  metaslab_flush_wait(metaslab_t *msp)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          while (msp->ms_flushing)
                  cv_wait(&msp->ms_flush_cv, &msp->ms_lock);
  }
  
  static unsigned int
  metaslab_idx_func(multilist_t *ml, void *arg)
  {
          metaslab_t *msp = arg;
  
          /*
           * ms_id values are allocated sequentially, so full 64bit
           * division would be a waste of time, so limit it to 32 bits.
           */
          return ((unsigned int)msp->ms_id % multilist_get_num_sublists(ml));
  }
  
  uint64_t
  metaslab_allocated_space(metaslab_t *msp)
  {
          return (msp->ms_allocated_space);
  }
  
  /*
   * Verify that the space accounting on disk matches the in-core range_trees.
   */
  static void
  metaslab_verify_space(metaslab_t *msp, uint64_t txg)
  {
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
          uint64_t allocating = 0;
          uint64_t sm_free_space, msp_free_space;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          ASSERT(!msp->ms_condensing);
  
          if ((zfs_flags & ZFS_DEBUG_METASLAB_VERIFY) == 0)
                  return;
  
          /*
           * We can only verify the metaslab space when we're called
           * from syncing context with a loaded metaslab that has an
           * allocated space map. Calling this in non-syncing context
           * does not provide a consistent view of the metaslab since
           * we're performing allocations in the future.
           */
          if (txg != spa_syncing_txg(spa) || msp->ms_sm == NULL ||
              !msp->ms_loaded)
                  return;
  
          /*
           * Even though the smp_alloc field can get negative,
           * when it comes to a metaslab's space map, that should
           * never be the case.
           */
          ASSERT3S(space_map_allocated(msp->ms_sm), >=, 0);
  
          ASSERT3U(space_map_allocated(msp->ms_sm), >=,
              range_tree_space(msp->ms_unflushed_frees));
  
          ASSERT3U(metaslab_allocated_space(msp), ==,
              space_map_allocated(msp->ms_sm) +
              range_tree_space(msp->ms_unflushed_allocs) -
              range_tree_space(msp->ms_unflushed_frees));
  
          sm_free_space = msp->ms_size - metaslab_allocated_space(msp);
  
          /*
           * Account for future allocations since we would have
           * already deducted that space from the ms_allocatable.
           */
          for (int t = 0; t < TXG_CONCURRENT_STATES; t++) {
                  allocating +=
                      range_tree_space(msp->ms_allocating[(txg + t) & TXG_MASK]);
          }
          ASSERT3U(allocating + msp->ms_allocated_this_txg, ==,
              msp->ms_allocating_total);
  
          ASSERT3U(msp->ms_deferspace, ==,
              range_tree_space(msp->ms_defer[0]) +
              range_tree_space(msp->ms_defer[1]));
  
          msp_free_space = range_tree_space(msp->ms_allocatable) + allocating +
              msp->ms_deferspace + range_tree_space(msp->ms_freed);
  
          VERIFY3U(sm_free_space, ==, msp_free_space);
  }
  
  static void
  metaslab_aux_histograms_clear(metaslab_t *msp)
  {
          /*
           * Auxiliary histograms are only cleared when resetting them,
           * which can only happen while the metaslab is loaded.
           */
          ASSERT(msp->ms_loaded);
  
          memset(msp->ms_synchist, 0, sizeof (msp->ms_synchist));
          for (int t = 0; t < TXG_DEFER_SIZE; t++)
                  memset(msp->ms_deferhist[t], 0, sizeof (msp->ms_deferhist[t]));
  }
  
  static void
  metaslab_aux_histogram_add(uint64_t *histogram, uint64_t shift,
      range_tree_t *rt)
  {
          /*
           * This is modeled after space_map_histogram_add(), so refer to that
           * function for implementation details. We want this to work like
           * the space map histogram, and not the range tree histogram, as we
           * are essentially constructing a delta that will be later subtracted
           * from the space map histogram.
           */
          int idx = 0;
          for (int i = shift; i < RANGE_TREE_HISTOGRAM_SIZE; i++) {
                  ASSERT3U(i, >=, idx + shift);
                  histogram[idx] += rt->rt_histogram[i] << (i - idx - shift);
  
                  if (idx < SPACE_MAP_HISTOGRAM_SIZE - 1) {
                          ASSERT3U(idx + shift, ==, i);
                          idx++;
                          ASSERT3U(idx, <, SPACE_MAP_HISTOGRAM_SIZE);
                  }
          }
  }
  
  /*
   * Called at every sync pass that the metaslab gets synced.
   *
   * The reason is that we want our auxiliary histograms to be updated
   * wherever the metaslab's space map histogram is updated. This way
   * we stay consistent on which parts of the metaslab space map's
   * histogram are currently not available for allocations (e.g because
   * they are in the defer, freed, and freeing trees).
   */
  static void
  metaslab_aux_histograms_update(metaslab_t *msp)
  {
          space_map_t *sm = msp->ms_sm;
          ASSERT(sm != NULL);
  
          /*
           * This is similar to the metaslab's space map histogram updates
           * that take place in metaslab_sync(). The only difference is that
           * we only care about segments that haven't made it into the
           * ms_allocatable tree yet.
           */
          if (msp->ms_loaded) {
                  metaslab_aux_histograms_clear(msp);
  
                  metaslab_aux_histogram_add(msp->ms_synchist,
                      sm->sm_shift, msp->ms_freed);
  
                  for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                          metaslab_aux_histogram_add(msp->ms_deferhist[t],
                              sm->sm_shift, msp->ms_defer[t]);
                  }
          }
  
          metaslab_aux_histogram_add(msp->ms_synchist,
              sm->sm_shift, msp->ms_freeing);
  }
  
  /*
   * Called every time we are done syncing (writing to) the metaslab,
   * i.e. at the end of each sync pass.
   * [see the comment in metaslab_impl.h for ms_synchist, ms_deferhist]
   */
  static void
  metaslab_aux_histograms_update_done(metaslab_t *msp, boolean_t defer_allowed)
  {
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
          space_map_t *sm = msp->ms_sm;
  
          if (sm == NULL) {
                  /*
                   * We came here from metaslab_init() when creating/opening a
                   * pool, looking at a metaslab that hasn't had any allocations
                   * yet.
                   */
                  return;
          }
  
          /*
           * This is similar to the actions that we take for the ms_freed
           * and ms_defer trees in metaslab_sync_done().
           */
          uint64_t hist_index = spa_syncing_txg(spa) % TXG_DEFER_SIZE;
          if (defer_allowed) {
                  memcpy(msp->ms_deferhist[hist_index], msp->ms_synchist,
                      sizeof (msp->ms_synchist));
          } else {
                  memset(msp->ms_deferhist[hist_index], 0,
                      sizeof (msp->ms_deferhist[hist_index]));
          }
          memset(msp->ms_synchist, 0, sizeof (msp->ms_synchist));
  }
  
  /*
   * Ensure that the metaslab's weight and fragmentation are consistent
   * with the contents of the histogram (either the range tree's histogram
   * or the space map's depending whether the metaslab is loaded).
   */
  static void
  metaslab_verify_weight_and_frag(metaslab_t *msp)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          if ((zfs_flags & ZFS_DEBUG_METASLAB_VERIFY) == 0)
                  return;
  
          /*
           * We can end up here from vdev_remove_complete(), in which case we
           * cannot do these assertions because we hold spa config locks and
           * thus we are not allowed to read from the DMU.
           *
           * We check if the metaslab group has been removed and if that's
           * the case we return immediately as that would mean that we are
           * here from the aforementioned code path.
           */
          if (msp->ms_group == NULL)
                  return;
  
          /*
           * Devices being removed always return a weight of 0 and leave
           * fragmentation and ms_max_size as is - there is nothing for
           * us to verify here.
           */
          vdev_t *vd = msp->ms_group->mg_vd;
          if (vd->vdev_removing)
                  return;
  
          /*
           * If the metaslab is dirty it probably means that we've done
           * some allocations or frees that have changed our histograms
           * and thus the weight.
           */
          for (int t = 0; t < TXG_SIZE; t++) {
                  if (txg_list_member(&vd->vdev_ms_list, msp, t))
                          return;
          }
  
          /*
           * This verification checks that our in-memory state is consistent
           * with what's on disk. If the pool is read-only then there aren't
           * any changes and we just have the initially-loaded state.
           */
          if (!spa_writeable(msp->ms_group->mg_vd->vdev_spa))
                  return;
  
          /* some extra verification for in-core tree if you can */
          if (msp->ms_loaded) {
                  range_tree_stat_verify(msp->ms_allocatable);
                  VERIFY(space_map_histogram_verify(msp->ms_sm,
                      msp->ms_allocatable));
          }
  
          uint64_t weight = msp->ms_weight;
          uint64_t was_active = msp->ms_weight & METASLAB_ACTIVE_MASK;
          boolean_t space_based = WEIGHT_IS_SPACEBASED(msp->ms_weight);
          uint64_t frag = msp->ms_fragmentation;
          uint64_t max_segsize = msp->ms_max_size;
  
          msp->ms_weight = 0;
          msp->ms_fragmentation = 0;
  
          /*
           * This function is used for verification purposes and thus should
           * not introduce any side-effects/mutations on the system's state.
           *
           * Regardless of whether metaslab_weight() thinks this metaslab
           * should be active or not, we want to ensure that the actual weight
           * (and therefore the value of ms_weight) would be the same if it
           * was to be recalculated at this point.
           *
           * In addition we set the nodirty flag so metaslab_weight() does
           * not dirty the metaslab for future TXGs (e.g. when trying to
           * force condensing to upgrade the metaslab spacemaps).
           */
          msp->ms_weight = metaslab_weight(msp, B_TRUE) | was_active;
  
          VERIFY3U(max_segsize, ==, msp->ms_max_size);
  
          /*
           * If the weight type changed then there is no point in doing
           * verification. Revert fields to their original values.
           */
          if ((space_based && !WEIGHT_IS_SPACEBASED(msp->ms_weight)) ||
              (!space_based && WEIGHT_IS_SPACEBASED(msp->ms_weight))) {
                  msp->ms_fragmentation = frag;
                  msp->ms_weight = weight;
                  return;
          }
  
          VERIFY3U(msp->ms_fragmentation, ==, frag);
          VERIFY3U(msp->ms_weight, ==, weight);
  }
  
  /*
   * If we're over the zfs_metaslab_mem_limit, select the loaded metaslab from
   * this class that was used longest ago, and attempt to unload it.  We don't
   * want to spend too much time in this loop to prevent performance
   * degradation, and we expect that most of the time this operation will
   * succeed. Between that and the normal unloading processing during txg sync,
   * we expect this to keep the metaslab memory usage under control.
   */
  static void
  metaslab_potentially_evict(metaslab_class_t *mc)
  {
  #ifdef _KERNEL
          uint64_t allmem = arc_all_memory();
          uint64_t inuse = spl_kmem_cache_inuse(zfs_btree_leaf_cache);
          uint64_t size = spl_kmem_cache_entry_size(zfs_btree_leaf_cache);
          uint_t tries = 0;
          for (; allmem * zfs_metaslab_mem_limit / 100 < inuse * size &&
              tries < multilist_get_num_sublists(&mc->mc_metaslab_txg_list) * 2;
              tries++) {
                  unsigned int idx = multilist_get_random_index(
                      &mc->mc_metaslab_txg_list);
                  multilist_sublist_t *mls =
                      multilist_sublist_lock(&mc->mc_metaslab_txg_list, idx);
                  metaslab_t *msp = multilist_sublist_head(mls);
                  multilist_sublist_unlock(mls);
                  while (msp != NULL && allmem * zfs_metaslab_mem_limit / 100 <
                      inuse * size) {
                          VERIFY3P(mls, ==, multilist_sublist_lock(
                              &mc->mc_metaslab_txg_list, idx));
                          ASSERT3U(idx, ==,
                              metaslab_idx_func(&mc->mc_metaslab_txg_list, msp));
  
                          if (!multilist_link_active(&msp->ms_class_txg_node)) {
                                  multilist_sublist_unlock(mls);
                                  break;
                          }
                          metaslab_t *next_msp = multilist_sublist_next(mls, msp);
                          multilist_sublist_unlock(mls);
                          /*
                           * If the metaslab is currently loading there are two
                           * cases. If it's the metaslab we're evicting, we
                           * can't continue on or we'll panic when we attempt to
                           * recursively lock the mutex. If it's another
                           * metaslab that's loading, it can be safely skipped,
                           * since we know it's very new and therefore not a
                           * good eviction candidate. We check later once the
                           * lock is held that the metaslab is fully loaded
                           * before actually unloading it.
                           */
                          if (msp->ms_loading) {
                                  msp = next_msp;
                                  inuse =
                                      spl_kmem_cache_inuse(zfs_btree_leaf_cache);
                                  continue;
                          }
                          /*
                           * We can't unload metaslabs with no spacemap because
                           * they're not ready to be unloaded yet. We can't
                           * unload metaslabs with outstanding allocations
                           * because doing so could cause the metaslab's weight
                           * to decrease while it's unloaded, which violates an
                           * invariant that we use to prevent unnecessary
                           * loading. We also don't unload metaslabs that are
                           * currently active because they are high-weight
                           * metaslabs that are likely to be used in the near
                           * future.
                           */
                          mutex_enter(&msp->ms_lock);
                          if (msp->ms_allocator == -1 && msp->ms_sm != NULL &&
                              msp->ms_allocating_total == 0) {
                                  metaslab_unload(msp);
                          }
                          mutex_exit(&msp->ms_lock);
                          msp = next_msp;
                          inuse = spl_kmem_cache_inuse(zfs_btree_leaf_cache);
                  }
          }
  #else
          (void) mc, (void) zfs_metaslab_mem_limit;
  #endif
  }
  
  static int
  metaslab_load_impl(metaslab_t *msp)
  {
          int error = 0;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          ASSERT(msp->ms_loading);
          ASSERT(!msp->ms_condensing);
  
          /*
           * We temporarily drop the lock to unblock other operations while we
           * are reading the space map. Therefore, metaslab_sync() and
           * metaslab_sync_done() can run at the same time as we do.
           *
           * If we are using the log space maps, metaslab_sync() can't write to
           * the metaslab's space map while we are loading as we only write to
           * it when we are flushing the metaslab, and that can't happen while
           * we are loading it.
           *
           * If we are not using log space maps though, metaslab_sync() can
           * append to the space map while we are loading. Therefore we load
           * only entries that existed when we started the load. Additionally,
           * metaslab_sync_done() has to wait for the load to complete because
           * there are potential races like metaslab_load() loading parts of the
           * space map that are currently being appended by metaslab_sync(). If
           * we didn't, the ms_allocatable would have entries that
           * metaslab_sync_done() would try to re-add later.
           *
           * That's why before dropping the lock we remember the synced length
           * of the metaslab and read up to that point of the space map,
           * ignoring entries appended by metaslab_sync() that happen after we
           * drop the lock.
           */
          uint64_t length = msp->ms_synced_length;
          mutex_exit(&msp->ms_lock);
  
          hrtime_t load_start = gethrtime();
          metaslab_rt_arg_t *mrap;
          if (msp->ms_allocatable->rt_arg == NULL) {
                  mrap = kmem_zalloc(sizeof (*mrap), KM_SLEEP);
          } else {
                  mrap = msp->ms_allocatable->rt_arg;
                  msp->ms_allocatable->rt_ops = NULL;
                  msp->ms_allocatable->rt_arg = NULL;
          }
          mrap->mra_bt = &msp->ms_allocatable_by_size;
          mrap->mra_floor_shift = metaslab_by_size_min_shift;
  
          if (msp->ms_sm != NULL) {
                  error = space_map_load_length(msp->ms_sm, msp->ms_allocatable,
                      SM_FREE, length);
  
                  /* Now, populate the size-sorted tree. */
                  metaslab_rt_create(msp->ms_allocatable, mrap);
                  msp->ms_allocatable->rt_ops = &metaslab_rt_ops;
                  msp->ms_allocatable->rt_arg = mrap;
  
                  struct mssa_arg arg = {0};
                  arg.rt = msp->ms_allocatable;
                  arg.mra = mrap;
                  range_tree_walk(msp->ms_allocatable, metaslab_size_sorted_add,
                      &arg);
          } else {
                  /*
                   * Add the size-sorted tree first, since we don't need to load
                   * the metaslab from the spacemap.
                   */
                  metaslab_rt_create(msp->ms_allocatable, mrap);
                  msp->ms_allocatable->rt_ops = &metaslab_rt_ops;
                  msp->ms_allocatable->rt_arg = mrap;
                  /*
                   * The space map has not been allocated yet, so treat
                   * all the space in the metaslab as free and add it to the
                   * ms_allocatable tree.
                   */
                  range_tree_add(msp->ms_allocatable,
                      msp->ms_start, msp->ms_size);
  
                  if (msp->ms_new) {
                          /*
                           * If the ms_sm doesn't exist, this means that this
                           * metaslab hasn't gone through metaslab_sync() and
                           * thus has never been dirtied. So we shouldn't
                           * expect any unflushed allocs or frees from previous
                           * TXGs.
                           */
                          ASSERT(range_tree_is_empty(msp->ms_unflushed_allocs));
                          ASSERT(range_tree_is_empty(msp->ms_unflushed_frees));
                  }
          }
  
          /*
           * We need to grab the ms_sync_lock to prevent metaslab_sync() from
           * changing the ms_sm (or log_sm) and the metaslab's range trees
           * while we are about to use them and populate the ms_allocatable.
           * The ms_lock is insufficient for this because metaslab_sync() doesn't
           * hold the ms_lock while writing the ms_checkpointing tree to disk.
           */
          mutex_enter(&msp->ms_sync_lock);
          mutex_enter(&msp->ms_lock);
  
          ASSERT(!msp->ms_condensing);
          ASSERT(!msp->ms_flushing);
  
          if (error != 0) {
                  mutex_exit(&msp->ms_sync_lock);
                  return (error);
          }
  
          ASSERT3P(msp->ms_group, !=, NULL);
          msp->ms_loaded = B_TRUE;
  
          /*
           * Apply all the unflushed changes to ms_allocatable right
           * away so any manipulations we do below have a clear view
           * of what is allocated and what is free.
           */
          range_tree_walk(msp->ms_unflushed_allocs,
              range_tree_remove, msp->ms_allocatable);
          range_tree_walk(msp->ms_unflushed_frees,
              range_tree_add, msp->ms_allocatable);
  
          ASSERT3P(msp->ms_group, !=, NULL);
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
          if (spa_syncing_log_sm(spa) != NULL) {
                  ASSERT(spa_feature_is_enabled(spa,
                      SPA_FEATURE_LOG_SPACEMAP));
  
                  /*
                   * If we use a log space map we add all the segments
                   * that are in ms_unflushed_frees so they are available
                   * for allocation.
                   *
                   * ms_allocatable needs to contain all free segments
                   * that are ready for allocations (thus not segments
                   * from ms_freeing, ms_freed, and the ms_defer trees).
                   * But if we grab the lock in this code path at a sync
                   * pass later that 1, then it also contains the
                   * segments of ms_freed (they were added to it earlier
                   * in this path through ms_unflushed_frees). So we
                   * need to remove all the segments that exist in
                   * ms_freed from ms_allocatable as they will be added
                   * later in metaslab_sync_done().
                   *
                   * When there's no log space map, the ms_allocatable
                   * correctly doesn't contain any segments that exist
                   * in ms_freed [see ms_synced_length].
                   */
                  range_tree_walk(msp->ms_freed,
                      range_tree_remove, msp->ms_allocatable);
          }
  
          /*
           * If we are not using the log space map, ms_allocatable
           * contains the segments that exist in the ms_defer trees
           * [see ms_synced_length]. Thus we need to remove them
           * from ms_allocatable as they will be added again in
           * metaslab_sync_done().
           *
           * If we are using the log space map, ms_allocatable still
           * contains the segments that exist in the ms_defer trees.
           * Not because it read them through the ms_sm though. But
           * because these segments are part of ms_unflushed_frees
           * whose segments we add to ms_allocatable earlier in this
           * code path.
           */
          for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                  range_tree_walk(msp->ms_defer[t],
                      range_tree_remove, msp->ms_allocatable);
          }
  
          /*
           * Call metaslab_recalculate_weight_and_sort() now that the
           * metaslab is loaded so we get the metaslab's real weight.
           *
           * Unless this metaslab was created with older software and
           * has not yet been converted to use segment-based weight, we
           * expect the new weight to be better or equal to the weight
           * that the metaslab had while it was not loaded. This is
           * because the old weight does not take into account the
           * consolidation of adjacent segments between TXGs. [see
           * comment for ms_synchist and ms_deferhist[] for more info]
           */
          uint64_t weight = msp->ms_weight;
          uint64_t max_size = msp->ms_max_size;
          metaslab_recalculate_weight_and_sort(msp);
          if (!WEIGHT_IS_SPACEBASED(weight))
                  ASSERT3U(weight, <=, msp->ms_weight);
          msp->ms_max_size = metaslab_largest_allocatable(msp);
          ASSERT3U(max_size, <=, msp->ms_max_size);
          hrtime_t load_end = gethrtime();
          msp->ms_load_time = load_end;
          zfs_dbgmsg("metaslab_load: txg %llu, spa %s, vdev_id %llu, "
              "ms_id %llu, smp_length %llu, "
              "unflushed_allocs %llu, unflushed_frees %llu, "
              "freed %llu, defer %llu + %llu, unloaded time %llu ms, "
              "loading_time %lld ms, ms_max_size %llu, "
              "max size error %lld, "
              "old_weight %llx, new_weight %llx",
              (u_longlong_t)spa_syncing_txg(spa), spa_name(spa),
              (u_longlong_t)msp->ms_group->mg_vd->vdev_id,
              (u_longlong_t)msp->ms_id,
              (u_longlong_t)space_map_length(msp->ms_sm),
              (u_longlong_t)range_tree_space(msp->ms_unflushed_allocs),
              (u_longlong_t)range_tree_space(msp->ms_unflushed_frees),
              (u_longlong_t)range_tree_space(msp->ms_freed),
              (u_longlong_t)range_tree_space(msp->ms_defer[0]),
              (u_longlong_t)range_tree_space(msp->ms_defer[1]),
              (longlong_t)((load_start - msp->ms_unload_time) / 1000000),
              (longlong_t)((load_end - load_start) / 1000000),
              (u_longlong_t)msp->ms_max_size,
              (u_longlong_t)msp->ms_max_size - max_size,
              (u_longlong_t)weight, (u_longlong_t)msp->ms_weight);
  
          metaslab_verify_space(msp, spa_syncing_txg(spa));
          mutex_exit(&msp->ms_sync_lock);
          return (0);
  }
  
  int
  metaslab_load(metaslab_t *msp)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          /*
           * There may be another thread loading the same metaslab, if that's
           * the case just wait until the other thread is done and return.
           */
          metaslab_load_wait(msp);
          if (msp->ms_loaded)
                  return (0);
          VERIFY(!msp->ms_loading);
          ASSERT(!msp->ms_condensing);
  
          /*
           * We set the loading flag BEFORE potentially dropping the lock to
           * wait for an ongoing flush (see ms_flushing below). This way other
           * threads know that there is already a thread that is loading this
           * metaslab.
           */
          msp->ms_loading = B_TRUE;
  
          /*
           * Wait for any in-progress flushing to finish as we drop the ms_lock
           * both here (during space_map_load()) and in metaslab_flush() (when
           * we flush our changes to the ms_sm).
           */
          if (msp->ms_flushing)
                  metaslab_flush_wait(msp);
  
          /*
           * In the possibility that we were waiting for the metaslab to be
           * flushed (where we temporarily dropped the ms_lock), ensure that
           * no one else loaded the metaslab somehow.
           */
          ASSERT(!msp->ms_loaded);
  
          /*
           * If we're loading a metaslab in the normal class, consider evicting
           * another one to keep our memory usage under the limit defined by the
           * zfs_metaslab_mem_limit tunable.
           */
          if (spa_normal_class(msp->ms_group->mg_class->mc_spa) ==
              msp->ms_group->mg_class) {
                  metaslab_potentially_evict(msp->ms_group->mg_class);
          }
  
          int error = metaslab_load_impl(msp);
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          msp->ms_loading = B_FALSE;
          cv_broadcast(&msp->ms_load_cv);
  
          return (error);
  }
  
  void
  metaslab_unload(metaslab_t *msp)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          /*
           * This can happen if a metaslab is selected for eviction (in
           * metaslab_potentially_evict) and then unloaded during spa_sync (via
           * metaslab_class_evict_old).
           */
          if (!msp->ms_loaded)
                  return;
  
          range_tree_vacate(msp->ms_allocatable, NULL, NULL);
          msp->ms_loaded = B_FALSE;
          msp->ms_unload_time = gethrtime();
  
          msp->ms_activation_weight = 0;
          msp->ms_weight &= ~METASLAB_ACTIVE_MASK;
  
          if (msp->ms_group != NULL) {
                  metaslab_class_t *mc = msp->ms_group->mg_class;
                  multilist_sublist_t *mls =
                      multilist_sublist_lock_obj(&mc->mc_metaslab_txg_list, msp);
                  if (multilist_link_active(&msp->ms_class_txg_node))
                          multilist_sublist_remove(mls, msp);
                  multilist_sublist_unlock(mls);
  
                  spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
                  zfs_dbgmsg("metaslab_unload: txg %llu, spa %s, vdev_id %llu, "
                      "ms_id %llu, weight %llx, "
                      "selected txg %llu (%llu ms ago), alloc_txg %llu, "
                      "loaded %llu ms ago, max_size %llu",
                      (u_longlong_t)spa_syncing_txg(spa), spa_name(spa),
                      (u_longlong_t)msp->ms_group->mg_vd->vdev_id,
                      (u_longlong_t)msp->ms_id,
                      (u_longlong_t)msp->ms_weight,
                      (u_longlong_t)msp->ms_selected_txg,
                      (u_longlong_t)(msp->ms_unload_time -
                      msp->ms_selected_time) / 1000 / 1000,
                      (u_longlong_t)msp->ms_alloc_txg,
                      (u_longlong_t)(msp->ms_unload_time -
                      msp->ms_load_time) / 1000 / 1000,
                      (u_longlong_t)msp->ms_max_size);
          }
  
          /*
           * We explicitly recalculate the metaslab's weight based on its space
           * map (as it is now not loaded). We want unload metaslabs to always
           * have their weights calculated from the space map histograms, while
           * loaded ones have it calculated from their in-core range tree
           * [see metaslab_load()]. This way, the weight reflects the information
           * available in-core, whether it is loaded or not.
           *
           * If ms_group == NULL means that we came here from metaslab_fini(),
           * at which point it doesn't make sense for us to do the recalculation
           * and the sorting.
           */
          if (msp->ms_group != NULL)
                  metaslab_recalculate_weight_and_sort(msp);
  }
  
  /*
   * We want to optimize the memory use of the per-metaslab range
   * trees. To do this, we store the segments in the range trees in
   * units of sectors, zero-indexing from the start of the metaslab. If
   * the vdev_ms_shift - the vdev_ashift is less than 32, we can store
   * the ranges using two uint32_ts, rather than two uint64_ts.
   */
  range_seg_type_t
  metaslab_calculate_range_tree_type(vdev_t *vdev, metaslab_t *msp,
      uint64_t *start, uint64_t *shift)
  {
          if (vdev->vdev_ms_shift - vdev->vdev_ashift < 32 &&
              !zfs_metaslab_force_large_segs) {
                  *shift = vdev->vdev_ashift;
                  *start = msp->ms_start;
                  return (RANGE_SEG32);
          } else {
                  *shift = 0;
                  *start = 0;
                  return (RANGE_SEG64);
          }
  }
  
  void
  metaslab_set_selected_txg(metaslab_t *msp, uint64_t txg)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          metaslab_class_t *mc = msp->ms_group->mg_class;
          multilist_sublist_t *mls =
              multilist_sublist_lock_obj(&mc->mc_metaslab_txg_list, msp);
          if (multilist_link_active(&msp->ms_class_txg_node))
                  multilist_sublist_remove(mls, msp);
          msp->ms_selected_txg = txg;
          msp->ms_selected_time = gethrtime();
          multilist_sublist_insert_tail(mls, msp);
          multilist_sublist_unlock(mls);
  }
  
  void
  metaslab_space_update(vdev_t *vd, metaslab_class_t *mc, int64_t alloc_delta,
      int64_t defer_delta, int64_t space_delta)
  {
          vdev_space_update(vd, alloc_delta, defer_delta, space_delta);
  
          ASSERT3P(vd->vdev_spa->spa_root_vdev, ==, vd->vdev_parent);
          ASSERT(vd->vdev_ms_count != 0);
  
          metaslab_class_space_update(mc, alloc_delta, defer_delta, space_delta,
              vdev_deflated_space(vd, space_delta));
  }
  
  int
  metaslab_init(metaslab_group_t *mg, uint64_t id, uint64_t object,
      uint64_t txg, metaslab_t **msp)
  {
          vdev_t *vd = mg->mg_vd;
          spa_t *spa = vd->vdev_spa;
          objset_t *mos = spa->spa_meta_objset;
          metaslab_t *ms;
          int error;
  
          ms = kmem_zalloc(sizeof (metaslab_t), KM_SLEEP);
          mutex_init(&ms->ms_lock, NULL, MUTEX_DEFAULT, NULL);
          mutex_init(&ms->ms_sync_lock, NULL, MUTEX_DEFAULT, NULL);
          cv_init(&ms->ms_load_cv, NULL, CV_DEFAULT, NULL);
          cv_init(&ms->ms_flush_cv, NULL, CV_DEFAULT, NULL);
          multilist_link_init(&ms->ms_class_txg_node);
  
          ms->ms_id = id;
          ms->ms_start = id << vd->vdev_ms_shift;
          ms->ms_size = 1ULL << vd->vdev_ms_shift;
          ms->ms_allocator = -1;
          ms->ms_new = B_TRUE;
  
          vdev_ops_t *ops = vd->vdev_ops;
          if (ops->vdev_op_metaslab_init != NULL)
                  ops->vdev_op_metaslab_init(vd, &ms->ms_start, &ms->ms_size);
  
          /*
           * We only open space map objects that already exist. All others
           * will be opened when we finally allocate an object for it. For
           * readonly pools there is no need to open the space map object.
           *
           * Note:
           * When called from vdev_expand(), we can't call into the DMU as
           * we are holding the spa_config_lock as a writer and we would
           * deadlock [see relevant comment in vdev_metaslab_init()]. in
           * that case, the object parameter is zero though, so we won't
           * call into the DMU.
           */
          if (object != 0 && !(spa->spa_mode == SPA_MODE_READ &&
              !spa->spa_read_spacemaps)) {
                  error = space_map_open(&ms->ms_sm, mos, object, ms->ms_start,
                      ms->ms_size, vd->vdev_ashift);
  
                  if (error != 0) {
                          kmem_free(ms, sizeof (metaslab_t));
                          return (error);
                  }
  
                  ASSERT(ms->ms_sm != NULL);
                  ms->ms_allocated_space = space_map_allocated(ms->ms_sm);
          }
  
          uint64_t shift, start;
          range_seg_type_t type =
              metaslab_calculate_range_tree_type(vd, ms, &start, &shift);
  
          ms->ms_allocatable = range_tree_create(NULL, type, NULL, start, shift);
          for (int t = 0; t < TXG_SIZE; t++) {
                  ms->ms_allocating[t] = range_tree_create(NULL, type,
                      NULL, start, shift);
          }
          ms->ms_freeing = range_tree_create(NULL, type, NULL, start, shift);
          ms->ms_freed = range_tree_create(NULL, type, NULL, start, shift);
          for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                  ms->ms_defer[t] = range_tree_create(NULL, type, NULL,
                      start, shift);
          }
          ms->ms_checkpointing =
              range_tree_create(NULL, type, NULL, start, shift);
          ms->ms_unflushed_allocs =
              range_tree_create(NULL, type, NULL, start, shift);
  
          metaslab_rt_arg_t *mrap = kmem_zalloc(sizeof (*mrap), KM_SLEEP);
          mrap->mra_bt = &ms->ms_unflushed_frees_by_size;
          mrap->mra_floor_shift = metaslab_by_size_min_shift;
          ms->ms_unflushed_frees = range_tree_create(&metaslab_rt_ops,
              type, mrap, start, shift);
  
          ms->ms_trim = range_tree_create(NULL, type, NULL, start, shift);
  
          metaslab_group_add(mg, ms);
          metaslab_set_fragmentation(ms, B_FALSE);
  
          /*
           * If we're opening an existing pool (txg == 0) or creating
           * a new one (txg == TXG_INITIAL), all space is available now.
           * If we're adding space to an existing pool, the new space
           * does not become available until after this txg has synced.
           * The metaslab's weight will also be initialized when we sync
           * out this txg. This ensures that we don't attempt to allocate
           * from it before we have initialized it completely.
           */
          if (txg <= TXG_INITIAL) {
                  metaslab_sync_done(ms, 0);
                  metaslab_space_update(vd, mg->mg_class,
                      metaslab_allocated_space(ms), 0, 0);
          }
  
          if (txg != 0) {
                  vdev_dirty(vd, 0, NULL, txg);
                  vdev_dirty(vd, VDD_METASLAB, ms, txg);
          }
  
          *msp = ms;
  
          return (0);
  }
  
  static void
  metaslab_fini_flush_data(metaslab_t *msp)
  {
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
  
          if (metaslab_unflushed_txg(msp) == 0) {
                  ASSERT3P(avl_find(&spa->spa_metaslabs_by_flushed, msp, NULL),
                      ==, NULL);
                  return;
          }
          ASSERT(spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP));
  
          mutex_enter(&spa->spa_flushed_ms_lock);
          avl_remove(&spa->spa_metaslabs_by_flushed, msp);
          mutex_exit(&spa->spa_flushed_ms_lock);
  
          spa_log_sm_decrement_mscount(spa, metaslab_unflushed_txg(msp));
          spa_log_summary_decrement_mscount(spa, metaslab_unflushed_txg(msp),
              metaslab_unflushed_dirty(msp));
  }
  
  uint64_t
  metaslab_unflushed_changes_memused(metaslab_t *ms)
  {
          return ((range_tree_numsegs(ms->ms_unflushed_allocs) +
              range_tree_numsegs(ms->ms_unflushed_frees)) *
              ms->ms_unflushed_allocs->rt_root.bt_elem_size);
  }
  
  void
  metaslab_fini(metaslab_t *msp)
  {
          metaslab_group_t *mg = msp->ms_group;
          vdev_t *vd = mg->mg_vd;
          spa_t *spa = vd->vdev_spa;
  
          metaslab_fini_flush_data(msp);
  
          metaslab_group_remove(mg, msp);
  
          mutex_enter(&msp->ms_lock);
          VERIFY(msp->ms_group == NULL);
  
          /*
           * If this metaslab hasn't been through metaslab_sync_done() yet its
           * space hasn't been accounted for in its vdev and doesn't need to be
           * subtracted.
           */
          if (!msp->ms_new) {
                  metaslab_space_update(vd, mg->mg_class,
                      -metaslab_allocated_space(msp), 0, -msp->ms_size);
  
          }
          space_map_close(msp->ms_sm);
          msp->ms_sm = NULL;
  
          metaslab_unload(msp);
  
          range_tree_destroy(msp->ms_allocatable);
          range_tree_destroy(msp->ms_freeing);
          range_tree_destroy(msp->ms_freed);
  
          ASSERT3U(spa->spa_unflushed_stats.sus_memused, >=,
              metaslab_unflushed_changes_memused(msp));
          spa->spa_unflushed_stats.sus_memused -=
              metaslab_unflushed_changes_memused(msp);
          range_tree_vacate(msp->ms_unflushed_allocs, NULL, NULL);
          range_tree_destroy(msp->ms_unflushed_allocs);
          range_tree_destroy(msp->ms_checkpointing);
          range_tree_vacate(msp->ms_unflushed_frees, NULL, NULL);
          range_tree_destroy(msp->ms_unflushed_frees);
  
          for (int t = 0; t < TXG_SIZE; t++) {
                  range_tree_destroy(msp->ms_allocating[t]);
          }
          for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                  range_tree_destroy(msp->ms_defer[t]);
          }
          ASSERT0(msp->ms_deferspace);
  
          for (int t = 0; t < TXG_SIZE; t++)
                  ASSERT(!txg_list_member(&vd->vdev_ms_list, msp, t));
  
          range_tree_vacate(msp->ms_trim, NULL, NULL);
          range_tree_destroy(msp->ms_trim);
  
          mutex_exit(&msp->ms_lock);
          cv_destroy(&msp->ms_load_cv);
          cv_destroy(&msp->ms_flush_cv);
          mutex_destroy(&msp->ms_lock);
          mutex_destroy(&msp->ms_sync_lock);
          ASSERT3U(msp->ms_allocator, ==, -1);
  
          kmem_free(msp, sizeof (metaslab_t));
  }
  
  #define FRAGMENTATION_TABLE_SIZE        17
  
  /*
   * This table defines a segment size based fragmentation metric that will
   * allow each metaslab to derive its own fragmentation value. This is done
   * by calculating the space in each bucket of the spacemap histogram and
   * multiplying that by the fragmentation metric in this table. Doing
   * this for all buckets and dividing it by the total amount of free
   * space in this metaslab (i.e. the total free space in all buckets) gives
   * us the fragmentation metric. This means that a high fragmentation metric
   * equates to most of the free space being comprised of small segments.
   * Conversely, if the metric is low, then most of the free space is in
   * large segments. A 10% change in fragmentation equates to approximately
   * double the number of segments.
   *
   * This table defines 0% fragmented space using 16MB segments. Testing has
   * shown that segments that are greater than or equal to 16MB do not suffer
   * from drastic performance problems. Using this value, we derive the rest
   * of the table. Since the fragmentation value is never stored on disk, it
   * is possible to change these calculations in the future.
   */
  static const int zfs_frag_table[FRAGMENTATION_TABLE_SIZE] = {
          100,    /* 512B */
          100,    /* 1K   */
          98,     /* 2K   */
          95,     /* 4K   */
          90,     /* 8K   */
          80,     /* 16K  */
          70,     /* 32K  */
          60,     /* 64K  */
          50,     /* 128K */
          40,     /* 256K */
          30,     /* 512K */
          20,     /* 1M   */
          15,     /* 2M   */
          10,     /* 4M   */
          5,      /* 8M   */
          0       /* 16M  */
  };
  
  /*
   * Calculate the metaslab's fragmentation metric and set ms_fragmentation.
   * Setting this value to ZFS_FRAG_INVALID means that the metaslab has not
   * been upgraded and does not support this metric. Otherwise, the return
   * value should be in the range [0, 100].
   */
  static void
  metaslab_set_fragmentation(metaslab_t *msp, boolean_t nodirty)
  {
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
          uint64_t fragmentation = 0;
          uint64_t total = 0;
          boolean_t feature_enabled = spa_feature_is_enabled(spa,
              SPA_FEATURE_SPACEMAP_HISTOGRAM);
  
          if (!feature_enabled) {
                  msp->ms_fragmentation = ZFS_FRAG_INVALID;
                  return;
          }
  
          /*
           * A null space map means that the entire metaslab is free
           * and thus is not fragmented.
           */
          if (msp->ms_sm == NULL) {
                  msp->ms_fragmentation = 0;
                  return;
          }
  
          /*
           * If this metaslab's space map has not been upgraded, flag it
           * so that we upgrade next time we encounter it.
           */
          if (msp->ms_sm->sm_dbuf->db_size != sizeof (space_map_phys_t)) {
                  uint64_t txg = spa_syncing_txg(spa);
                  vdev_t *vd = msp->ms_group->mg_vd;
  
                  /*
                   * If we've reached the final dirty txg, then we must
                   * be shutting down the pool. We don't want to dirty
                   * any data past this point so skip setting the condense
                   * flag. We can retry this action the next time the pool
                   * is imported. We also skip marking this metaslab for
                   * condensing if the caller has explicitly set nodirty.
                   */
                  if (!nodirty &&
                      spa_writeable(spa) && txg < spa_final_dirty_txg(spa)) {
                          msp->ms_condense_wanted = B_TRUE;
                          vdev_dirty(vd, VDD_METASLAB, msp, txg + 1);
                          zfs_dbgmsg("txg %llu, requesting force condense: "
                              "ms_id %llu, vdev_id %llu", (u_longlong_t)txg,
                              (u_longlong_t)msp->ms_id,
                              (u_longlong_t)vd->vdev_id);
                  }
                  msp->ms_fragmentation = ZFS_FRAG_INVALID;
                  return;
          }
  
          for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
                  uint64_t space = 0;
                  uint8_t shift = msp->ms_sm->sm_shift;
  
                  int idx = MIN(shift - SPA_MINBLOCKSHIFT + i,
                      FRAGMENTATION_TABLE_SIZE - 1);
  
                  if (msp->ms_sm->sm_phys->smp_histogram[i] == 0)
                          continue;
  
                  space = msp->ms_sm->sm_phys->smp_histogram[i] << (i + shift);
                  total += space;
  
                  ASSERT3U(idx, <, FRAGMENTATION_TABLE_SIZE);
                  fragmentation += space * zfs_frag_table[idx];
          }
  
          if (total > 0)
                  fragmentation /= total;
          ASSERT3U(fragmentation, <=, 100);
  
          msp->ms_fragmentation = fragmentation;
  }
  
  /*
   * Compute a weight -- a selection preference value -- for the given metaslab.
   * This is based on the amount of free space, the level of fragmentation,
   * the LBA range, and whether the metaslab is loaded.
   */
  static uint64_t
  metaslab_space_weight(metaslab_t *msp)
  {
          metaslab_group_t *mg = msp->ms_group;
          vdev_t *vd = mg->mg_vd;
          uint64_t weight, space;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          /*
           * The baseline weight is the metaslab's free space.
           */
          space = msp->ms_size - metaslab_allocated_space(msp);
  
          if (metaslab_fragmentation_factor_enabled &&
              msp->ms_fragmentation != ZFS_FRAG_INVALID) {
                  /*
                   * Use the fragmentation information to inversely scale
                   * down the baseline weight. We need to ensure that we
                   * don't exclude this metaslab completely when it's 100%
                   * fragmented. To avoid this we reduce the fragmented value
                   * by 1.
                   */
                  space = (space * (100 - (msp->ms_fragmentation - 1))) / 100;
  
                  /*
                   * If space < SPA_MINBLOCKSIZE, then we will not allocate from
                   * this metaslab again. The fragmentation metric may have
                   * decreased the space to something smaller than
                   * SPA_MINBLOCKSIZE, so reset the space to SPA_MINBLOCKSIZE
                   * so that we can consume any remaining space.
                   */
                  if (space > 0 && space < SPA_MINBLOCKSIZE)
                          space = SPA_MINBLOCKSIZE;
          }
          weight = space;
  
          /*
           * Modern disks have uniform bit density and constant angular velocity.
           * Therefore, the outer recording zones are faster (higher bandwidth)
           * than the inner zones by the ratio of outer to inner track diameter,
           * which is typically around 2:1.  We account for this by assigning
           * higher weight to lower metaslabs (multiplier ranging from 2x to 1x).
           * In effect, this means that we'll select the metaslab with the most
           * free bandwidth rather than simply the one with the most free space.
           */
          if (!vd->vdev_nonrot && metaslab_lba_weighting_enabled) {
                  weight = 2 * weight - (msp->ms_id * weight) / vd->vdev_ms_count;
                  ASSERT(weight >= space && weight <= 2 * space);
          }
  
          /*
           * If this metaslab is one we're actively using, adjust its
           * weight to make it preferable to any inactive metaslab so
           * we'll polish it off. If the fragmentation on this metaslab
           * has exceed our threshold, then don't mark it active.
           */
          if (msp->ms_loaded && msp->ms_fragmentation != ZFS_FRAG_INVALID &&
              msp->ms_fragmentation <= zfs_metaslab_fragmentation_threshold) {
                  weight |= (msp->ms_weight & METASLAB_ACTIVE_MASK);
          }
  
          WEIGHT_SET_SPACEBASED(weight);
          return (weight);
  }
  
  /*
   * Return the weight of the specified metaslab, according to the segment-based
   * weighting algorithm. The metaslab must be loaded. This function can
   * be called within a sync pass since it relies only on the metaslab's
   * range tree which is always accurate when the metaslab is loaded.
   */
  static uint64_t
  metaslab_weight_from_range_tree(metaslab_t *msp)
  {
          uint64_t weight = 0;
          uint32_t segments = 0;
  
          ASSERT(msp->ms_loaded);
  
          for (int i = RANGE_TREE_HISTOGRAM_SIZE - 1; i >= SPA_MINBLOCKSHIFT;
              i--) {
                  uint8_t shift = msp->ms_group->mg_vd->vdev_ashift;
                  int max_idx = SPACE_MAP_HISTOGRAM_SIZE + shift - 1;
  
                  segments <<= 1;
                  segments += msp->ms_allocatable->rt_histogram[i];
  
                  /*
                   * The range tree provides more precision than the space map
                   * and must be downgraded so that all values fit within the
                   * space map's histogram. This allows us to compare loaded
                   * vs. unloaded metaslabs to determine which metaslab is
                   * considered "best".
                   */
                  if (i > max_idx)
                          continue;
  
                  if (segments != 0) {
                          WEIGHT_SET_COUNT(weight, segments);
                          WEIGHT_SET_INDEX(weight, i);
                          WEIGHT_SET_ACTIVE(weight, 0);
                          break;
                  }
          }
          return (weight);
  }
  
  /*
   * Calculate the weight based on the on-disk histogram. Should be applied
   * only to unloaded metaslabs  (i.e no incoming allocations) in-order to
   * give results consistent with the on-disk state
   */
  static uint64_t
  metaslab_weight_from_spacemap(metaslab_t *msp)
  {
          space_map_t *sm = msp->ms_sm;
          ASSERT(!msp->ms_loaded);
          ASSERT(sm != NULL);
          ASSERT3U(space_map_object(sm), !=, 0);
          ASSERT3U(sm->sm_dbuf->db_size, ==, sizeof (space_map_phys_t));
  
          /*
           * Create a joint histogram from all the segments that have made
           * it to the metaslab's space map histogram, that are not yet
           * available for allocation because they are still in the freeing
           * pipeline (e.g. freeing, freed, and defer trees). Then subtract
           * these segments from the space map's histogram to get a more
           * accurate weight.
           */
          uint64_t deferspace_histogram[SPACE_MAP_HISTOGRAM_SIZE] = {0};
          for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++)
                  deferspace_histogram[i] += msp->ms_synchist[i];
          for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                  for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
                          deferspace_histogram[i] += msp->ms_deferhist[t][i];
                  }
          }
  
          uint64_t weight = 0;
          for (int i = SPACE_MAP_HISTOGRAM_SIZE - 1; i >= 0; i--) {
                  ASSERT3U(sm->sm_phys->smp_histogram[i], >=,
                      deferspace_histogram[i]);
                  uint64_t count =
                      sm->sm_phys->smp_histogram[i] - deferspace_histogram[i];
                  if (count != 0) {
                          WEIGHT_SET_COUNT(weight, count);
                          WEIGHT_SET_INDEX(weight, i + sm->sm_shift);
                          WEIGHT_SET_ACTIVE(weight, 0);
                          break;
                  }
          }
          return (weight);
  }
  
  /*
   * Compute a segment-based weight for the specified metaslab. The weight
   * is determined by highest bucket in the histogram. The information
   * for the highest bucket is encoded into the weight value.
   */
  static uint64_t
  metaslab_segment_weight(metaslab_t *msp)
  {
          metaslab_group_t *mg = msp->ms_group;
          uint64_t weight = 0;
          uint8_t shift = mg->mg_vd->vdev_ashift;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          /*
           * The metaslab is completely free.
           */
          if (metaslab_allocated_space(msp) == 0) {
                  int idx = highbit64(msp->ms_size) - 1;
                  int max_idx = SPACE_MAP_HISTOGRAM_SIZE + shift - 1;
  
                  if (idx < max_idx) {
                          WEIGHT_SET_COUNT(weight, 1ULL);
                          WEIGHT_SET_INDEX(weight, idx);
                  } else {
                          WEIGHT_SET_COUNT(weight, 1ULL << (idx - max_idx));
                          WEIGHT_SET_INDEX(weight, max_idx);
                  }
                  WEIGHT_SET_ACTIVE(weight, 0);
                  ASSERT(!WEIGHT_IS_SPACEBASED(weight));
                  return (weight);
          }
  
          ASSERT3U(msp->ms_sm->sm_dbuf->db_size, ==, sizeof (space_map_phys_t));
  
          /*
           * If the metaslab is fully allocated then just make the weight 0.
           */
          if (metaslab_allocated_space(msp) == msp->ms_size)
                  return (0);
          /*
           * If the metaslab is already loaded, then use the range tree to
           * determine the weight. Otherwise, we rely on the space map information
           * to generate the weight.
           */
          if (msp->ms_loaded) {
                  weight = metaslab_weight_from_range_tree(msp);
          } else {
                  weight = metaslab_weight_from_spacemap(msp);
          }
  
          /*
           * If the metaslab was active the last time we calculated its weight
           * then keep it active. We want to consume the entire region that
           * is associated with this weight.
           */
          if (msp->ms_activation_weight != 0 && weight != 0)
                  WEIGHT_SET_ACTIVE(weight, WEIGHT_GET_ACTIVE(msp->ms_weight));
          return (weight);
  }
  
  /*
   * Determine if we should attempt to allocate from this metaslab. If the
   * metaslab is loaded, then we can determine if the desired allocation
   * can be satisfied by looking at the size of the maximum free segment
   * on that metaslab. Otherwise, we make our decision based on the metaslab's
   * weight. For segment-based weighting we can determine the maximum
   * allocation based on the index encoded in its value. For space-based
   * weights we rely on the entire weight (excluding the weight-type bit).
   */
  static boolean_t
  metaslab_should_allocate(metaslab_t *msp, uint64_t asize, boolean_t try_hard)
  {
          /*
           * If the metaslab is loaded, ms_max_size is definitive and we can use
           * the fast check. If it's not, the ms_max_size is a lower bound (once
           * set), and we should use the fast check as long as we're not in
           * try_hard and it's been less than zfs_metaslab_max_size_cache_sec
           * seconds since the metaslab was unloaded.
           */
          if (msp->ms_loaded ||
              (msp->ms_max_size != 0 && !try_hard && gethrtime() <
              msp->ms_unload_time + SEC2NSEC(zfs_metaslab_max_size_cache_sec)))
                  return (msp->ms_max_size >= asize);
  
          boolean_t should_allocate;
          if (!WEIGHT_IS_SPACEBASED(msp->ms_weight)) {
                  /*
                   * The metaslab segment weight indicates segments in the
                   * range [2^i, 2^(i+1)), where i is the index in the weight.
                   * Since the asize might be in the middle of the range, we
                   * should attempt the allocation if asize < 2^(i+1).
                   */
                  should_allocate = (asize <
                      1ULL << (WEIGHT_GET_INDEX(msp->ms_weight) + 1));
          } else {
                  should_allocate = (asize <=
                      (msp->ms_weight & ~METASLAB_WEIGHT_TYPE));
          }
  
          return (should_allocate);
  }
  
  static uint64_t
  metaslab_weight(metaslab_t *msp, boolean_t nodirty)
  {
          vdev_t *vd = msp->ms_group->mg_vd;
          spa_t *spa = vd->vdev_spa;
          uint64_t weight;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          metaslab_set_fragmentation(msp, nodirty);
  
          /*
           * Update the maximum size. If the metaslab is loaded, this will
           * ensure that we get an accurate maximum size if newly freed space
           * has been added back into the free tree. If the metaslab is
           * unloaded, we check if there's a larger free segment in the
           * unflushed frees. This is a lower bound on the largest allocatable
           * segment size. Coalescing of adjacent entries may reveal larger
           * allocatable segments, but we aren't aware of those until loading
           * the space map into a range tree.
           */
          if (msp->ms_loaded) {
                  msp->ms_max_size = metaslab_largest_allocatable(msp);
          } else {
                  msp->ms_max_size = MAX(msp->ms_max_size,
                      metaslab_largest_unflushed_free(msp));
          }
  
          /*
           * Segment-based weighting requires space map histogram support.
           */
          if (zfs_metaslab_segment_weight_enabled &&
              spa_feature_is_enabled(spa, SPA_FEATURE_SPACEMAP_HISTOGRAM) &&
              (msp->ms_sm == NULL || msp->ms_sm->sm_dbuf->db_size ==
              sizeof (space_map_phys_t))) {
                  weight = metaslab_segment_weight(msp);
          } else {
                  weight = metaslab_space_weight(msp);
          }
          return (weight);
  }
  
  void
  metaslab_recalculate_weight_and_sort(metaslab_t *msp)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          /* note: we preserve the mask (e.g. indication of primary, etc..) */
          uint64_t was_active = msp->ms_weight & METASLAB_ACTIVE_MASK;
          metaslab_group_sort(msp->ms_group, msp,
              metaslab_weight(msp, B_FALSE) | was_active);
  }
  
  static int
  metaslab_activate_allocator(metaslab_group_t *mg, metaslab_t *msp,
      int allocator, uint64_t activation_weight)
  {
          metaslab_group_allocator_t *mga = &mg->mg_allocator[allocator];
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          /*
           * If we're activating for the claim code, we don't want to actually
           * set the metaslab up for a specific allocator.
           */
          if (activation_weight == METASLAB_WEIGHT_CLAIM) {
                  ASSERT0(msp->ms_activation_weight);
                  msp->ms_activation_weight = msp->ms_weight;
                  metaslab_group_sort(mg, msp, msp->ms_weight |
                      activation_weight);
                  return (0);
          }
  
          metaslab_t **mspp = (activation_weight == METASLAB_WEIGHT_PRIMARY ?
              &mga->mga_primary : &mga->mga_secondary);
  
          mutex_enter(&mg->mg_lock);
          if (*mspp != NULL) {
                  mutex_exit(&mg->mg_lock);
                  return (EEXIST);
          }
  
          *mspp = msp;
          ASSERT3S(msp->ms_allocator, ==, -1);
          msp->ms_allocator = allocator;
          msp->ms_primary = (activation_weight == METASLAB_WEIGHT_PRIMARY);
  
          ASSERT0(msp->ms_activation_weight);
          msp->ms_activation_weight = msp->ms_weight;
          metaslab_group_sort_impl(mg, msp,
              msp->ms_weight | activation_weight);
          mutex_exit(&mg->mg_lock);
  
          return (0);
  }
  
  static int
  metaslab_activate(metaslab_t *msp, int allocator, uint64_t activation_weight)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          /*
           * The current metaslab is already activated for us so there
           * is nothing to do. Already activated though, doesn't mean
           * that this metaslab is activated for our allocator nor our
           * requested activation weight. The metaslab could have started
           * as an active one for our allocator but changed allocators
           * while we were waiting to grab its ms_lock or we stole it
           * [see find_valid_metaslab()]. This means that there is a
           * possibility of passivating a metaslab of another allocator
           * or from a different activation mask, from this thread.
           */
          if ((msp->ms_weight & METASLAB_ACTIVE_MASK) != 0) {
                  ASSERT(msp->ms_loaded);
                  return (0);
          }
  
          int error = metaslab_load(msp);
          if (error != 0) {
                  metaslab_group_sort(msp->ms_group, msp, 0);
                  return (error);
          }
  
          /*
           * When entering metaslab_load() we may have dropped the
           * ms_lock because we were loading this metaslab, or we
           * were waiting for another thread to load it for us. In
           * that scenario, we recheck the weight of the metaslab
           * to see if it was activated by another thread.
           *
           * If the metaslab was activated for another allocator or
           * it was activated with a different activation weight (e.g.
           * we wanted to make it a primary but it was activated as
           * secondary) we return error (EBUSY).
           *
           * If the metaslab was activated for the same allocator
           * and requested activation mask, skip activating it.
           */
          if ((msp->ms_weight & METASLAB_ACTIVE_MASK) != 0) {
                  if (msp->ms_allocator != allocator)
                          return (EBUSY);
  
                  if ((msp->ms_weight & activation_weight) == 0)
                          return (SET_ERROR(EBUSY));
  
                  EQUIV((activation_weight == METASLAB_WEIGHT_PRIMARY),
                      msp->ms_primary);
                  return (0);
          }
  
          /*
           * If the metaslab has literally 0 space, it will have weight 0. In
           * that case, don't bother activating it. This can happen if the
           * metaslab had space during find_valid_metaslab, but another thread
           * loaded it and used all that space while we were waiting to grab the
           * lock.
           */
          if (msp->ms_weight == 0) {
                  ASSERT0(range_tree_space(msp->ms_allocatable));
                  return (SET_ERROR(ENOSPC));
          }
  
          if ((error = metaslab_activate_allocator(msp->ms_group, msp,
              allocator, activation_weight)) != 0) {
                  return (error);
          }
  
          ASSERT(msp->ms_loaded);
          ASSERT(msp->ms_weight & METASLAB_ACTIVE_MASK);
  
          return (0);
  }
  
  static void
  metaslab_passivate_allocator(metaslab_group_t *mg, metaslab_t *msp,
      uint64_t weight)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          ASSERT(msp->ms_loaded);
  
          if (msp->ms_weight & METASLAB_WEIGHT_CLAIM) {
                  metaslab_group_sort(mg, msp, weight);
                  return;
          }
  
          mutex_enter(&mg->mg_lock);
          ASSERT3P(msp->ms_group, ==, mg);
          ASSERT3S(0, <=, msp->ms_allocator);
          ASSERT3U(msp->ms_allocator, <, mg->mg_allocators);
  
          metaslab_group_allocator_t *mga = &mg->mg_allocator[msp->ms_allocator];
          if (msp->ms_primary) {
                  ASSERT3P(mga->mga_primary, ==, msp);
                  ASSERT(msp->ms_weight & METASLAB_WEIGHT_PRIMARY);
                  mga->mga_primary = NULL;
          } else {
                  ASSERT3P(mga->mga_secondary, ==, msp);
                  ASSERT(msp->ms_weight & METASLAB_WEIGHT_SECONDARY);
                  mga->mga_secondary = NULL;
          }
          msp->ms_allocator = -1;
          metaslab_group_sort_impl(mg, msp, weight);
          mutex_exit(&mg->mg_lock);
  }
  
  static void
  metaslab_passivate(metaslab_t *msp, uint64_t weight)
  {
          uint64_t size __maybe_unused = weight & ~METASLAB_WEIGHT_TYPE;
  
          /*
           * If size < SPA_MINBLOCKSIZE, then we will not allocate from
           * this metaslab again.  In that case, it had better be empty,
           * or we would be leaving space on the table.
           */
          ASSERT(!WEIGHT_IS_SPACEBASED(msp->ms_weight) ||
              size >= SPA_MINBLOCKSIZE ||
              range_tree_space(msp->ms_allocatable) == 0);
          ASSERT0(weight & METASLAB_ACTIVE_MASK);
  
          ASSERT(msp->ms_activation_weight != 0);
          msp->ms_activation_weight = 0;
          metaslab_passivate_allocator(msp->ms_group, msp, weight);
          ASSERT0(msp->ms_weight & METASLAB_ACTIVE_MASK);
  }
  
  /*
   * Segment-based metaslabs are activated once and remain active until
   * we either fail an allocation attempt (similar to space-based metaslabs)
   * or have exhausted the free space in zfs_metaslab_switch_threshold
   * buckets since the metaslab was activated. This function checks to see
   * if we've exhausted the zfs_metaslab_switch_threshold buckets in the
   * metaslab and passivates it proactively. This will allow us to select a
   * metaslab with a larger contiguous region, if any, remaining within this
   * metaslab group. If we're in sync pass > 1, then we continue using this
   * metaslab so that we don't dirty more block and cause more sync passes.
   */
  static void
  metaslab_segment_may_passivate(metaslab_t *msp)
  {
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
  
          if (WEIGHT_IS_SPACEBASED(msp->ms_weight) || spa_sync_pass(spa) > 1)
                  return;
  
          /*
           * Since we are in the middle of a sync pass, the most accurate
           * information that is accessible to us is the in-core range tree
           * histogram; calculate the new weight based on that information.
           */
          uint64_t weight = metaslab_weight_from_range_tree(msp);
          int activation_idx = WEIGHT_GET_INDEX(msp->ms_activation_weight);
          int current_idx = WEIGHT_GET_INDEX(weight);
  
          if (current_idx <= activation_idx - zfs_metaslab_switch_threshold)
                  metaslab_passivate(msp, weight);
  }
  
  static void
  metaslab_preload(void *arg)
  {
          metaslab_t *msp = arg;
          metaslab_class_t *mc = msp->ms_group->mg_class;
          spa_t *spa = mc->mc_spa;
          fstrans_cookie_t cookie = spl_fstrans_mark();
  
          ASSERT(!MUTEX_HELD(&msp->ms_group->mg_lock));
  
          mutex_enter(&msp->ms_lock);
          (void) metaslab_load(msp);
          metaslab_set_selected_txg(msp, spa_syncing_txg(spa));
          mutex_exit(&msp->ms_lock);
          spl_fstrans_unmark(cookie);
  }
  
  static void
  metaslab_group_preload(metaslab_group_t *mg)
  {
          spa_t *spa = mg->mg_vd->vdev_spa;
          metaslab_t *msp;
          avl_tree_t *t = &mg->mg_metaslab_tree;
          int m = 0;
  
          if (spa_shutting_down(spa) || !metaslab_preload_enabled) {
                  taskq_wait_outstanding(mg->mg_taskq, 0);
                  return;
          }
  
          mutex_enter(&mg->mg_lock);
  
          /*
           * Load the next potential metaslabs
           */
          for (msp = avl_first(t); msp != NULL; msp = AVL_NEXT(t, msp)) {
                  ASSERT3P(msp->ms_group, ==, mg);
  
                  /*
                   * We preload only the maximum number of metaslabs specified
                   * by metaslab_preload_limit. If a metaslab is being forced
                   * to condense then we preload it too. This will ensure
                   * that force condensing happens in the next txg.
                   */
                  if (++m > metaslab_preload_limit && !msp->ms_condense_wanted) {
                          continue;
                  }
  
                  VERIFY(taskq_dispatch(mg->mg_taskq, metaslab_preload,
                      msp, TQ_SLEEP) != TASKQID_INVALID);
          }
          mutex_exit(&mg->mg_lock);
  }
  
  /*
   * Determine if the space map's on-disk footprint is past our tolerance for
   * inefficiency. We would like to use the following criteria to make our
   * decision:
   *
   * 1. Do not condense if the size of the space map object would dramatically
   *    increase as a result of writing out the free space range tree.
   *
   * 2. Condense if the on on-disk space map representation is at least
   *    zfs_condense_pct/100 times the size of the optimal representation
   *    (i.e. zfs_condense_pct = 110 and in-core = 1MB, optimal = 1.1MB).
   *
   * 3. Do not condense if the on-disk size of the space map does not actually
   *    decrease.
   *
   * Unfortunately, we cannot compute the on-disk size of the space map in this
   * context because we cannot accurately compute the effects of compression, etc.
   * Instead, we apply the heuristic described in the block comment for
   * zfs_metaslab_condense_block_threshold - we only condense if the space used
   * is greater than a threshold number of blocks.
   */
  static boolean_t
  metaslab_should_condense(metaslab_t *msp)
  {
          space_map_t *sm = msp->ms_sm;
          vdev_t *vd = msp->ms_group->mg_vd;
          uint64_t vdev_blocksize = 1ULL << vd->vdev_ashift;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          ASSERT(msp->ms_loaded);
          ASSERT(sm != NULL);
          ASSERT3U(spa_sync_pass(vd->vdev_spa), ==, 1);
  
          /*
           * We always condense metaslabs that are empty and metaslabs for
           * which a condense request has been made.
           */
          if (range_tree_numsegs(msp->ms_allocatable) == 0 ||
              msp->ms_condense_wanted)
                  return (B_TRUE);
  
          uint64_t record_size = MAX(sm->sm_blksz, vdev_blocksize);
          uint64_t object_size = space_map_length(sm);
          uint64_t optimal_size = space_map_estimate_optimal_size(sm,
              msp->ms_allocatable, SM_NO_VDEVID);
  
          return (object_size >= (optimal_size * zfs_condense_pct / 100) &&
              object_size > zfs_metaslab_condense_block_threshold * record_size);
  }
  
  /*
   * Condense the on-disk space map representation to its minimized form.
   * The minimized form consists of a small number of allocations followed
   * by the entries of the free range tree (ms_allocatable). The condensed
   * spacemap contains all the entries of previous TXGs (including those in
   * the pool-wide log spacemaps; thus this is effectively a superset of
   * metaslab_flush()), but this TXG's entries still need to be written.
   */
  static void
  metaslab_condense(metaslab_t *msp, dmu_tx_t *tx)
  {
          range_tree_t *condense_tree;
          space_map_t *sm = msp->ms_sm;
          uint64_t txg = dmu_tx_get_txg(tx);
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          ASSERT(msp->ms_loaded);
          ASSERT(msp->ms_sm != NULL);
  
          /*
           * In order to condense the space map, we need to change it so it
           * only describes which segments are currently allocated and free.
           *
           * All the current free space resides in the ms_allocatable, all
           * the ms_defer trees, and all the ms_allocating trees. We ignore
           * ms_freed because it is empty because we're in sync pass 1. We
           * ignore ms_freeing because these changes are not yet reflected
           * in the spacemap (they will be written later this txg).
           *
           * So to truncate the space map to represent all the entries of
           * previous TXGs we do the following:
           *
           * 1] We create a range tree (condense tree) that is 100% empty.
           * 2] We add to it all segments found in the ms_defer trees
           *    as those segments are marked as free in the original space
           *    map. We do the same with the ms_allocating trees for the same
           *    reason. Adding these segments should be a relatively
           *    inexpensive operation since we expect these trees to have a
           *    small number of nodes.
           * 3] We vacate any unflushed allocs, since they are not frees we
           *    need to add to the condense tree. Then we vacate any
           *    unflushed frees as they should already be part of ms_allocatable.
           * 4] At this point, we would ideally like to add all segments
           *    in the ms_allocatable tree from the condense tree. This way
           *    we would write all the entries of the condense tree as the
           *    condensed space map, which would only contain freed
           *    segments with everything else assumed to be allocated.
           *
           *    Doing so can be prohibitively expensive as ms_allocatable can
           *    be large, and therefore computationally expensive to add to
           *    the condense_tree. Instead we first sync out an entry marking
           *    everything as allocated, then the condense_tree and then the
           *    ms_allocatable, in the condensed space map. While this is not
           *    optimal, it is typically close to optimal and more importantly
           *    much cheaper to compute.
           *
           * 5] Finally, as both of the unflushed trees were written to our
           *    new and condensed metaslab space map, we basically flushed
           *    all the unflushed changes to disk, thus we call
           *    metaslab_flush_update().
           */
          ASSERT3U(spa_sync_pass(spa), ==, 1);
          ASSERT(range_tree_is_empty(msp->ms_freed)); /* since it is pass 1 */
  
          zfs_dbgmsg("condensing: txg %llu, msp[%llu] %px, vdev id %llu, "
              "spa %s, smp size %llu, segments %llu, forcing condense=%s",
              (u_longlong_t)txg, (u_longlong_t)msp->ms_id, msp,
              (u_longlong_t)msp->ms_group->mg_vd->vdev_id,
              spa->spa_name, (u_longlong_t)space_map_length(msp->ms_sm),
              (u_longlong_t)range_tree_numsegs(msp->ms_allocatable),
              msp->ms_condense_wanted ? "TRUE" : "FALSE");
  
          msp->ms_condense_wanted = B_FALSE;
  
          range_seg_type_t type;
          uint64_t shift, start;
          type = metaslab_calculate_range_tree_type(msp->ms_group->mg_vd, msp,
              &start, &shift);
  
          condense_tree = range_tree_create(NULL, type, NULL, start, shift);
  
          for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                  range_tree_walk(msp->ms_defer[t],
                      range_tree_add, condense_tree);
          }
  
          for (int t = 0; t < TXG_CONCURRENT_STATES; t++) {
                  range_tree_walk(msp->ms_allocating[(txg + t) & TXG_MASK],
                      range_tree_add, condense_tree);
          }
  
          ASSERT3U(spa->spa_unflushed_stats.sus_memused, >=,
              metaslab_unflushed_changes_memused(msp));
          spa->spa_unflushed_stats.sus_memused -=
              metaslab_unflushed_changes_memused(msp);
          range_tree_vacate(msp->ms_unflushed_allocs, NULL, NULL);
          range_tree_vacate(msp->ms_unflushed_frees, NULL, NULL);
  
          /*
           * We're about to drop the metaslab's lock thus allowing other
           * consumers to change it's content. Set the metaslab's ms_condensing
           * flag to ensure that allocations on this metaslab do not occur
           * while we're in the middle of committing it to disk. This is only
           * critical for ms_allocatable as all other range trees use per TXG
           * views of their content.
           */
          msp->ms_condensing = B_TRUE;
  
          mutex_exit(&msp->ms_lock);
          uint64_t object = space_map_object(msp->ms_sm);
          space_map_truncate(sm,
              spa_feature_is_enabled(spa, SPA_FEATURE_LOG_SPACEMAP) ?
              zfs_metaslab_sm_blksz_with_log : zfs_metaslab_sm_blksz_no_log, tx);
  
          /*
           * space_map_truncate() may have reallocated the spacemap object.
           * If so, update the vdev_ms_array.
           */
          if (space_map_object(msp->ms_sm) != object) {
                  object = space_map_object(msp->ms_sm);
                  dmu_write(spa->spa_meta_objset,
                      msp->ms_group->mg_vd->vdev_ms_array, sizeof (uint64_t) *
                      msp->ms_id, sizeof (uint64_t), &object, tx);
          }
  
          /*
           * Note:
           * When the log space map feature is enabled, each space map will
           * always have ALLOCS followed by FREES for each sync pass. This is
           * typically true even when the log space map feature is disabled,
           * except from the case where a metaslab goes through metaslab_sync()
           * and gets condensed. In that case the metaslab's space map will have
           * ALLOCS followed by FREES (due to condensing) followed by ALLOCS
           * followed by FREES (due to space_map_write() in metaslab_sync()) for
           * sync pass 1.
           */
          range_tree_t *tmp_tree = range_tree_create(NULL, type, NULL, start,
              shift);
          range_tree_add(tmp_tree, msp->ms_start, msp->ms_size);
          space_map_write(sm, tmp_tree, SM_ALLOC, SM_NO_VDEVID, tx);
          space_map_write(sm, msp->ms_allocatable, SM_FREE, SM_NO_VDEVID, tx);
          space_map_write(sm, condense_tree, SM_FREE, SM_NO_VDEVID, tx);
  
          range_tree_vacate(condense_tree, NULL, NULL);
          range_tree_destroy(condense_tree);
          range_tree_vacate(tmp_tree, NULL, NULL);
          range_tree_destroy(tmp_tree);
          mutex_enter(&msp->ms_lock);
  
          msp->ms_condensing = B_FALSE;
          metaslab_flush_update(msp, tx);
  }
  
  static void
  metaslab_unflushed_add(metaslab_t *msp, dmu_tx_t *tx)
  {
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
          ASSERT(spa_syncing_log_sm(spa) != NULL);
          ASSERT(msp->ms_sm != NULL);
          ASSERT(range_tree_is_empty(msp->ms_unflushed_allocs));
          ASSERT(range_tree_is_empty(msp->ms_unflushed_frees));
  
          mutex_enter(&spa->spa_flushed_ms_lock);
          metaslab_set_unflushed_txg(msp, spa_syncing_txg(spa), tx);
          metaslab_set_unflushed_dirty(msp, B_TRUE);
          avl_add(&spa->spa_metaslabs_by_flushed, msp);
          mutex_exit(&spa->spa_flushed_ms_lock);
  
          spa_log_sm_increment_current_mscount(spa);
          spa_log_summary_add_flushed_metaslab(spa, B_TRUE);
  }
  
  void
  metaslab_unflushed_bump(metaslab_t *msp, dmu_tx_t *tx, boolean_t dirty)
  {
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
          ASSERT(spa_syncing_log_sm(spa) != NULL);
          ASSERT(msp->ms_sm != NULL);
          ASSERT(metaslab_unflushed_txg(msp) != 0);
          ASSERT3P(avl_find(&spa->spa_metaslabs_by_flushed, msp, NULL), ==, msp);
          ASSERT(range_tree_is_empty(msp->ms_unflushed_allocs));
          ASSERT(range_tree_is_empty(msp->ms_unflushed_frees));
  
          VERIFY3U(tx->tx_txg, <=, spa_final_dirty_txg(spa));
  
          /* update metaslab's position in our flushing tree */
          uint64_t ms_prev_flushed_txg = metaslab_unflushed_txg(msp);
          boolean_t ms_prev_flushed_dirty = metaslab_unflushed_dirty(msp);
          mutex_enter(&spa->spa_flushed_ms_lock);
          avl_remove(&spa->spa_metaslabs_by_flushed, msp);
          metaslab_set_unflushed_txg(msp, spa_syncing_txg(spa), tx);
          metaslab_set_unflushed_dirty(msp, dirty);
          avl_add(&spa->spa_metaslabs_by_flushed, msp);
          mutex_exit(&spa->spa_flushed_ms_lock);
  
          /* update metaslab counts of spa_log_sm_t nodes */
          spa_log_sm_decrement_mscount(spa, ms_prev_flushed_txg);
          spa_log_sm_increment_current_mscount(spa);
  
          /* update log space map summary */
          spa_log_summary_decrement_mscount(spa, ms_prev_flushed_txg,
              ms_prev_flushed_dirty);
          spa_log_summary_add_flushed_metaslab(spa, dirty);
  
          /* cleanup obsolete logs if any */
          spa_cleanup_old_sm_logs(spa, tx);
  }
  
  /*
   * Called when the metaslab has been flushed (its own spacemap now reflects
   * all the contents of the pool-wide spacemap log). Updates the metaslab's
   * metadata and any pool-wide related log space map data (e.g. summary,
   * obsolete logs, etc..) to reflect that.
   */
  static void
  metaslab_flush_update(metaslab_t *msp, dmu_tx_t *tx)
  {
          metaslab_group_t *mg = msp->ms_group;
          spa_t *spa = mg->mg_vd->vdev_spa;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          ASSERT3U(spa_sync_pass(spa), ==, 1);
  
          /*
           * Just because a metaslab got flushed, that doesn't mean that
           * it will pass through metaslab_sync_done(). Thus, make sure to
           * update ms_synced_length here in case it doesn't.
           */
          msp->ms_synced_length = space_map_length(msp->ms_sm);
  
          /*
           * We may end up here from metaslab_condense() without the
           * feature being active. In that case this is a no-op.
           */
          if (!spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP) ||
              metaslab_unflushed_txg(msp) == 0)
                  return;
  
          metaslab_unflushed_bump(msp, tx, B_FALSE);
  }
  
  boolean_t
  metaslab_flush(metaslab_t *msp, dmu_tx_t *tx)
  {
          spa_t *spa = msp->ms_group->mg_vd->vdev_spa;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          ASSERT3U(spa_sync_pass(spa), ==, 1);
          ASSERT(spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP));
  
          ASSERT(msp->ms_sm != NULL);
          ASSERT(metaslab_unflushed_txg(msp) != 0);
          ASSERT(avl_find(&spa->spa_metaslabs_by_flushed, msp, NULL) != NULL);
  
          /*
           * There is nothing wrong with flushing the same metaslab twice, as
           * this codepath should work on that case. However, the current
           * flushing scheme makes sure to avoid this situation as we would be
           * making all these calls without having anything meaningful to write
           * to disk. We assert this behavior here.
           */
          ASSERT3U(metaslab_unflushed_txg(msp), <, dmu_tx_get_txg(tx));
  
          /*
           * We can not flush while loading, because then we would
           * not load the ms_unflushed_{allocs,frees}.
           */
          if (msp->ms_loading)
                  return (B_FALSE);
  
          metaslab_verify_space(msp, dmu_tx_get_txg(tx));
          metaslab_verify_weight_and_frag(msp);
  
          /*
           * Metaslab condensing is effectively flushing. Therefore if the
           * metaslab can be condensed we can just condense it instead of
           * flushing it.
           *
           * Note that metaslab_condense() does call metaslab_flush_update()
           * so we can just return immediately after condensing. We also
           * don't need to care about setting ms_flushing or broadcasting
           * ms_flush_cv, even if we temporarily drop the ms_lock in
           * metaslab_condense(), as the metaslab is already loaded.
           */
          if (msp->ms_loaded && metaslab_should_condense(msp)) {
                  metaslab_group_t *mg = msp->ms_group;
  
                  /*
                   * For all histogram operations below refer to the
                   * comments of metaslab_sync() where we follow a
                   * similar procedure.
                   */
                  metaslab_group_histogram_verify(mg);
                  metaslab_class_histogram_verify(mg->mg_class);
                  metaslab_group_histogram_remove(mg, msp);
  
                  metaslab_condense(msp, tx);
  
                  space_map_histogram_clear(msp->ms_sm);
                  space_map_histogram_add(msp->ms_sm, msp->ms_allocatable, tx);
                  ASSERT(range_tree_is_empty(msp->ms_freed));
                  for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                          space_map_histogram_add(msp->ms_sm,
                              msp->ms_defer[t], tx);
                  }
                  metaslab_aux_histograms_update(msp);
  
                  metaslab_group_histogram_add(mg, msp);
                  metaslab_group_histogram_verify(mg);
                  metaslab_class_histogram_verify(mg->mg_class);
  
                  metaslab_verify_space(msp, dmu_tx_get_txg(tx));
  
                  /*
                   * Since we recreated the histogram (and potentially
                   * the ms_sm too while condensing) ensure that the
                   * weight is updated too because we are not guaranteed
                   * that this metaslab is dirty and will go through
                   * metaslab_sync_done().
                   */
                  metaslab_recalculate_weight_and_sort(msp);
                  return (B_TRUE);
          }
  
          msp->ms_flushing = B_TRUE;
          uint64_t sm_len_before = space_map_length(msp->ms_sm);
  
          mutex_exit(&msp->ms_lock);
          space_map_write(msp->ms_sm, msp->ms_unflushed_allocs, SM_ALLOC,
              SM_NO_VDEVID, tx);
          space_map_write(msp->ms_sm, msp->ms_unflushed_frees, SM_FREE,
              SM_NO_VDEVID, tx);
          mutex_enter(&msp->ms_lock);
  
          uint64_t sm_len_after = space_map_length(msp->ms_sm);
          if (zfs_flags & ZFS_DEBUG_LOG_SPACEMAP) {
                  zfs_dbgmsg("flushing: txg %llu, spa %s, vdev_id %llu, "
                      "ms_id %llu, unflushed_allocs %llu, unflushed_frees %llu, "
                      "appended %llu bytes", (u_longlong_t)dmu_tx_get_txg(tx),
                      spa_name(spa),
                      (u_longlong_t)msp->ms_group->mg_vd->vdev_id,
                      (u_longlong_t)msp->ms_id,
                      (u_longlong_t)range_tree_space(msp->ms_unflushed_allocs),
                      (u_longlong_t)range_tree_space(msp->ms_unflushed_frees),
                      (u_longlong_t)(sm_len_after - sm_len_before));
          }
  
          ASSERT3U(spa->spa_unflushed_stats.sus_memused, >=,
              metaslab_unflushed_changes_memused(msp));
          spa->spa_unflushed_stats.sus_memused -=
              metaslab_unflushed_changes_memused(msp);
          range_tree_vacate(msp->ms_unflushed_allocs, NULL, NULL);
          range_tree_vacate(msp->ms_unflushed_frees, NULL, NULL);
  
          metaslab_verify_space(msp, dmu_tx_get_txg(tx));
          metaslab_verify_weight_and_frag(msp);
  
          metaslab_flush_update(msp, tx);
  
          metaslab_verify_space(msp, dmu_tx_get_txg(tx));
          metaslab_verify_weight_and_frag(msp);
  
          msp->ms_flushing = B_FALSE;
          cv_broadcast(&msp->ms_flush_cv);
          return (B_TRUE);
  }
  
  /*
   * Write a metaslab to disk in the context of the specified transaction group.
   */
  void
  metaslab_sync(metaslab_t *msp, uint64_t txg)
  {
          metaslab_group_t *mg = msp->ms_group;
          vdev_t *vd = mg->mg_vd;
          spa_t *spa = vd->vdev_spa;
          objset_t *mos = spa_meta_objset(spa);
          range_tree_t *alloctree = msp->ms_allocating[txg & TXG_MASK];
          dmu_tx_t *tx;
  
          ASSERT(!vd->vdev_ishole);
  
          /*
           * This metaslab has just been added so there's no work to do now.
           */
          if (msp->ms_new) {
                  ASSERT0(range_tree_space(alloctree));
                  ASSERT0(range_tree_space(msp->ms_freeing));
                  ASSERT0(range_tree_space(msp->ms_freed));
                  ASSERT0(range_tree_space(msp->ms_checkpointing));
                  ASSERT0(range_tree_space(msp->ms_trim));
                  return;
          }
  
          /*
           * Normally, we don't want to process a metaslab if there are no
           * allocations or frees to perform. However, if the metaslab is being
           * forced to condense, it's loaded and we're not beyond the final
           * dirty txg, we need to let it through. Not condensing beyond the
           * final dirty txg prevents an issue where metaslabs that need to be
           * condensed but were loaded for other reasons could cause a panic
           * here. By only checking the txg in that branch of the conditional,
           * we preserve the utility of the VERIFY statements in all other
           * cases.
           */
          if (range_tree_is_empty(alloctree) &&
              range_tree_is_empty(msp->ms_freeing) &&
              range_tree_is_empty(msp->ms_checkpointing) &&
              !(msp->ms_loaded && msp->ms_condense_wanted &&
              txg <= spa_final_dirty_txg(spa)))
                  return;
  
  
          VERIFY3U(txg, <=, spa_final_dirty_txg(spa));
  
          /*
           * The only state that can actually be changing concurrently
           * with metaslab_sync() is the metaslab's ms_allocatable. No
           * other thread can be modifying this txg's alloc, freeing,
           * freed, or space_map_phys_t.  We drop ms_lock whenever we
           * could call into the DMU, because the DMU can call down to
           * us (e.g. via zio_free()) at any time.
           *
           * The spa_vdev_remove_thread() can be reading metaslab state
           * concurrently, and it is locked out by the ms_sync_lock.
           * Note that the ms_lock is insufficient for this, because it
           * is dropped by space_map_write().
           */
          tx = dmu_tx_create_assigned(spa_get_dsl(spa), txg);
  
          /*
           * Generate a log space map if one doesn't exist already.
           */
          spa_generate_syncing_log_sm(spa, tx);
  
          if (msp->ms_sm == NULL) {
                  uint64_t new_object = space_map_alloc(mos,
                      spa_feature_is_enabled(spa, SPA_FEATURE_LOG_SPACEMAP) ?
                      zfs_metaslab_sm_blksz_with_log :
                      zfs_metaslab_sm_blksz_no_log, tx);
                  VERIFY3U(new_object, !=, 0);
  
                  dmu_write(mos, vd->vdev_ms_array, sizeof (uint64_t) *
                      msp->ms_id, sizeof (uint64_t), &new_object, tx);
  
                  VERIFY0(space_map_open(&msp->ms_sm, mos, new_object,
                      msp->ms_start, msp->ms_size, vd->vdev_ashift));
                  ASSERT(msp->ms_sm != NULL);
  
                  ASSERT(range_tree_is_empty(msp->ms_unflushed_allocs));
                  ASSERT(range_tree_is_empty(msp->ms_unflushed_frees));
                  ASSERT0(metaslab_allocated_space(msp));
          }
  
          if (!range_tree_is_empty(msp->ms_checkpointing) &&
              vd->vdev_checkpoint_sm == NULL) {
                  ASSERT(spa_has_checkpoint(spa));
  
                  uint64_t new_object = space_map_alloc(mos,
                      zfs_vdev_standard_sm_blksz, tx);
                  VERIFY3U(new_object, !=, 0);
  
                  VERIFY0(space_map_open(&vd->vdev_checkpoint_sm,
                      mos, new_object, 0, vd->vdev_asize, vd->vdev_ashift));
                  ASSERT3P(vd->vdev_checkpoint_sm, !=, NULL);
  
                  /*
                   * We save the space map object as an entry in vdev_top_zap
                   * so it can be retrieved when the pool is reopened after an
                   * export or through zdb.
                   */
                  VERIFY0(zap_add(vd->vdev_spa->spa_meta_objset,
                      vd->vdev_top_zap, VDEV_TOP_ZAP_POOL_CHECKPOINT_SM,
                      sizeof (new_object), 1, &new_object, tx));
          }
  
          mutex_enter(&msp->ms_sync_lock);
          mutex_enter(&msp->ms_lock);
  
          /*
           * Note: metaslab_condense() clears the space map's histogram.
           * Therefore we must verify and remove this histogram before
           * condensing.
           */
          metaslab_group_histogram_verify(mg);
          metaslab_class_histogram_verify(mg->mg_class);
          metaslab_group_histogram_remove(mg, msp);
  
          if (spa->spa_sync_pass == 1 && msp->ms_loaded &&
              metaslab_should_condense(msp))
                  metaslab_condense(msp, tx);
  
          /*
           * We'll be going to disk to sync our space accounting, thus we
           * drop the ms_lock during that time so allocations coming from
           * open-context (ZIL) for future TXGs do not block.
           */
          mutex_exit(&msp->ms_lock);
          space_map_t *log_sm = spa_syncing_log_sm(spa);
          if (log_sm != NULL) {
                  ASSERT(spa_feature_is_enabled(spa, SPA_FEATURE_LOG_SPACEMAP));
                  if (metaslab_unflushed_txg(msp) == 0)
                          metaslab_unflushed_add(msp, tx);
                  else if (!metaslab_unflushed_dirty(msp))
                          metaslab_unflushed_bump(msp, tx, B_TRUE);
  
                  space_map_write(log_sm, alloctree, SM_ALLOC,
                      vd->vdev_id, tx);
                  space_map_write(log_sm, msp->ms_freeing, SM_FREE,
                      vd->vdev_id, tx);
                  mutex_enter(&msp->ms_lock);
  
                  ASSERT3U(spa->spa_unflushed_stats.sus_memused, >=,
                      metaslab_unflushed_changes_memused(msp));
                  spa->spa_unflushed_stats.sus_memused -=
                      metaslab_unflushed_changes_memused(msp);
                  range_tree_remove_xor_add(alloctree,
                      msp->ms_unflushed_frees, msp->ms_unflushed_allocs);
                  range_tree_remove_xor_add(msp->ms_freeing,
                      msp->ms_unflushed_allocs, msp->ms_unflushed_frees);
                  spa->spa_unflushed_stats.sus_memused +=
                      metaslab_unflushed_changes_memused(msp);
          } else {
                  ASSERT(!spa_feature_is_enabled(spa, SPA_FEATURE_LOG_SPACEMAP));
  
                  space_map_write(msp->ms_sm, alloctree, SM_ALLOC,
                      SM_NO_VDEVID, tx);
                  space_map_write(msp->ms_sm, msp->ms_freeing, SM_FREE,
                      SM_NO_VDEVID, tx);
                  mutex_enter(&msp->ms_lock);
          }
  
          msp->ms_allocated_space += range_tree_space(alloctree);
          ASSERT3U(msp->ms_allocated_space, >=,
              range_tree_space(msp->ms_freeing));
          msp->ms_allocated_space -= range_tree_space(msp->ms_freeing);
  
          if (!range_tree_is_empty(msp->ms_checkpointing)) {
                  ASSERT(spa_has_checkpoint(spa));
                  ASSERT3P(vd->vdev_checkpoint_sm, !=, NULL);
  
                  /*
                   * Since we are doing writes to disk and the ms_checkpointing
                   * tree won't be changing during that time, we drop the
                   * ms_lock while writing to the checkpoint space map, for the
                   * same reason mentioned above.
                   */
                  mutex_exit(&msp->ms_lock);
                  space_map_write(vd->vdev_checkpoint_sm,
                      msp->ms_checkpointing, SM_FREE, SM_NO_VDEVID, tx);
                  mutex_enter(&msp->ms_lock);
  
                  spa->spa_checkpoint_info.sci_dspace +=
                      range_tree_space(msp->ms_checkpointing);
                  vd->vdev_stat.vs_checkpoint_space +=
                      range_tree_space(msp->ms_checkpointing);
                  ASSERT3U(vd->vdev_stat.vs_checkpoint_space, ==,
                      -space_map_allocated(vd->vdev_checkpoint_sm));
  
                  range_tree_vacate(msp->ms_checkpointing, NULL, NULL);
          }
  
          if (msp->ms_loaded) {
                  /*
                   * When the space map is loaded, we have an accurate
                   * histogram in the range tree. This gives us an opportunity
                   * to bring the space map's histogram up-to-date so we clear
                   * it first before updating it.
                   */
                  space_map_histogram_clear(msp->ms_sm);
                  space_map_histogram_add(msp->ms_sm, msp->ms_allocatable, tx);
  
                  /*
                   * Since we've cleared the histogram we need to add back
                   * any free space that has already been processed, plus
                   * any deferred space. This allows the on-disk histogram
                   * to accurately reflect all free space even if some space
                   * is not yet available for allocation (i.e. deferred).
                   */
                  space_map_histogram_add(msp->ms_sm, msp->ms_freed, tx);
  
                  /*
                   * Add back any deferred free space that has not been
                   * added back into the in-core free tree yet. This will
                   * ensure that we don't end up with a space map histogram
                   * that is completely empty unless the metaslab is fully
                   * allocated.
                   */
                  for (int t = 0; t < TXG_DEFER_SIZE; t++) {
                          space_map_histogram_add(msp->ms_sm,
                              msp->ms_defer[t], tx);
                  }
          }
  
          /*
           * Always add the free space from this sync pass to the space
           * map histogram. We want to make sure that the on-disk histogram
           * accounts for all free space. If the space map is not loaded,
           * then we will lose some accuracy but will correct it the next
           * time we load the space map.
           */
          space_map_histogram_add(msp->ms_sm, msp->ms_freeing, tx);
          metaslab_aux_histograms_update(msp);
  
          metaslab_group_histogram_add(mg, msp);
          metaslab_group_histogram_verify(mg);
          metaslab_class_histogram_verify(mg->mg_class);
  
          /*
           * For sync pass 1, we avoid traversing this txg's free range tree
           * and instead will just swap the pointers for freeing and freed.
           * We can safely do this since the freed_tree is guaranteed to be
           * empty on the initial pass.
           *
           * Keep in mind that even if we are currently using a log spacemap
           * we want current frees to end up in the ms_allocatable (but not
           * get appended to the ms_sm) so their ranges can be reused as usual.
           */
          if (spa_sync_pass(spa) == 1) {
                  range_tree_swap(&msp->ms_freeing, &msp->ms_freed);
                  ASSERT0(msp->ms_allocated_this_txg);
          } else {
                  range_tree_vacate(msp->ms_freeing,
                      range_tree_add, msp->ms_freed);
          }
          msp->ms_allocated_this_txg += range_tree_space(alloctree);
          range_tree_vacate(alloctree, NULL, NULL);
  
          ASSERT0(range_tree_space(msp->ms_allocating[txg & TXG_MASK]));
          ASSERT0(range_tree_space(msp->ms_allocating[TXG_CLEAN(txg)
              & TXG_MASK]));
          ASSERT0(range_tree_space(msp->ms_freeing));
          ASSERT0(range_tree_space(msp->ms_checkpointing));
  
          mutex_exit(&msp->ms_lock);
  
          /*
           * Verify that the space map object ID has been recorded in the
           * vdev_ms_array.
           */
          uint64_t object;
          VERIFY0(dmu_read(mos, vd->vdev_ms_array,
              msp->ms_id * sizeof (uint64_t), sizeof (uint64_t), &object, 0));
          VERIFY3U(object, ==, space_map_object(msp->ms_sm));
  
          mutex_exit(&msp->ms_sync_lock);
          dmu_tx_commit(tx);
  }
  
  static void
  metaslab_evict(metaslab_t *msp, uint64_t txg)
  {
          if (!msp->ms_loaded || msp->ms_disabled != 0)
                  return;
  
          for (int t = 1; t < TXG_CONCURRENT_STATES; t++) {
                  VERIFY0(range_tree_space(
                      msp->ms_allocating[(txg + t) & TXG_MASK]));
          }
          if (msp->ms_allocator != -1)
                  metaslab_passivate(msp, msp->ms_weight & ~METASLAB_ACTIVE_MASK);
  
          if (!metaslab_debug_unload)
                  metaslab_unload(msp);
  }
  
  /*
   * Called after a transaction group has completely synced to mark
   * all of the metaslab's free space as usable.
   */
  void
  metaslab_sync_done(metaslab_t *msp, uint64_t txg)
  {
          metaslab_group_t *mg = msp->ms_group;
          vdev_t *vd = mg->mg_vd;
          spa_t *spa = vd->vdev_spa;
          range_tree_t **defer_tree;
          int64_t alloc_delta, defer_delta;
          boolean_t defer_allowed = B_TRUE;
  
          ASSERT(!vd->vdev_ishole);
  
          mutex_enter(&msp->ms_lock);
  
          if (msp->ms_new) {
                  /* this is a new metaslab, add its capacity to the vdev */
                  metaslab_space_update(vd, mg->mg_class, 0, 0, msp->ms_size);
  
                  /* there should be no allocations nor frees at this point */
                  VERIFY0(msp->ms_allocated_this_txg);
                  VERIFY0(range_tree_space(msp->ms_freed));
          }
  
          ASSERT0(range_tree_space(msp->ms_freeing));
          ASSERT0(range_tree_space(msp->ms_checkpointing));
  
          defer_tree = &msp->ms_defer[txg % TXG_DEFER_SIZE];
  
          uint64_t free_space = metaslab_class_get_space(spa_normal_class(spa)) -
              metaslab_class_get_alloc(spa_normal_class(spa));
          if (free_space <= spa_get_slop_space(spa) || vd->vdev_removing) {
                  defer_allowed = B_FALSE;
          }
  
          defer_delta = 0;
          alloc_delta = msp->ms_allocated_this_txg -
              range_tree_space(msp->ms_freed);
  
          if (defer_allowed) {
                  defer_delta = range_tree_space(msp->ms_freed) -
                      range_tree_space(*defer_tree);
          } else {
                  defer_delta -= range_tree_space(*defer_tree);
          }
          metaslab_space_update(vd, mg->mg_class, alloc_delta + defer_delta,
              defer_delta, 0);
  
          if (spa_syncing_log_sm(spa) == NULL) {
                  /*
                   * If there's a metaslab_load() in progress and we don't have
                   * a log space map, it means that we probably wrote to the
                   * metaslab's space map. If this is the case, we need to
                   * make sure that we wait for the load to complete so that we
                   * have a consistent view at the in-core side of the metaslab.
                   */
                  metaslab_load_wait(msp);
          } else {
                  ASSERT(spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP));
          }
  
          /*
           * When auto-trimming is enabled, free ranges which are added to
           * ms_allocatable are also be added to ms_trim.  The ms_trim tree is
           * periodically consumed by the vdev_autotrim_thread() which issues
           * trims for all ranges and then vacates the tree.  The ms_trim tree
           * can be discarded at any time with the sole consequence of recent
           * frees not being trimmed.
           */
          if (spa_get_autotrim(spa) == SPA_AUTOTRIM_ON) {
                  range_tree_walk(*defer_tree, range_tree_add, msp->ms_trim);
                  if (!defer_allowed) {
                          range_tree_walk(msp->ms_freed, range_tree_add,
                              msp->ms_trim);
                  }
          } else {
                  range_tree_vacate(msp->ms_trim, NULL, NULL);
          }
  
          /*
           * Move the frees from the defer_tree back to the free
           * range tree (if it's loaded). Swap the freed_tree and
           * the defer_tree -- this is safe to do because we've
           * just emptied out the defer_tree.
           */
          range_tree_vacate(*defer_tree,
              msp->ms_loaded ? range_tree_add : NULL, msp->ms_allocatable);
          if (defer_allowed) {
                  range_tree_swap(&msp->ms_freed, defer_tree);
          } else {
                  range_tree_vacate(msp->ms_freed,
                      msp->ms_loaded ? range_tree_add : NULL,
                      msp->ms_allocatable);
          }
  
          msp->ms_synced_length = space_map_length(msp->ms_sm);
  
          msp->ms_deferspace += defer_delta;
          ASSERT3S(msp->ms_deferspace, >=, 0);
          ASSERT3S(msp->ms_deferspace, <=, msp->ms_size);
          if (msp->ms_deferspace != 0) {
                  /*
                   * Keep syncing this metaslab until all deferred frees
                   * are back in circulation.
                   */
                  vdev_dirty(vd, VDD_METASLAB, msp, txg + 1);
          }
          metaslab_aux_histograms_update_done(msp, defer_allowed);
  
          if (msp->ms_new) {
                  msp->ms_new = B_FALSE;
                  mutex_enter(&mg->mg_lock);
                  mg->mg_ms_ready++;
                  mutex_exit(&mg->mg_lock);
          }
  
          /*
           * Re-sort metaslab within its group now that we've adjusted
           * its allocatable space.
           */
          metaslab_recalculate_weight_and_sort(msp);
  
          ASSERT0(range_tree_space(msp->ms_allocating[txg & TXG_MASK]));
          ASSERT0(range_tree_space(msp->ms_freeing));
          ASSERT0(range_tree_space(msp->ms_freed));
          ASSERT0(range_tree_space(msp->ms_checkpointing));
          msp->ms_allocating_total -= msp->ms_allocated_this_txg;
          msp->ms_allocated_this_txg = 0;
          mutex_exit(&msp->ms_lock);
  }
  
  void
  metaslab_sync_reassess(metaslab_group_t *mg)
  {
          spa_t *spa = mg->mg_class->mc_spa;
  
          spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);
          metaslab_group_alloc_update(mg);
          mg->mg_fragmentation = metaslab_group_fragmentation(mg);
  
          /*
           * Preload the next potential metaslabs but only on active
           * metaslab groups. We can get into a state where the metaslab
           * is no longer active since we dirty metaslabs as we remove a
           * a device, thus potentially making the metaslab group eligible
           * for preloading.
           */
          if (mg->mg_activation_count > 0) {
                  metaslab_group_preload(mg);
          }
          spa_config_exit(spa, SCL_ALLOC, FTAG);
  }
  
  /*
   * When writing a ditto block (i.e. more than one DVA for a given BP) on
   * the same vdev as an existing DVA of this BP, then try to allocate it
   * on a different metaslab than existing DVAs (i.e. a unique metaslab).
   */
  static boolean_t
  metaslab_is_unique(metaslab_t *msp, dva_t *dva)
  {
          uint64_t dva_ms_id;
  
          if (DVA_GET_ASIZE(dva) == 0)
                  return (B_TRUE);
  
          if (msp->ms_group->mg_vd->vdev_id != DVA_GET_VDEV(dva))
                  return (B_TRUE);
  
          dva_ms_id = DVA_GET_OFFSET(dva) >> msp->ms_group->mg_vd->vdev_ms_shift;
  
          return (msp->ms_id != dva_ms_id);
  }
  
  /*
   * ==========================================================================
   * Metaslab allocation tracing facility
   * ==========================================================================
   */
  
  /*
   * Add an allocation trace element to the allocation tracing list.
   */
  static void
  metaslab_trace_add(zio_alloc_list_t *zal, metaslab_group_t *mg,
      metaslab_t *msp, uint64_t psize, uint32_t dva_id, uint64_t offset,
      int allocator)
  {
          metaslab_alloc_trace_t *mat;
  
          if (!metaslab_trace_enabled)
                  return;
  
          /*
           * When the tracing list reaches its maximum we remove
           * the second element in the list before adding a new one.
           * By removing the second element we preserve the original
           * entry as a clue to what allocations steps have already been
           * performed.
           */
          if (zal->zal_size == metaslab_trace_max_entries) {
                  metaslab_alloc_trace_t *mat_next;
  #ifdef ZFS_DEBUG
                  panic("too many entries in allocation list");
  #endif
                  METASLABSTAT_BUMP(metaslabstat_trace_over_limit);
                  zal->zal_size--;
                  mat_next = list_next(&zal->zal_list, list_head(&zal->zal_list));
                  list_remove(&zal->zal_list, mat_next);
                  kmem_cache_free(metaslab_alloc_trace_cache, mat_next);
          }
  
          mat = kmem_cache_alloc(metaslab_alloc_trace_cache, KM_SLEEP);
          list_link_init(&mat->mat_list_node);
          mat->mat_mg = mg;
          mat->mat_msp = msp;
          mat->mat_size = psize;
          mat->mat_dva_id = dva_id;
          mat->mat_offset = offset;
          mat->mat_weight = 0;
          mat->mat_allocator = allocator;
  
          if (msp != NULL)
                  mat->mat_weight = msp->ms_weight;
  
          /*
           * The list is part of the zio so locking is not required. Only
           * a single thread will perform allocations for a given zio.
           */
          list_insert_tail(&zal->zal_list, mat);
          zal->zal_size++;
  
          ASSERT3U(zal->zal_size, <=, metaslab_trace_max_entries);
  }
  
  void
  metaslab_trace_init(zio_alloc_list_t *zal)
  {
          list_create(&zal->zal_list, sizeof (metaslab_alloc_trace_t),
              offsetof(metaslab_alloc_trace_t, mat_list_node));
          zal->zal_size = 0;
  }
  
  void
  metaslab_trace_fini(zio_alloc_list_t *zal)
  {
          metaslab_alloc_trace_t *mat;
  
          while ((mat = list_remove_head(&zal->zal_list)) != NULL)
                  kmem_cache_free(metaslab_alloc_trace_cache, mat);
          list_destroy(&zal->zal_list);
          zal->zal_size = 0;
  }
  
  /*
   * ==========================================================================
   * Metaslab block operations
   * ==========================================================================
   */
  
  static void
  metaslab_group_alloc_increment(spa_t *spa, uint64_t vdev, const void *tag,
      int flags, int allocator)
  {
          if (!(flags & METASLAB_ASYNC_ALLOC) ||
              (flags & METASLAB_DONT_THROTTLE))
                  return;
  
          metaslab_group_t *mg = vdev_lookup_top(spa, vdev)->vdev_mg;
          if (!mg->mg_class->mc_alloc_throttle_enabled)
                  return;
  
          metaslab_group_allocator_t *mga = &mg->mg_allocator[allocator];
          (void) zfs_refcount_add(&mga->mga_alloc_queue_depth, tag);
  }
  
  static void
  metaslab_group_increment_qdepth(metaslab_group_t *mg, int allocator)
  {
          metaslab_group_allocator_t *mga = &mg->mg_allocator[allocator];
          metaslab_class_allocator_t *mca =
              &mg->mg_class->mc_allocator[allocator];
          uint64_t max = mg->mg_max_alloc_queue_depth;
          uint64_t cur = mga->mga_cur_max_alloc_queue_depth;
          while (cur < max) {
                  if (atomic_cas_64(&mga->mga_cur_max_alloc_queue_depth,
                      cur, cur + 1) == cur) {
                          atomic_inc_64(&mca->mca_alloc_max_slots);
                          return;
                  }
                  cur = mga->mga_cur_max_alloc_queue_depth;
          }
  }
  
  void
  metaslab_group_alloc_decrement(spa_t *spa, uint64_t vdev, const void *tag,
      int flags, int allocator, boolean_t io_complete)
  {
          if (!(flags & METASLAB_ASYNC_ALLOC) ||
              (flags & METASLAB_DONT_THROTTLE))
                  return;
  
          metaslab_group_t *mg = vdev_lookup_top(spa, vdev)->vdev_mg;
          if (!mg->mg_class->mc_alloc_throttle_enabled)
                  return;
  
          metaslab_group_allocator_t *mga = &mg->mg_allocator[allocator];
          (void) zfs_refcount_remove(&mga->mga_alloc_queue_depth, tag);
          if (io_complete)
                  metaslab_group_increment_qdepth(mg, allocator);
  }
  
  void
  metaslab_group_alloc_verify(spa_t *spa, const blkptr_t *bp, const void *tag,
      int allocator)
  {
  #ifdef ZFS_DEBUG
          const dva_t *dva = bp->blk_dva;
          int ndvas = BP_GET_NDVAS(bp);
  
          for (int d = 0; d < ndvas; d++) {
                  uint64_t vdev = DVA_GET_VDEV(&dva[d]);
                  metaslab_group_t *mg = vdev_lookup_top(spa, vdev)->vdev_mg;
                  metaslab_group_allocator_t *mga = &mg->mg_allocator[allocator];
                  VERIFY(zfs_refcount_not_held(&mga->mga_alloc_queue_depth, tag));
          }
  #endif
  }
  
  static uint64_t
  metaslab_block_alloc(metaslab_t *msp, uint64_t size, uint64_t txg)
  {
          uint64_t start;
          range_tree_t *rt = msp->ms_allocatable;
          metaslab_class_t *mc = msp->ms_group->mg_class;
  
          ASSERT(MUTEX_HELD(&msp->ms_lock));
          VERIFY(!msp->ms_condensing);
          VERIFY0(msp->ms_disabled);
  
          start = mc->mc_ops->msop_alloc(msp, size);
          if (start != -1ULL) {
                  metaslab_group_t *mg = msp->ms_group;
                  vdev_t *vd = mg->mg_vd;
  
                  VERIFY0(P2PHASE(start, 1ULL << vd->vdev_ashift));
                  VERIFY0(P2PHASE(size, 1ULL << vd->vdev_ashift));
                  VERIFY3U(range_tree_space(rt) - size, <=, msp->ms_size);
                  range_tree_remove(rt, start, size);
                  range_tree_clear(msp->ms_trim, start, size);
  
                  if (range_tree_is_empty(msp->ms_allocating[txg & TXG_MASK]))
                          vdev_dirty(mg->mg_vd, VDD_METASLAB, msp, txg);
  
                  range_tree_add(msp->ms_allocating[txg & TXG_MASK], start, size);
                  msp->ms_allocating_total += size;
  
                  /* Track the last successful allocation */
                  msp->ms_alloc_txg = txg;
                  metaslab_verify_space(msp, txg);
          }
  
          /*
           * Now that we've attempted the allocation we need to update the
           * metaslab's maximum block size since it may have changed.
           */
          msp->ms_max_size = metaslab_largest_allocatable(msp);
          return (start);
  }
  
  /*
   * Find the metaslab with the highest weight that is less than what we've
   * already tried.  In the common case, this means that we will examine each
   * metaslab at most once. Note that concurrent callers could reorder metaslabs
   * by activation/passivation once we have dropped the mg_lock. If a metaslab is
   * activated by another thread, and we fail to allocate from the metaslab we
   * have selected, we may not try the newly-activated metaslab, and instead
   * activate another metaslab.  This is not optimal, but generally does not cause
   * any problems (a possible exception being if every metaslab is completely full
   * except for the newly-activated metaslab which we fail to examine).
   */
  static metaslab_t *
  find_valid_metaslab(metaslab_group_t *mg, uint64_t activation_weight,
      dva_t *dva, int d, boolean_t want_unique, uint64_t asize, int allocator,
      boolean_t try_hard, zio_alloc_list_t *zal, metaslab_t *search,
      boolean_t *was_active)
  {
          avl_index_t idx;
          avl_tree_t *t = &mg->mg_metaslab_tree;
          metaslab_t *msp = avl_find(t, search, &idx);
          if (msp == NULL)
                  msp = avl_nearest(t, idx, AVL_AFTER);
  
          uint_t tries = 0;
          for (; msp != NULL; msp = AVL_NEXT(t, msp)) {
                  int i;
  
                  if (!try_hard && tries > zfs_metaslab_find_max_tries) {
                          METASLABSTAT_BUMP(metaslabstat_too_many_tries);
                          return (NULL);
                  }
                  tries++;
  
                  if (!metaslab_should_allocate(msp, asize, try_hard)) {
                          metaslab_trace_add(zal, mg, msp, asize, d,
                              TRACE_TOO_SMALL, allocator);
                          continue;
                  }
  
                  /*
                   * If the selected metaslab is condensing or disabled,
                   * skip it.
                   */
                  if (msp->ms_condensing || msp->ms_disabled > 0)
                          continue;
  
                  *was_active = msp->ms_allocator != -1;
                  /*
                   * If we're activating as primary, this is our first allocation
                   * from this disk, so we don't need to check how close we are.
                   * If the metaslab under consideration was already active,
                   * we're getting desperate enough to steal another allocator's
                   * metaslab, so we still don't care about distances.
                   */
                  if (activation_weight == METASLAB_WEIGHT_PRIMARY || *was_active)
                          break;
  
                  for (i = 0; i < d; i++) {
                          if (want_unique &&
                              !metaslab_is_unique(msp, &dva[i]))
                                  break;  /* try another metaslab */
                  }
                  if (i == d)
                          break;
          }
  
          if (msp != NULL) {
                  search->ms_weight = msp->ms_weight;
                  search->ms_start = msp->ms_start + 1;
                  search->ms_allocator = msp->ms_allocator;
                  search->ms_primary = msp->ms_primary;
          }
          return (msp);
  }
  
  static void
  metaslab_active_mask_verify(metaslab_t *msp)
  {
          ASSERT(MUTEX_HELD(&msp->ms_lock));
  
          if ((zfs_flags & ZFS_DEBUG_METASLAB_VERIFY) == 0)
                  return;
  
          if ((msp->ms_weight & METASLAB_ACTIVE_MASK) == 0)
                  return;
  
          if (msp->ms_weight & METASLAB_WEIGHT_PRIMARY) {
                  VERIFY0(msp->ms_weight & METASLAB_WEIGHT_SECONDARY);
                  VERIFY0(msp->ms_weight & METASLAB_WEIGHT_CLAIM);
                  VERIFY3S(msp->ms_allocator, !=, -1);
                  VERIFY(msp->ms_primary);
                  return;
          }
  
          if (msp->ms_weight & METASLAB_WEIGHT_SECONDARY) {
                  VERIFY0(msp->ms_weight & METASLAB_WEIGHT_PRIMARY);
                  VERIFY0(msp->ms_weight & METASLAB_WEIGHT_CLAIM);
                  VERIFY3S(msp->ms_allocator, !=, -1);
                  VERIFY(!msp->ms_primary);
                  return;
          }
  
          if (msp->ms_weight & METASLAB_WEIGHT_CLAIM) {
                  VERIFY0(msp->ms_weight & METASLAB_WEIGHT_PRIMARY);
                  VERIFY0(msp->ms_weight & METASLAB_WEIGHT_SECONDARY);
                  VERIFY3S(msp->ms_allocator, ==, -1);
                  return;
          }
  }
  
  static uint64_t
  metaslab_group_alloc_normal(metaslab_group_t *mg, zio_alloc_list_t *zal,
      uint64_t asize, uint64_t txg, boolean_t want_unique, dva_t *dva, int d,
      int allocator, boolean_t try_hard)
  {
          metaslab_t *msp = NULL;
          uint64_t offset = -1ULL;
  
          uint64_t activation_weight = METASLAB_WEIGHT_PRIMARY;
          for (int i = 0; i < d; i++) {
                  if (activation_weight == METASLAB_WEIGHT_PRIMARY &&
                      DVA_GET_VDEV(&dva[i]) == mg->mg_vd->vdev_id) {
                          activation_weight = METASLAB_WEIGHT_SECONDARY;
                  } else if (activation_weight == METASLAB_WEIGHT_SECONDARY &&
                      DVA_GET_VDEV(&dva[i]) == mg->mg_vd->vdev_id) {
                          activation_weight = METASLAB_WEIGHT_CLAIM;
                          break;
                  }
          }
  
          /*
           * If we don't have enough metaslabs active to fill the entire array, we
           * just use the 0th slot.
           */
          if (mg->mg_ms_ready < mg->mg_allocators * 3)
                  allocator = 0;
          metaslab_group_allocator_t *mga = &mg->mg_allocator[allocator];
  
          ASSERT3U(mg->mg_vd->vdev_ms_count, >=, 2);
  
          metaslab_t *search = kmem_alloc(sizeof (*search), KM_SLEEP);
          search->ms_weight = UINT64_MAX;
          search->ms_start = 0;
          /*
           * At the end of the metaslab tree are the already-active metaslabs,
           * first the primaries, then the secondaries. When we resume searching
           * through the tree, we need to consider ms_allocator and ms_primary so
           * we start in the location right after where we left off, and don't
           * accidentally loop forever considering the same metaslabs.
           */
          search->ms_allocator = -1;
          search->ms_primary = B_TRUE;
          for (;;) {
                  boolean_t was_active = B_FALSE;
  
                  mutex_enter(&mg->mg_lock);
  
                  if (activation_weight == METASLAB_WEIGHT_PRIMARY &&
                      mga->mga_primary != NULL) {
                          msp = mga->mga_primary;
  
                          /*
                           * Even though we don't hold the ms_lock for the
                           * primary metaslab, those fields should not
                           * change while we hold the mg_lock. Thus it is
                           * safe to make assertions on them.
                           */
                          ASSERT(msp->ms_primary);
                          ASSERT3S(msp->ms_allocator, ==, allocator);
                          ASSERT(msp->ms_loaded);
  
                          was_active = B_TRUE;
                          ASSERT(msp->ms_weight & METASLAB_ACTIVE_MASK);
                  } else if (activation_weight == METASLAB_WEIGHT_SECONDARY &&
                      mga->mga_secondary != NULL) {
                          msp = mga->mga_secondary;
  
                          /*
                           * See comment above about the similar assertions
                           * for the primary metaslab.
                           */
                          ASSERT(!msp->ms_primary);
                          ASSERT3S(msp->ms_allocator, ==, allocator);
                          ASSERT(msp->ms_loaded);
  
                          was_active = B_TRUE;
                          ASSERT(msp->ms_weight & METASLAB_ACTIVE_MASK);
                  } else {
                          msp = find_valid_metaslab(mg, activation_weight, dva, d,
                              want_unique, asize, allocator, try_hard, zal,
                              search, &was_active);
                  }
  
                  mutex_exit(&mg->mg_lock);
                  if (msp == NULL) {
                          kmem_free(search, sizeof (*search));
                          return (-1ULL);
                  }
                  mutex_enter(&msp->ms_lock);
  
                  metaslab_active_mask_verify(msp);
  
                  /*
                   * This code is disabled out because of issues with
                   * tracepoints in non-gpl kernel modules.
                   */
  #if 0
                  DTRACE_PROBE3(ms__activation__attempt,
                      metaslab_t *, msp, uint64_t, activation_weight,
                      boolean_t, was_active);
  #endif
  
                  /*
                   * Ensure that the metaslab we have selected is still
                   * capable of handling our request. It's possible that
                   * another thread may have changed the weight while we
                   * were blocked on the metaslab lock. We check the
                   * active status first to see if we need to set_selected_txg
                   * a new metaslab.
                   */
                  if (was_active && !(msp->ms_weight & METASLAB_ACTIVE_MASK)) {
                          ASSERT3S(msp->ms_allocator, ==, -1);
                          mutex_exit(&msp->ms_lock);
                          continue;
                  }
  
                  /*
                   * If the metaslab was activated for another allocator
                   * while we were waiting in the ms_lock above, or it's
                   * a primary and we're seeking a secondary (or vice versa),
                   * we go back and select a new metaslab.
                   */
                  if (!was_active && (msp->ms_weight & METASLAB_ACTIVE_MASK) &&
                      (msp->ms_allocator != -1) &&
                      (msp->ms_allocator != allocator || ((activation_weight ==
                      METASLAB_WEIGHT_PRIMARY) != msp->ms_primary))) {
                          ASSERT(msp->ms_loaded);
                          ASSERT((msp->ms_weight & METASLAB_WEIGHT_CLAIM) ||
                              msp->ms_allocator != -1);
                          mutex_exit(&msp->ms_lock);
                          continue;
                  }
  
                  /*
                   * This metaslab was used for claiming regions allocated
                   * by the ZIL during pool import. Once these regions are
                   * claimed we don't need to keep the CLAIM bit set
                   * anymore. Passivate this metaslab to zero its activation
                   * mask.
                   */
                  if (msp->ms_weight & METASLAB_WEIGHT_CLAIM &&
                      activation_weight != METASLAB_WEIGHT_CLAIM) {
                          ASSERT(msp->ms_loaded);
                          ASSERT3S(msp->ms_allocator, ==, -1);
                          metaslab_passivate(msp, msp->ms_weight &
                              ~METASLAB_WEIGHT_CLAIM);
                          mutex_exit(&msp->ms_lock);
                          continue;
                  }
  
                  metaslab_set_selected_txg(msp, txg);
  
                  int activation_error =
                      metaslab_activate(msp, allocator, activation_weight);
                  metaslab_active_mask_verify(msp);
  
                  /*
                   * If the metaslab was activated by another thread for
                   * another allocator or activation_weight (EBUSY), or it
                   * failed because another metaslab was assigned as primary
                   * for this allocator (EEXIST) we continue using this
                   * metaslab for our allocation, rather than going on to a
                   * worse metaslab (we waited for that metaslab to be loaded
                   * after all).
                   *
                   * If the activation failed due to an I/O error or ENOSPC we
                   * skip to the next metaslab.
                   */
                  boolean_t activated;
                  if (activation_error == 0) {
                          activated = B_TRUE;
                  } else if (activation_error == EBUSY ||
                      activation_error == EEXIST) {
                          activated = B_FALSE;
                  } else {
                          mutex_exit(&msp->ms_lock);
                          continue;
                  }
                  ASSERT(msp->ms_loaded);
  
                  /*
                   * Now that we have the lock, recheck to see if we should
                   * continue to use this metaslab for this allocation. The
                   * the metaslab is now loaded so metaslab_should_allocate()
                   * can accurately determine if the allocation attempt should
                   * proceed.
                   */
                  if (!metaslab_should_allocate(msp, asize, try_hard)) {
                          /* Passivate this metaslab and select a new one. */
                          metaslab_trace_add(zal, mg, msp, asize, d,
                              TRACE_TOO_SMALL, allocator);
                          goto next;
                  }
  
                  /*
                   * If this metaslab is currently condensing then pick again
                   * as we can't manipulate this metaslab until it's committed
                   * to disk. If this metaslab is being initialized, we shouldn't
                   * allocate from it since the allocated region might be
                   * overwritten after allocation.
                   */
                  if (msp->ms_condensing) {
                          metaslab_trace_add(zal, mg, msp, asize, d,
                              TRACE_CONDENSING, allocator);
                          if (activated) {
                                  metaslab_passivate(msp, msp->ms_weight &
                                      ~METASLAB_ACTIVE_MASK);
                          }
                          mutex_exit(&msp->ms_lock);
                          continue;
                  } else if (msp->ms_disabled > 0) {
                          metaslab_trace_add(zal, mg, msp, asize, d,
                              TRACE_DISABLED, allocator);
                          if (activated) {
                                  metaslab_passivate(msp, msp->ms_weight &
                                      ~METASLAB_ACTIVE_MASK);
                          }
                          mutex_exit(&msp->ms_lock);
                          continue;
                  }
  
                  offset = metaslab_block_alloc(msp, asize, txg);
                  metaslab_trace_add(zal, mg, msp, asize, d, offset, allocator);
  
                  if (offset != -1ULL) {
                          /* Proactively passivate the metaslab, if needed */
                          if (activated)
                                  metaslab_segment_may_passivate(msp);
                          break;
                  }
  next:
                  ASSERT(msp->ms_loaded);
  
                  /*
                   * This code is disabled out because of issues with
                   * tracepoints in non-gpl kernel modules.
                   */
  #if 0
                  DTRACE_PROBE2(ms__alloc__failure, metaslab_t *, msp,
                      uint64_t, asize);
  #endif
  
                  /*
                   * We were unable to allocate from this metaslab so determine
                   * a new weight for this metaslab. Now that we have loaded
                   * the metaslab we can provide a better hint to the metaslab
                   * selector.
                   *
                   * For space-based metaslabs, we use the maximum block size.
                   * This information is only available when the metaslab
                   * is loaded and is more accurate than the generic free
                   * space weight that was calculated by metaslab_weight().
                   * This information allows us to quickly compare the maximum
                   * available allocation in the metaslab to the allocation
                   * size being requested.
                   *
                   * For segment-based metaslabs, determine the new weight
                   * based on the highest bucket in the range tree. We
                   * explicitly use the loaded segment weight (i.e. the range
                   * tree histogram) since it contains the space that is
                   * currently available for allocation and is accurate
                   * even within a sync pass.
                   */
                  uint64_t weight;
                  if (WEIGHT_IS_SPACEBASED(msp->ms_weight)) {
                          weight = metaslab_largest_allocatable(msp);
                          WEIGHT_SET_SPACEBASED(weight);
                  } else {
                          weight = metaslab_weight_from_range_tree(msp);
                  }
  
                  if (activated) {
                          metaslab_passivate(msp, weight);
                  } else {
                          /*
                           * For the case where we use the metaslab that is
                           * active for another allocator we want to make
                           * sure that we retain the activation mask.
                           *
                           * Note that we could attempt to use something like
                           * metaslab_recalculate_weight_and_sort() that
                           * retains the activation mask here. That function
                           * uses metaslab_weight() to set the weight though
                           * which is not as accurate as the calculations
                           * above.
                           */
                          weight |= msp->ms_weight & METASLAB_ACTIVE_MASK;
                          metaslab_group_sort(mg, msp, weight);
                  }
                  metaslab_active_mask_verify(msp);
  
                  /*
                   * We have just failed an allocation attempt, check
                   * that metaslab_should_allocate() agrees. Otherwise,
                   * we may end up in an infinite loop retrying the same
                   * metaslab.
                   */
                  ASSERT(!metaslab_should_allocate(msp, asize, try_hard));
  
                  mutex_exit(&msp->ms_lock);
          }
          mutex_exit(&msp->ms_lock);
          kmem_free(search, sizeof (*search));
          return (offset);
  }
  
  static uint64_t
  metaslab_group_alloc(metaslab_group_t *mg, zio_alloc_list_t *zal,
      uint64_t asize, uint64_t txg, boolean_t want_unique, dva_t *dva, int d,
      int allocator, boolean_t try_hard)
  {
          uint64_t offset;
          ASSERT(mg->mg_initialized);
  
          offset = metaslab_group_alloc_normal(mg, zal, asize, txg, want_unique,
              dva, d, allocator, try_hard);
  
          mutex_enter(&mg->mg_lock);
          if (offset == -1ULL) {
                  mg->mg_failed_allocations++;
                  metaslab_trace_add(zal, mg, NULL, asize, d,
                      TRACE_GROUP_FAILURE, allocator);
                  if (asize == SPA_GANGBLOCKSIZE) {
                          /*
                           * This metaslab group was unable to allocate
                           * the minimum gang block size so it must be out of
                           * space. We must notify the allocation throttle
                           * to start skipping allocation attempts to this
                           * metaslab group until more space becomes available.
                           * Note: this failure cannot be caused by the
                           * allocation throttle since the allocation throttle
                           * is only responsible for skipping devices and
                           * not failing block allocations.
                           */
                          mg->mg_no_free_space = B_TRUE;
                  }
          }
          mg->mg_allocations++;
          mutex_exit(&mg->mg_lock);
          return (offset);
  }
  
  /*
   * Allocate a block for the specified i/o.
   */
  int
  metaslab_alloc_dva(spa_t *spa, metaslab_class_t *mc, uint64_t psize,
      dva_t *dva, int d, dva_t *hintdva, uint64_t txg, int flags,
      zio_alloc_list_t *zal, int allocator)
  {
          metaslab_class_allocator_t *mca = &mc->mc_allocator[allocator];
          metaslab_group_t *mg, *fast_mg, *rotor;
          vdev_t *vd;
          boolean_t try_hard = B_FALSE;
  
          ASSERT(!DVA_IS_VALID(&dva[d]));
  
          /*
           * For testing, make some blocks above a certain size be gang blocks.
           * This will result in more split blocks when using device removal,
           * and a large number of split blocks coupled with ztest-induced
           * damage can result in extremely long reconstruction times.  This
           * will also test spilling from special to normal.
           */
          if (psize >= metaslab_force_ganging && (random_in_range(100) < 3)) {
                  metaslab_trace_add(zal, NULL, NULL, psize, d, TRACE_FORCE_GANG,
                      allocator);
                  return (SET_ERROR(ENOSPC));
          }
  
          /*
           * Start at the rotor and loop through all mgs until we find something.
           * Note that there's no locking on mca_rotor or mca_aliquot because
           * nothing actually breaks if we miss a few updates -- we just won't
           * allocate quite as evenly.  It all balances out over time.
           *
           * If we are doing ditto or log blocks, try to spread them across
           * consecutive vdevs.  If we're forced to reuse a vdev before we've
           * allocated all of our ditto blocks, then try and spread them out on
           * that vdev as much as possible.  If it turns out to not be possible,
           * gradually lower our standards until anything becomes acceptable.
           * Also, allocating on consecutive vdevs (as opposed to random vdevs)
           * gives us hope of containing our fault domains to something we're
           * able to reason about.  Otherwise, any two top-level vdev failures
           * will guarantee the loss of data.  With consecutive allocation,
           * only two adjacent top-level vdev failures will result in data loss.
           *
           * If we are doing gang blocks (hintdva is non-NULL), try to keep
           * ourselves on the same vdev as our gang block header.  That
           * way, we can hope for locality in vdev_cache, plus it makes our
           * fault domains something tractable.
           */
          if (hintdva) {
                  vd = vdev_lookup_top(spa, DVA_GET_VDEV(&hintdva[d]));
  
                  /*
                   * It's possible the vdev we're using as the hint no
                   * longer exists or its mg has been closed (e.g. by
                   * device removal).  Consult the rotor when
                   * all else fails.
                   */
                  if (vd != NULL && vd->vdev_mg != NULL) {
                          mg = vdev_get_mg(vd, mc);
  
                          if (flags & METASLAB_HINTBP_AVOID)
                                  mg = mg->mg_next;
                  } else {
                          mg = mca->mca_rotor;
                  }
          } else if (d != 0) {
                  vd = vdev_lookup_top(spa, DVA_GET_VDEV(&dva[d - 1]));
                  mg = vd->vdev_mg->mg_next;
          } else if (flags & METASLAB_FASTWRITE) {
                  mg = fast_mg = mca->mca_rotor;
  
                  do {
                          if (fast_mg->mg_vd->vdev_pending_fastwrite <
                              mg->mg_vd->vdev_pending_fastwrite)
                                  mg = fast_mg;
                  } while ((fast_mg = fast_mg->mg_next) != mca->mca_rotor);
  
          } else {
                  ASSERT(mca->mca_rotor != NULL);
                  mg = mca->mca_rotor;
          }
  
          /*
           * If the hint put us into the wrong metaslab class, or into a
           * metaslab group that has been passivated, just follow the rotor.
           */
          if (mg->mg_class != mc || mg->mg_activation_count <= 0)
                  mg = mca->mca_rotor;
  
          rotor = mg;
  top:
          do {
                  boolean_t allocatable;
  
                  ASSERT(mg->mg_activation_count == 1);
                  vd = mg->mg_vd;
  
                  /*
                   * Don't allocate from faulted devices.
                   */
                  if (try_hard) {
                          spa_config_enter(spa, SCL_ZIO, FTAG, RW_READER);
                          allocatable = vdev_allocatable(vd);
                          spa_config_exit(spa, SCL_ZIO, FTAG);
                  } else {
                          allocatable = vdev_allocatable(vd);
                  }
  
                  /*
                   * Determine if the selected metaslab group is eligible
                   * for allocations. If we're ganging then don't allow
                   * this metaslab group to skip allocations since that would
                   * inadvertently return ENOSPC and suspend the pool
                   * even though space is still available.
                   */
                  if (allocatable && !GANG_ALLOCATION(flags) && !try_hard) {
                          allocatable = metaslab_group_allocatable(mg, rotor,
                              flags, psize, allocator, d);
                  }
  
                  if (!allocatable) {
                          metaslab_trace_add(zal, mg, NULL, psize, d,
                              TRACE_NOT_ALLOCATABLE, allocator);
                          goto next;
                  }
  
                  ASSERT(mg->mg_initialized);
  
                  /*
                   * Avoid writing single-copy data to an unhealthy,
                   * non-redundant vdev, unless we've already tried all
                   * other vdevs.
                   */
                  if (vd->vdev_state < VDEV_STATE_HEALTHY &&
                      d == 0 && !try_hard && vd->vdev_children == 0) {
                          metaslab_trace_add(zal, mg, NULL, psize, d,
                              TRACE_VDEV_ERROR, allocator);
                          goto next;
                  }
  
                  ASSERT(mg->mg_class == mc);
  
                  uint64_t asize = vdev_psize_to_asize(vd, psize);
                  ASSERT(P2PHASE(asize, 1ULL << vd->vdev_ashift) == 0);
  
                  /*
                   * If we don't need to try hard, then require that the
                   * block be on a different metaslab from any other DVAs
                   * in this BP (unique=true).  If we are trying hard, then
                   * allow any metaslab to be used (unique=false).
                   */
                  uint64_t offset = metaslab_group_alloc(mg, zal, asize, txg,
                      !try_hard, dva, d, allocator, try_hard);
  
                  if (offset != -1ULL) {
                          /*
                           * If we've just selected this metaslab group,
                           * figure out whether the corresponding vdev is
                           * over- or under-used relative to the pool,
                           * and set an allocation bias to even it out.
                           *
                           * Bias is also used to compensate for unequally
                           * sized vdevs so that space is allocated fairly.
                           */
                          if (mca->mca_aliquot == 0 && metaslab_bias_enabled) {
                                  vdev_stat_t *vs = &vd->vdev_stat;
                                  int64_t vs_free = vs->vs_space - vs->vs_alloc;
                                  int64_t mc_free = mc->mc_space - mc->mc_alloc;
                                  int64_t ratio;
  
                                  /*
                                   * Calculate how much more or less we should
                                   * try to allocate from this device during
                                   * this iteration around the rotor.
                                   *
                                   * This basically introduces a zero-centered
                                   * bias towards the devices with the most
                                   * free space, while compensating for vdev
                                   * size differences.
                                   *
                                   * Examples:
                                   *  vdev V1 = 16M/128M
                                   *  vdev V2 = 16M/128M
                                   *  ratio(V1) = 100% ratio(V2) = 100%
                                   *
                                   *  vdev V1 = 16M/128M
                                   *  vdev V2 = 64M/128M
                                   *  ratio(V1) = 127% ratio(V2) =  72%
                                   *
                                   *  vdev V1 = 16M/128M
                                   *  vdev V2 = 64M/512M
                                   *  ratio(V1) =  40% ratio(V2) = 160%
                                   */
                                  ratio = (vs_free * mc->mc_alloc_groups * 100) /
                                      (mc_free + 1);
                                  mg->mg_bias = ((ratio - 100) *
                                      (int64_t)mg->mg_aliquot) / 100;
                          } else if (!metaslab_bias_enabled) {
                                  mg->mg_bias = 0;
                          }
  
                          if ((flags & METASLAB_FASTWRITE) ||
                              atomic_add_64_nv(&mca->mca_aliquot, asize) >=
                              mg->mg_aliquot + mg->mg_bias) {
                                  mca->mca_rotor = mg->mg_next;
                                  mca->mca_aliquot = 0;
                          }
  
                          DVA_SET_VDEV(&dva[d], vd->vdev_id);
                          DVA_SET_OFFSET(&dva[d], offset);
                          DVA_SET_GANG(&dva[d],
                              ((flags & METASLAB_GANG_HEADER) ? 1 : 0));
                          DVA_SET_ASIZE(&dva[d], asize);
  
                          if (flags & METASLAB_FASTWRITE) {
                                  atomic_add_64(&vd->vdev_pending_fastwrite,
                                      psize);
                          }
  
                          return (0);
                  }
  next:
                  mca->mca_rotor = mg->mg_next;
                  mca->mca_aliquot = 0;
          } while ((mg = mg->mg_next) != rotor);
  
          /*
           * If we haven't tried hard, perhaps do so now.
           */
          if (!try_hard && (zfs_metaslab_try_hard_before_gang ||
              GANG_ALLOCATION(flags) || (flags & METASLAB_ZIL) != 0 ||
              psize <= 1 << spa->spa_min_ashift)) {
                  METASLABSTAT_BUMP(metaslabstat_try_hard);
                  try_hard = B_TRUE;
                  goto top;
          }
  
          memset(&dva[d], 0, sizeof (dva_t));
  
          metaslab_trace_add(zal, rotor, NULL, psize, d, TRACE_ENOSPC, allocator);
          return (SET_ERROR(ENOSPC));
  }
  
  void
  metaslab_free_concrete(vdev_t *vd, uint64_t offset, uint64_t asize,
      boolean_t checkpoint)
  {
          metaslab_t *msp;
          spa_t *spa = vd->vdev_spa;
  
          ASSERT(vdev_is_concrete(vd));
          ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);
          ASSERT3U(offset >> vd->vdev_ms_shift, <, vd->vdev_ms_count);
  
          msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];
  
          VERIFY(!msp->ms_condensing);
          VERIFY3U(offset, >=, msp->ms_start);
          VERIFY3U(offset + asize, <=, msp->ms_start + msp->ms_size);
          VERIFY0(P2PHASE(offset, 1ULL << vd->vdev_ashift));
          VERIFY0(P2PHASE(asize, 1ULL << vd->vdev_ashift));
  
          metaslab_check_free_impl(vd, offset, asize);
  
          mutex_enter(&msp->ms_lock);
          if (range_tree_is_empty(msp->ms_freeing) &&
              range_tree_is_empty(msp->ms_checkpointing)) {
                  vdev_dirty(vd, VDD_METASLAB, msp, spa_syncing_txg(spa));
          }
  
          if (checkpoint) {
                  ASSERT(spa_has_checkpoint(spa));
                  range_tree_add(msp->ms_checkpointing, offset, asize);
          } else {
                  range_tree_add(msp->ms_freeing, offset, asize);
          }
          mutex_exit(&msp->ms_lock);
  }
  
  void
  metaslab_free_impl_cb(uint64_t inner_offset, vdev_t *vd, uint64_t offset,
      uint64_t size, void *arg)
  {
          (void) inner_offset;
          boolean_t *checkpoint = arg;
  
          ASSERT3P(checkpoint, !=, NULL);
  
          if (vd->vdev_ops->vdev_op_remap != NULL)
                  vdev_indirect_mark_obsolete(vd, offset, size);
          else
                  metaslab_free_impl(vd, offset, size, *checkpoint);
  }
  
  static void
  metaslab_free_impl(vdev_t *vd, uint64_t offset, uint64_t size,
      boolean_t checkpoint)
  {
          spa_t *spa = vd->vdev_spa;
  
          ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);
  
          if (spa_syncing_txg(spa) > spa_freeze_txg(spa))
                  return;
  
          if (spa->spa_vdev_removal != NULL &&
              spa->spa_vdev_removal->svr_vdev_id == vd->vdev_id &&
              vdev_is_concrete(vd)) {
                  /*
                   * Note: we check if the vdev is concrete because when
                   * we complete the removal, we first change the vdev to be
                   * an indirect vdev (in open context), and then (in syncing
                   * context) clear spa_vdev_removal.
                   */
                  free_from_removing_vdev(vd, offset, size);
          } else if (vd->vdev_ops->vdev_op_remap != NULL) {
                  vdev_indirect_mark_obsolete(vd, offset, size);
                  vd->vdev_ops->vdev_op_remap(vd, offset, size,
                      metaslab_free_impl_cb, &checkpoint);
          } else {
                  metaslab_free_concrete(vd, offset, size, checkpoint);
          }
  }
  
  typedef struct remap_blkptr_cb_arg {
          blkptr_t *rbca_bp;
          spa_remap_cb_t rbca_cb;
          vdev_t *rbca_remap_vd;
          uint64_t rbca_remap_offset;
          void *rbca_cb_arg;
  } remap_blkptr_cb_arg_t;
  
  static void
  remap_blkptr_cb(uint64_t inner_offset, vdev_t *vd, uint64_t offset,
      uint64_t size, void *arg)
  {
          remap_blkptr_cb_arg_t *rbca = arg;
          blkptr_t *bp = rbca->rbca_bp;
  
          /* We can not remap split blocks. */
          if (size != DVA_GET_ASIZE(&bp->blk_dva[0]))
                  return;
          ASSERT0(inner_offset);
  
          if (rbca->rbca_cb != NULL) {
                  /*
                   * At this point we know that we are not handling split
                   * blocks and we invoke the callback on the previous
                   * vdev which must be indirect.
                   */
                  ASSERT3P(rbca->rbca_remap_vd->vdev_ops, ==, &vdev_indirect_ops);
  
                  rbca->rbca_cb(rbca->rbca_remap_vd->vdev_id,
                      rbca->rbca_remap_offset, size, rbca->rbca_cb_arg);
  
                  /* set up remap_blkptr_cb_arg for the next call */
                  rbca->rbca_remap_vd = vd;
                  rbca->rbca_remap_offset = offset;
          }
  
          /*
           * The phys birth time is that of dva[0].  This ensures that we know
           * when each dva was written, so that resilver can determine which
           * blocks need to be scrubbed (i.e. those written during the time
           * the vdev was offline).  It also ensures that the key used in
           * the ARC hash table is unique (i.e. dva[0] + phys_birth).  If
           * we didn't change the phys_birth, a lookup in the ARC for a
           * remapped BP could find the data that was previously stored at
           * this vdev + offset.
           */
          vdev_t *oldvd = vdev_lookup_top(vd->vdev_spa,
              DVA_GET_VDEV(&bp->blk_dva[0]));
          vdev_indirect_births_t *vib = oldvd->vdev_indirect_births;
          bp->blk_phys_birth = vdev_indirect_births_physbirth(vib,
              DVA_GET_OFFSET(&bp->blk_dva[0]), DVA_GET_ASIZE(&bp->blk_dva[0]));
  
          DVA_SET_VDEV(&bp->blk_dva[0], vd->vdev_id);
          DVA_SET_OFFSET(&bp->blk_dva[0], offset);
  }
  
  /*
   * If the block pointer contains any indirect DVAs, modify them to refer to
   * concrete DVAs.  Note that this will sometimes not be possible, leaving
   * the indirect DVA in place.  This happens if the indirect DVA spans multiple
   * segments in the mapping (i.e. it is a "split block").
   *
   * If the BP was remapped, calls the callback on the original dva (note the
   * callback can be called multiple times if the original indirect DVA refers
   * to another indirect DVA, etc).
   *
   * Returns TRUE if the BP was remapped.
   */
  boolean_t
  spa_remap_blkptr(spa_t *spa, blkptr_t *bp, spa_remap_cb_t callback, void *arg)
  {
          remap_blkptr_cb_arg_t rbca;
  
          if (!zfs_remap_blkptr_enable)
                  return (B_FALSE);
  
          if (!spa_feature_is_enabled(spa, SPA_FEATURE_OBSOLETE_COUNTS))
                  return (B_FALSE);
  
          /*
           * Dedup BP's can not be remapped, because ddt_phys_select() depends
           * on DVA[0] being the same in the BP as in the DDT (dedup table).
           */
          if (BP_GET_DEDUP(bp))
                  return (B_FALSE);
  
          /*
           * Gang blocks can not be remapped, because
           * zio_checksum_gang_verifier() depends on the DVA[0] that's in
           * the BP used to read the gang block header (GBH) being the same
           * as the DVA[0] that we allocated for the GBH.
           */
          if (BP_IS_GANG(bp))
                  return (B_FALSE);
  
          /*
           * Embedded BP's have no DVA to remap.
           */
          if (BP_GET_NDVAS(bp) < 1)
                  return (B_FALSE);
  
          /*
           * Note: we only remap dva[0].  If we remapped other dvas, we
           * would no longer know what their phys birth txg is.
           */
          dva_t *dva = &bp->blk_dva[0];
  
          uint64_t offset = DVA_GET_OFFSET(dva);
          uint64_t size = DVA_GET_ASIZE(dva);
          vdev_t *vd = vdev_lookup_top(spa, DVA_GET_VDEV(dva));
  
          if (vd->vdev_ops->vdev_op_remap == NULL)
                  return (B_FALSE);
  
          rbca.rbca_bp = bp;
          rbca.rbca_cb = callback;
          rbca.rbca_remap_vd = vd;
          rbca.rbca_remap_offset = offset;
          rbca.rbca_cb_arg = arg;
  
          /*
           * remap_blkptr_cb() will be called in order for each level of
           * indirection, until a concrete vdev is reached or a split block is
           * encountered. old_vd and old_offset are updated within the callback
           * as we go from the one indirect vdev to the next one (either concrete
           * or indirect again) in that order.
           */
          vd->vdev_ops->vdev_op_remap(vd, offset, size, remap_blkptr_cb, &rbca);
  
          /* Check if the DVA wasn't remapped because it is a split block */
          if (DVA_GET_VDEV(&rbca.rbca_bp->blk_dva[0]) == vd->vdev_id)
                  return (B_FALSE);
  
          return (B_TRUE);
  }
  
  /*
   * Undo the allocation of a DVA which happened in the given transaction group.
   */
  void
  metaslab_unalloc_dva(spa_t *spa, const dva_t *dva, uint64_t txg)
  {
          metaslab_t *msp;
          vdev_t *vd;
          uint64_t vdev = DVA_GET_VDEV(dva);
          uint64_t offset = DVA_GET_OFFSET(dva);
          uint64_t size = DVA_GET_ASIZE(dva);
  
          ASSERT(DVA_IS_VALID(dva));
          ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);
  
          if (txg > spa_freeze_txg(spa))
                  return;
  
          if ((vd = vdev_lookup_top(spa, vdev)) == NULL || !DVA_IS_VALID(dva) ||
              (offset >> vd->vdev_ms_shift) >= vd->vdev_ms_count) {
                  zfs_panic_recover("metaslab_free_dva(): bad DVA %llu:%llu:%llu",
                      (u_longlong_t)vdev, (u_longlong_t)offset,
                      (u_longlong_t)size);
                  return;
          }
  
          ASSERT(!vd->vdev_removing);
          ASSERT(vdev_is_concrete(vd));
          ASSERT0(vd->vdev_indirect_config.vic_mapping_object);
          ASSERT3P(vd->vdev_indirect_mapping, ==, NULL);
  
          if (DVA_GET_GANG(dva))
                  size = vdev_gang_header_asize(vd);
  
          msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];
  
          mutex_enter(&msp->ms_lock);
          range_tree_remove(msp->ms_allocating[txg & TXG_MASK],
              offset, size);
          msp->ms_allocating_total -= size;
  
          VERIFY(!msp->ms_condensing);
          VERIFY3U(offset, >=, msp->ms_start);
          VERIFY3U(offset + size, <=, msp->ms_start + msp->ms_size);
          VERIFY3U(range_tree_space(msp->ms_allocatable) + size, <=,
              msp->ms_size);
          VERIFY0(P2PHASE(offset, 1ULL << vd->vdev_ashift));
          VERIFY0(P2PHASE(size, 1ULL << vd->vdev_ashift));
          range_tree_add(msp->ms_allocatable, offset, size);
          mutex_exit(&msp->ms_lock);
  }
  
  /*
   * Free the block represented by the given DVA.
   */
  void
  metaslab_free_dva(spa_t *spa, const dva_t *dva, boolean_t checkpoint)
  {
          uint64_t vdev = DVA_GET_VDEV(dva);
          uint64_t offset = DVA_GET_OFFSET(dva);
          uint64_t size = DVA_GET_ASIZE(dva);
          vdev_t *vd = vdev_lookup_top(spa, vdev);
  
          ASSERT(DVA_IS_VALID(dva));
          ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);
  
          if (DVA_GET_GANG(dva)) {
                  size = vdev_gang_header_asize(vd);
          }
  
          metaslab_free_impl(vd, offset, size, checkpoint);
  }
  
  /*
   * Reserve some allocation slots. The reservation system must be called
   * before we call into the allocator. If there aren't any available slots
   * then the I/O will be throttled until an I/O completes and its slots are
   * freed up. The function returns true if it was successful in placing
   * the reservation.
   */
  boolean_t
  metaslab_class_throttle_reserve(metaslab_class_t *mc, int slots, int allocator,
      zio_t *zio, int flags)
  {
          metaslab_class_allocator_t *mca = &mc->mc_allocator[allocator];
          uint64_t max = mca->mca_alloc_max_slots;
  
          ASSERT(mc->mc_alloc_throttle_enabled);
          if (GANG_ALLOCATION(flags) || (flags & METASLAB_MUST_RESERVE) ||
              zfs_refcount_count(&mca->mca_alloc_slots) + slots <= max) {
                  /*
                   * The potential race between _count() and _add() is covered
                   * by the allocator lock in most cases, or irrelevant due to
                   * GANG_ALLOCATION() or METASLAB_MUST_RESERVE set in others.
                   * But even if we assume some other non-existing scenario, the
                   * worst that can happen is few more I/Os get to allocation
                   * earlier, that is not a problem.
                   *
                   * We reserve the slots individually so that we can unreserve
                   * them individually when an I/O completes.
                   */
                  for (int d = 0; d < slots; d++)
                          zfs_refcount_add(&mca->mca_alloc_slots, zio);
                  zio->io_flags |= ZIO_FLAG_IO_ALLOCATING;
                  return (B_TRUE);
          }
          return (B_FALSE);
  }
  
  void
  metaslab_class_throttle_unreserve(metaslab_class_t *mc, int slots,
      int allocator, zio_t *zio)
  {
          metaslab_class_allocator_t *mca = &mc->mc_allocator[allocator];
  
          ASSERT(mc->mc_alloc_throttle_enabled);
          for (int d = 0; d < slots; d++)
                  zfs_refcount_remove(&mca->mca_alloc_slots, zio);
  }
  
  static int
  metaslab_claim_concrete(vdev_t *vd, uint64_t offset, uint64_t size,
      uint64_t txg)
  {
          metaslab_t *msp;
          spa_t *spa = vd->vdev_spa;
          int error = 0;
  
          if (offset >> vd->vdev_ms_shift >= vd->vdev_ms_count)
                  return (SET_ERROR(ENXIO));
  
          ASSERT3P(vd->vdev_ms, !=, NULL);
          msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];
  
          mutex_enter(&msp->ms_lock);
  
          if ((txg != 0 && spa_writeable(spa)) || !msp->ms_loaded) {
                  error = metaslab_activate(msp, 0, METASLAB_WEIGHT_CLAIM);
                  if (error == EBUSY) {
                          ASSERT(msp->ms_loaded);
                          ASSERT(msp->ms_weight & METASLAB_ACTIVE_MASK);
                          error = 0;
                  }
          }
  
          if (error == 0 &&
              !range_tree_contains(msp->ms_allocatable, offset, size))
                  error = SET_ERROR(ENOENT);
  
          if (error || txg == 0) {        /* txg == 0 indicates dry run */
                  mutex_exit(&msp->ms_lock);
                  return (error);
          }
  
          VERIFY(!msp->ms_condensing);
          VERIFY0(P2PHASE(offset, 1ULL << vd->vdev_ashift));
          VERIFY0(P2PHASE(size, 1ULL << vd->vdev_ashift));
          VERIFY3U(range_tree_space(msp->ms_allocatable) - size, <=,
              msp->ms_size);
          range_tree_remove(msp->ms_allocatable, offset, size);
          range_tree_clear(msp->ms_trim, offset, size);
  
          if (spa_writeable(spa)) {       /* don't dirty if we're zdb(8) */
                  metaslab_class_t *mc = msp->ms_group->mg_class;
                  multilist_sublist_t *mls =
                      multilist_sublist_lock_obj(&mc->mc_metaslab_txg_list, msp);
                  if (!multilist_link_active(&msp->ms_class_txg_node)) {
                          msp->ms_selected_txg = txg;
                          multilist_sublist_insert_head(mls, msp);
                  }
                  multilist_sublist_unlock(mls);
  
                  if (range_tree_is_empty(msp->ms_allocating[txg & TXG_MASK]))
                          vdev_dirty(vd, VDD_METASLAB, msp, txg);
                  range_tree_add(msp->ms_allocating[txg & TXG_MASK],
                      offset, size);
                  msp->ms_allocating_total += size;
          }
  
          mutex_exit(&msp->ms_lock);
  
          return (0);
  }
  
  typedef struct metaslab_claim_cb_arg_t {
          uint64_t        mcca_txg;
          int             mcca_error;
  } metaslab_claim_cb_arg_t;
  
  static void
  metaslab_claim_impl_cb(uint64_t inner_offset, vdev_t *vd, uint64_t offset,
      uint64_t size, void *arg)
  {
          (void) inner_offset;
          metaslab_claim_cb_arg_t *mcca_arg = arg;
  
          if (mcca_arg->mcca_error == 0) {
                  mcca_arg->mcca_error = metaslab_claim_concrete(vd, offset,
                      size, mcca_arg->mcca_txg);
          }
  }
  
  int
  metaslab_claim_impl(vdev_t *vd, uint64_t offset, uint64_t size, uint64_t txg)
  {
          if (vd->vdev_ops->vdev_op_remap != NULL) {
                  metaslab_claim_cb_arg_t arg;
  
                  /*
                   * Only zdb(8) can claim on indirect vdevs.  This is used
                   * to detect leaks of mapped space (that are not accounted
                   * for in the obsolete counts, spacemap, or bpobj).
                   */
                  ASSERT(!spa_writeable(vd->vdev_spa));
                  arg.mcca_error = 0;
                  arg.mcca_txg = txg;
  
                  vd->vdev_ops->vdev_op_remap(vd, offset, size,
                      metaslab_claim_impl_cb, &arg);
  
                  if (arg.mcca_error == 0) {
                          arg.mcca_error = metaslab_claim_concrete(vd,
                              offset, size, txg);
                  }
                  return (arg.mcca_error);
          } else {
                  return (metaslab_claim_concrete(vd, offset, size, txg));
          }
  }
  
  /*
   * Intent log support: upon opening the pool after a crash, notify the SPA
   * of blocks that the intent log has allocated for immediate write, but
   * which are still considered free by the SPA because the last transaction
   * group didn't commit yet.
   */
  static int
  metaslab_claim_dva(spa_t *spa, const dva_t *dva, uint64_t txg)
  {
          uint64_t vdev = DVA_GET_VDEV(dva);
          uint64_t offset = DVA_GET_OFFSET(dva);
          uint64_t size = DVA_GET_ASIZE(dva);
          vdev_t *vd;
  
          if ((vd = vdev_lookup_top(spa, vdev)) == NULL) {
                  return (SET_ERROR(ENXIO));
          }
  
          ASSERT(DVA_IS_VALID(dva));
  
          if (DVA_GET_GANG(dva))
                  size = vdev_gang_header_asize(vd);
  
          return (metaslab_claim_impl(vd, offset, size, txg));
  }
  
  int
  metaslab_alloc(spa_t *spa, metaslab_class_t *mc, uint64_t psize, blkptr_t *bp,
      int ndvas, uint64_t txg, blkptr_t *hintbp, int flags,
      zio_alloc_list_t *zal, zio_t *zio, int allocator)
  {
          dva_t *dva = bp->blk_dva;
          dva_t *hintdva = (hintbp != NULL) ? hintbp->blk_dva : NULL;
          int error = 0;
  
          ASSERT(bp->blk_birth == 0);
          ASSERT(BP_PHYSICAL_BIRTH(bp) == 0);
  
          spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);
  
          if (mc->mc_allocator[allocator].mca_rotor == NULL) {
                  /* no vdevs in this class */
                  spa_config_exit(spa, SCL_ALLOC, FTAG);
                  return (SET_ERROR(ENOSPC));
          }
  
          ASSERT(ndvas > 0 && ndvas <= spa_max_replication(spa));
          ASSERT(BP_GET_NDVAS(bp) == 0);
          ASSERT(hintbp == NULL || ndvas <= BP_GET_NDVAS(hintbp));
          ASSERT3P(zal, !=, NULL);
  
          for (int d = 0; d < ndvas; d++) {
                  error = metaslab_alloc_dva(spa, mc, psize, dva, d, hintdva,
                      txg, flags, zal, allocator);
                  if (error != 0) {
                          for (d--; d >= 0; d--) {
                                  metaslab_unalloc_dva(spa, &dva[d], txg);
                                  metaslab_group_alloc_decrement(spa,
                                      DVA_GET_VDEV(&dva[d]), zio, flags,
                                      allocator, B_FALSE);
                                  memset(&dva[d], 0, sizeof (dva_t));
                          }
                          spa_config_exit(spa, SCL_ALLOC, FTAG);
                          return (error);
                  } else {
                          /*
                           * Update the metaslab group's queue depth
                           * based on the newly allocated dva.
                           */
                          metaslab_group_alloc_increment(spa,
                              DVA_GET_VDEV(&dva[d]), zio, flags, allocator);
                  }
          }
          ASSERT(error == 0);
          ASSERT(BP_GET_NDVAS(bp) == ndvas);
  
          spa_config_exit(spa, SCL_ALLOC, FTAG);
  
          BP_SET_BIRTH(bp, txg, 0);
  
          return (0);
  }
  
  void
  metaslab_free(spa_t *spa, const blkptr_t *bp, uint64_t txg, boolean_t now)
  {
          const dva_t *dva = bp->blk_dva;
          int ndvas = BP_GET_NDVAS(bp);
  
          ASSERT(!BP_IS_HOLE(bp));
          ASSERT(!now || bp->blk_birth >= spa_syncing_txg(spa));
  
          /*
           * If we have a checkpoint for the pool we need to make sure that
           * the blocks that we free that are part of the checkpoint won't be
           * reused until the checkpoint is discarded or we revert to it.
           *
           * The checkpoint flag is passed down the metaslab_free code path
           * and is set whenever we want to add a block to the checkpoint's
           * accounting. That is, we "checkpoint" blocks that existed at the
           * time the checkpoint was created and are therefore referenced by
           * the checkpointed uberblock.
           *
           * Note that, we don't checkpoint any blocks if the current
           * syncing txg <= spa_checkpoint_txg. We want these frees to sync
           * normally as they will be referenced by the checkpointed uberblock.
           */
          boolean_t checkpoint = B_FALSE;
          if (bp->blk_birth <= spa->spa_checkpoint_txg &&
              spa_syncing_txg(spa) > spa->spa_checkpoint_txg) {
                  /*
                   * At this point, if the block is part of the checkpoint
                   * there is no way it was created in the current txg.
                   */
                  ASSERT(!now);
                  ASSERT3U(spa_syncing_txg(spa), ==, txg);
                  checkpoint = B_TRUE;
          }
  
          spa_config_enter(spa, SCL_FREE, FTAG, RW_READER);
  
          for (int d = 0; d < ndvas; d++) {
                  if (now) {
                          metaslab_unalloc_dva(spa, &dva[d], txg);
                  } else {
                          ASSERT3U(txg, ==, spa_syncing_txg(spa));
                          metaslab_free_dva(spa, &dva[d], checkpoint);
                  }
          }
  
          spa_config_exit(spa, SCL_FREE, FTAG);
  }
  
  int
  metaslab_claim(spa_t *spa, const blkptr_t *bp, uint64_t txg)
  {
          const dva_t *dva = bp->blk_dva;
          int ndvas = BP_GET_NDVAS(bp);
          int error = 0;
  
          ASSERT(!BP_IS_HOLE(bp));
  
          if (txg != 0) {
                  /*
                   * First do a dry run to make sure all DVAs are claimable,
                   * so we don't have to unwind from partial failures below.
                   */
                  if ((error = metaslab_claim(spa, bp, 0)) != 0)
                          return (error);
          }
  
          spa_config_enter(spa, SCL_ALLOC, FTAG, RW_READER);
  
          for (int d = 0; d < ndvas; d++) {
                  error = metaslab_claim_dva(spa, &dva[d], txg);
                  if (error != 0)
                          break;
          }
  
          spa_config_exit(spa, SCL_ALLOC, FTAG);
  
          ASSERT(error == 0 || txg == 0);
  
          return (error);
  }
  
  void
  metaslab_fastwrite_mark(spa_t *spa, const blkptr_t *bp)
  {
          const dva_t *dva = bp->blk_dva;
          int ndvas = BP_GET_NDVAS(bp);
          uint64_t psize = BP_GET_PSIZE(bp);
          int d;
          vdev_t *vd;
  
          ASSERT(!BP_IS_HOLE(bp));
          ASSERT(!BP_IS_EMBEDDED(bp));
          ASSERT(psize > 0);
  
          spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
  
          for (d = 0; d < ndvas; d++) {
                  if ((vd = vdev_lookup_top(spa, DVA_GET_VDEV(&dva[d]))) == NULL)
                          continue;
                  atomic_add_64(&vd->vdev_pending_fastwrite, psize);
          }
  
          spa_config_exit(spa, SCL_VDEV, FTAG);
  }
  
  void
  metaslab_fastwrite_unmark(spa_t *spa, const blkptr_t *bp)
  {
          const dva_t *dva = bp->blk_dva;
          int ndvas = BP_GET_NDVAS(bp);
          uint64_t psize = BP_GET_PSIZE(bp);
          int d;
          vdev_t *vd;
  
          ASSERT(!BP_IS_HOLE(bp));
          ASSERT(!BP_IS_EMBEDDED(bp));
          ASSERT(psize > 0);
  
          spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
  
          for (d = 0; d < ndvas; d++) {
                  if ((vd = vdev_lookup_top(spa, DVA_GET_VDEV(&dva[d]))) == NULL)
                          continue;
                  ASSERT3U(vd->vdev_pending_fastwrite, >=, psize);
                  atomic_sub_64(&vd->vdev_pending_fastwrite, psize);
          }
  
          spa_config_exit(spa, SCL_VDEV, FTAG);
  }
  
  static void
  metaslab_check_free_impl_cb(uint64_t inner, vdev_t *vd, uint64_t offset,
      uint64_t size, void *arg)
  {
          (void) inner, (void) arg;
  
          if (vd->vdev_ops == &vdev_indirect_ops)
                  return;
  
          metaslab_check_free_impl(vd, offset, size);
  }
  
  static void
  metaslab_check_free_impl(vdev_t *vd, uint64_t offset, uint64_t size)
  {
          metaslab_t *msp;
          spa_t *spa __maybe_unused = vd->vdev_spa;
  
          if ((zfs_flags & ZFS_DEBUG_ZIO_FREE) == 0)
                  return;
  
          if (vd->vdev_ops->vdev_op_remap != NULL) {
                  vd->vdev_ops->vdev_op_remap(vd, offset, size,
                      metaslab_check_free_impl_cb, NULL);
                  return;
          }
  
          ASSERT(vdev_is_concrete(vd));
          ASSERT3U(offset >> vd->vdev_ms_shift, <, vd->vdev_ms_count);
          ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);
  
          msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];
  
          mutex_enter(&msp->ms_lock);
          if (msp->ms_loaded) {
                  range_tree_verify_not_present(msp->ms_allocatable,
                      offset, size);
          }
  
          /*
           * Check all segments that currently exist in the freeing pipeline.
           *
           * It would intuitively make sense to also check the current allocating
           * tree since metaslab_unalloc_dva() exists for extents that are
           * allocated and freed in the same sync pass within the same txg.
           * Unfortunately there are places (e.g. the ZIL) where we allocate a
           * segment but then we free part of it within the same txg
           * [see zil_sync()]. Thus, we don't call range_tree_verify() in the
           * current allocating tree.
           */
          range_tree_verify_not_present(msp->ms_freeing, offset, size);
          range_tree_verify_not_present(msp->ms_checkpointing, offset, size);
          range_tree_verify_not_present(msp->ms_freed, offset, size);
          for (int j = 0; j < TXG_DEFER_SIZE; j++)
                  range_tree_verify_not_present(msp->ms_defer[j], offset, size);
          range_tree_verify_not_present(msp->ms_trim, offset, size);
          mutex_exit(&msp->ms_lock);
  }
  
  void
  metaslab_check_free(spa_t *spa, const blkptr_t *bp)
  {
          if ((zfs_flags & ZFS_DEBUG_ZIO_FREE) == 0)
                  return;
  
          spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
          for (int i = 0; i < BP_GET_NDVAS(bp); i++) {
                  uint64_t vdev = DVA_GET_VDEV(&bp->blk_dva[i]);
                  vdev_t *vd = vdev_lookup_top(spa, vdev);
                  uint64_t offset = DVA_GET_OFFSET(&bp->blk_dva[i]);
                  uint64_t size = DVA_GET_ASIZE(&bp->blk_dva[i]);
  
                  if (DVA_GET_GANG(&bp->blk_dva[i]))
                          size = vdev_gang_header_asize(vd);
  
                  ASSERT3P(vd, !=, NULL);
  
                  metaslab_check_free_impl(vd, offset, size);
          }
          spa_config_exit(spa, SCL_VDEV, FTAG);
  }
  
  static void
  metaslab_group_disable_wait(metaslab_group_t *mg)
  {
          ASSERT(MUTEX_HELD(&mg->mg_ms_disabled_lock));
          while (mg->mg_disabled_updating) {
                  cv_wait(&mg->mg_ms_disabled_cv, &mg->mg_ms_disabled_lock);
          }
  }
  
  static void
  metaslab_group_disabled_increment(metaslab_group_t *mg)
  {
          ASSERT(MUTEX_HELD(&mg->mg_ms_disabled_lock));
          ASSERT(mg->mg_disabled_updating);
  
          while (mg->mg_ms_disabled >= max_disabled_ms) {
                  cv_wait(&mg->mg_ms_disabled_cv, &mg->mg_ms_disabled_lock);
          }
          mg->mg_ms_disabled++;
          ASSERT3U(mg->mg_ms_disabled, <=, max_disabled_ms);
  }
  
  /*
   * Mark the metaslab as disabled to prevent any allocations on this metaslab.
   * We must also track how many metaslabs are currently disabled within a
   * metaslab group and limit them to prevent allocation failures from
   * occurring because all metaslabs are disabled.
   */
  void
  metaslab_disable(metaslab_t *msp)
  {
          ASSERT(!MUTEX_HELD(&msp->ms_lock));
          metaslab_group_t *mg = msp->ms_group;
  
          mutex_enter(&mg->mg_ms_disabled_lock);
  
          /*
           * To keep an accurate count of how many threads have disabled
           * a specific metaslab group, we only allow one thread to mark
           * the metaslab group at a time. This ensures that the value of
           * ms_disabled will be accurate when we decide to mark a metaslab
           * group as disabled. To do this we force all other threads
           * to wait till the metaslab's mg_disabled_updating flag is no
           * longer set.
           */
          metaslab_group_disable_wait(mg);
          mg->mg_disabled_updating = B_TRUE;
          if (msp->ms_disabled == 0) {
                  metaslab_group_disabled_increment(mg);
          }
          mutex_enter(&msp->ms_lock);
          msp->ms_disabled++;
          mutex_exit(&msp->ms_lock);
  
          mg->mg_disabled_updating = B_FALSE;
          cv_broadcast(&mg->mg_ms_disabled_cv);
          mutex_exit(&mg->mg_ms_disabled_lock);
  }
  
  void
  metaslab_enable(metaslab_t *msp, boolean_t sync, boolean_t unload)
  {
          metaslab_group_t *mg = msp->ms_group;
          spa_t *spa = mg->mg_vd->vdev_spa;
  
          /*
           * Wait for the outstanding IO to be synced to prevent newly
           * allocated blocks from being overwritten.  This used by
           * initialize and TRIM which are modifying unallocated space.
           */
          if (sync)
                  txg_wait_synced(spa_get_dsl(spa), 0);
  
          mutex_enter(&mg->mg_ms_disabled_lock);
          mutex_enter(&msp->ms_lock);
          if (--msp->ms_disabled == 0) {
                  mg->mg_ms_disabled--;
                  cv_broadcast(&mg->mg_ms_disabled_cv);
                  if (unload)
                          metaslab_unload(msp);
          }
          mutex_exit(&msp->ms_lock);
          mutex_exit(&mg->mg_ms_disabled_lock);
  }
  
  void
  metaslab_set_unflushed_dirty(metaslab_t *ms, boolean_t dirty)
  {
          ms->ms_unflushed_dirty = dirty;
  }
  
  static void
  metaslab_update_ondisk_flush_data(metaslab_t *ms, dmu_tx_t *tx)
  {
          vdev_t *vd = ms->ms_group->mg_vd;
          spa_t *spa = vd->vdev_spa;
          objset_t *mos = spa_meta_objset(spa);
  
          ASSERT(spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP));
  
          metaslab_unflushed_phys_t entry = {
                  .msp_unflushed_txg = metaslab_unflushed_txg(ms),
          };
          uint64_t entry_size = sizeof (entry);
          uint64_t entry_offset = ms->ms_id * entry_size;
  
          uint64_t object = 0;
          int err = zap_lookup(mos, vd->vdev_top_zap,
              VDEV_TOP_ZAP_MS_UNFLUSHED_PHYS_TXGS, sizeof (uint64_t), 1,
              &object);
          if (err == ENOENT) {
                  object = dmu_object_alloc(mos, DMU_OTN_UINT64_METADATA,
                      SPA_OLD_MAXBLOCKSIZE, DMU_OT_NONE, 0, tx);
                  VERIFY0(zap_add(mos, vd->vdev_top_zap,
                      VDEV_TOP_ZAP_MS_UNFLUSHED_PHYS_TXGS, sizeof (uint64_t), 1,
                      &object, tx));
          } else {
                  VERIFY0(err);
          }
  
          dmu_write(spa_meta_objset(spa), object, entry_offset, entry_size,
              &entry, tx);
  }
  
  void
  metaslab_set_unflushed_txg(metaslab_t *ms, uint64_t txg, dmu_tx_t *tx)
  {
          ms->ms_unflushed_txg = txg;
          metaslab_update_ondisk_flush_data(ms, tx);
  }
  
  boolean_t
  metaslab_unflushed_dirty(metaslab_t *ms)
  {
          return (ms->ms_unflushed_dirty);
  }
  
  uint64_t
  metaslab_unflushed_txg(metaslab_t *ms)
  {
          return (ms->ms_unflushed_txg);
  }
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, aliquot, U64, ZMOD_RW,
          "Allocation granularity (a.k.a. stripe size)");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, debug_load, INT, ZMOD_RW,
          "Load all metaslabs when pool is first opened");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, debug_unload, INT, ZMOD_RW,
          "Prevent metaslabs from being unloaded");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, preload_enabled, INT, ZMOD_RW,
          "Preload potential metaslabs during reassessment");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, unload_delay, UINT, ZMOD_RW,
          "Delay in txgs after metaslab was last used before unloading");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, unload_delay_ms, UINT, ZMOD_RW,
          "Delay in milliseconds after metaslab was last used before unloading");
  
  /* BEGIN CSTYLED */
  ZFS_MODULE_PARAM(zfs_mg, zfs_mg_, noalloc_threshold, UINT, ZMOD_RW,
          "Percentage of metaslab group size that should be free to make it "
          "eligible for allocation");
  
  ZFS_MODULE_PARAM(zfs_mg, zfs_mg_, fragmentation_threshold, UINT, ZMOD_RW,
          "Percentage of metaslab group size that should be considered eligible "
          "for allocations unless all metaslab groups within the metaslab class "
          "have also crossed this threshold");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, fragmentation_factor_enabled, INT,
          ZMOD_RW,
          "Use the fragmentation metric to prefer less fragmented metaslabs");
  /* END CSTYLED */
  
  ZFS_MODULE_PARAM(zfs_metaslab, zfs_metaslab_, fragmentation_threshold, UINT,
          ZMOD_RW, "Fragmentation for metaslab to allow allocation");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, lba_weighting_enabled, INT, ZMOD_RW,
          "Prefer metaslabs with lower LBAs");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, bias_enabled, INT, ZMOD_RW,
          "Enable metaslab group biasing");
  
  ZFS_MODULE_PARAM(zfs_metaslab, zfs_metaslab_, segment_weight_enabled, INT,
          ZMOD_RW, "Enable segment-based metaslab selection");
  
  ZFS_MODULE_PARAM(zfs_metaslab, zfs_metaslab_, switch_threshold, INT, ZMOD_RW,
          "Segment-based metaslab selection maximum buckets before switching");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, force_ganging, U64, ZMOD_RW,
          "Blocks larger than this size are forced to be gang blocks");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, df_max_search, UINT, ZMOD_RW,
          "Max distance (bytes) to search forward before using size tree");
  
  ZFS_MODULE_PARAM(zfs_metaslab, metaslab_, df_use_largest_segment, INT, ZMOD_RW,
          "When looking in size tree, use largest segment instead of exact fit");
  
  ZFS_MODULE_PARAM(zfs_metaslab, zfs_metaslab_, max_size_cache_sec, U64,
          ZMOD_RW, "How long to trust the cached max chunk size of a metaslab");
  
  ZFS_MODULE_PARAM(zfs_metaslab, zfs_metaslab_, mem_limit, UINT, ZMOD_RW,
          "Percentage of memory that can be used to store metaslab range trees");
  
  ZFS_MODULE_PARAM(zfs_metaslab, zfs_metaslab_, try_hard_before_gang, INT,
          ZMOD_RW, "Try hard to allocate before ganging");
  
  ZFS_MODULE_PARAM(zfs_metaslab, zfs_metaslab_, find_max_tries, UINT, ZMOD_RW,
          "Normally only consider this many of the best metaslabs in each vdev");