FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/module/zfs/spa_log_spacemap.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]
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     * CDDL HEADER END
     */
    
    /*
     * Copyright (c) 2018, 2019 by Delphix. All rights reserved.
     */
    
    #include <sys/dmu_objset.h>
    #include <sys/metaslab.h>
    #include <sys/metaslab_impl.h>
    #include <sys/spa.h>
    #include <sys/spa_impl.h>
    #include <sys/spa_log_spacemap.h>
    #include <sys/vdev_impl.h>
    #include <sys/zap.h>
    
    /*
     * Log Space Maps
     *
     * Log space maps are an optimization in ZFS metadata allocations for pools
     * whose workloads are primarily random-writes. Random-write workloads are also
     * typically random-free, meaning that they are freeing from locations scattered
     * throughout the pool. This means that each TXG we will have to append some
     * FREE records to almost every metaslab. With log space maps, we hold their
     * changes in memory and log them altogether in one pool-wide space map on-disk
     * for persistence. As more blocks are accumulated in the log space maps and
     * more unflushed changes are accounted in memory, we flush a selected group
     * of metaslabs every TXG to relieve memory pressure and potential overheads
     * when loading the pool. Flushing a metaslab to disk relieves memory as we
     * flush any unflushed changes from memory to disk (i.e. the metaslab's space
     * map) and saves import time by making old log space maps obsolete and
     * eventually destroying them. [A log space map is said to be obsolete when all
     * its entries have made it to their corresponding metaslab space maps].
     *
     * == On disk data structures used ==
     *
     * - The pool has a new feature flag and a new entry in the MOS. The feature
     *   is activated when we create the first log space map and remains active
     *   for the lifetime of the pool. The new entry in the MOS Directory [refer
     *   to DMU_POOL_LOG_SPACEMAP_ZAP] is populated with a ZAP whose key-value
     *   pairs are of the form <key: txg, value: log space map object for that txg>.
     *   This entry is our on-disk reference of the log space maps that exist in
     *   the pool for each TXG and it is used during import to load all the
     *   metaslab unflushed changes in memory. To see how this structure is first
     *   created and later populated refer to spa_generate_syncing_log_sm(). To see
     *   how it is used during import time refer to spa_ld_log_sm_metadata().
     *
     * - Each vdev has a new entry in its vdev_top_zap (see field
     *   VDEV_TOP_ZAP_MS_UNFLUSHED_PHYS_TXGS) which holds the msp_unflushed_txg of
     *   each metaslab in this vdev. This field is the on-disk counterpart of the
     *   in-memory field ms_unflushed_txg which tells us from which TXG and onwards
     *   the metaslab haven't had its changes flushed. During import, we use this
     *   to ignore any entries in the space map log that are for this metaslab but
     *   from a TXG before msp_unflushed_txg. At that point, we also populate its
     *   in-memory counterpart and from there both fields are updated every time
     *   we flush that metaslab.
     *
     * - A space map is created every TXG and, during that TXG, it is used to log
     *   all incoming changes (the log space map). When created, the log space map
     *   is referenced in memory by spa_syncing_log_sm and its object ID is inserted
     *   to the space map ZAP mentioned above. The log space map is closed at the
     *   end of the TXG and will be destroyed when it becomes fully obsolete. We
     *   know when a log space map has become obsolete by looking at the oldest
     *   (and smallest) ms_unflushed_txg in the pool. If the value of that is bigger
     *   than the log space map's TXG, then it means that there is no metaslab who
     *   doesn't have the changes from that log and we can therefore destroy it.
     *   [see spa_cleanup_old_sm_logs()].
     *
     * == Important in-memory structures ==
     *
     * - The per-spa field spa_metaslabs_by_flushed sorts all the metaslabs in
     *   the pool by their ms_unflushed_txg field. It is primarily used for three
     *   reasons. First of all, it is used during flushing where we try to flush
     *   metaslabs in-order from the oldest-flushed to the most recently flushed
     *   every TXG. Secondly, it helps us to lookup the ms_unflushed_txg of the
     *   oldest flushed metaslab to distinguish which log space maps have become
     *   obsolete and which ones are still relevant. Finally it tells us which
     *   metaslabs have unflushed changes in a pool where this feature was just
     *   enabled, as we don't immediately add all of the pool's metaslabs but we
     *   add them over time as they go through metaslab_sync(). The reason that
     *   we do that is to ease these pools into the behavior of the flushing
    *   algorithm (described later on).
    *
    * - The per-spa field spa_sm_logs_by_txg can be thought as the in-memory
    *   counterpart of the space map ZAP mentioned above. It's an AVL tree whose
    *   nodes represent the log space maps in the pool. This in-memory
    *   representation of log space maps in the pool sorts the log space maps by
    *   the TXG that they were created (which is also the TXG of their unflushed
    *   changes). It also contains the following extra information for each
    *   space map:
    *   [1] The number of metaslabs that were last flushed on that TXG. This is
    *       important because if that counter is zero and this is the oldest
    *       log then it means that it is also obsolete.
    *   [2] The number of blocks of that space map. This field is used by the
    *       block heuristic of our flushing algorithm (described later on).
    *       It represents how many blocks of metadata changes ZFS had to write
    *       to disk for that TXG.
    *
    * - The per-spa field spa_log_summary is a list of entries that summarizes
    *   the metaslab and block counts of all the nodes of the spa_sm_logs_by_txg
    *   AVL tree mentioned above. The reason this exists is that our flushing
    *   algorithm (described later) tries to estimate how many metaslabs to flush
    *   in each TXG by iterating over all the log space maps and looking at their
    *   block counts. Summarizing that information means that don't have to
    *   iterate through each space map, minimizing the runtime overhead of the
    *   flushing algorithm which would be induced in syncing context. In terms of
    *   implementation the log summary is used as a queue:
    *   * we modify or pop entries from its head when we flush metaslabs
    *   * we modify or append entries to its tail when we sync changes.
    *
    * - Each metaslab has two new range trees that hold its unflushed changes,
    *   ms_unflushed_allocs and ms_unflushed_frees. These are always disjoint.
    *
    * == Flushing algorithm ==
    *
    * The decision of how many metaslabs to flush on a give TXG is guided by
    * two heuristics:
    *
    * [1] The memory heuristic -
    * We keep track of the memory used by the unflushed trees from all the
    * metaslabs [see sus_memused of spa_unflushed_stats] and we ensure that it
    * stays below a certain threshold which is determined by an arbitrary hard
    * limit and an arbitrary percentage of the system's memory [see
    * spa_log_exceeds_memlimit()]. When we see that the memory usage of the
    * unflushed changes are passing that threshold, we flush metaslabs, which
    * empties their unflushed range trees, reducing the memory used.
    *
    * [2] The block heuristic -
    * We try to keep the total number of blocks in the log space maps in check
    * so the log doesn't grow indefinitely and we don't induce a lot of overhead
    * when loading the pool. At the same time we don't want to flush a lot of
    * metaslabs too often as this would defeat the purpose of the log space map.
    * As a result we set a limit in the amount of blocks that we think it's
    * acceptable for the log space maps to have and try not to cross it.
    * [see sus_blocklimit from spa_unflushed_stats].
    *
    * In order to stay below the block limit every TXG we have to estimate how
    * many metaslabs we need to flush based on the current rate of incoming blocks
    * and our history of log space map blocks. The main idea here is to answer
    * the question of how many metaslabs do we need to flush in order to get rid
    * at least an X amount of log space map blocks. We can answer this question
    * by iterating backwards from the oldest log space map to the newest one
    * and looking at their metaslab and block counts. At this point the log summary
    * mentioned above comes handy as it reduces the amount of things that we have
    * to iterate (even though it may reduce the preciseness of our estimates due
    * to its aggregation of data). So with that in mind, we project the incoming
    * rate of the current TXG into the future and attempt to approximate how many
    * metaslabs would we need to flush from now in order to avoid exceeding our
    * block limit in different points in the future (granted that we would keep
    * flushing the same number of metaslabs for every TXG). Then we take the
    * maximum number from all these estimates to be on the safe side. For the
    * exact implementation details of algorithm refer to
    * spa_estimate_metaslabs_to_flush.
    */
   
   /*
    * This is used as the block size for the space maps used for the
    * log space map feature. These space maps benefit from a bigger
    * block size as we expect to be writing a lot of data to them at
    * once.
    */
   static const unsigned long zfs_log_sm_blksz = 1ULL << 17;
   
   /*
    * Percentage of the overall system's memory that ZFS allows to be
    * used for unflushed changes (e.g. the sum of size of all the nodes
    * in the unflushed trees).
    *
    * Note that this value is calculated over 1000000 for finer granularity
    * (thus the _ppm suffix; reads as "parts per million"). As an example,
    * the default of 1000 allows 0.1% of memory to be used.
    */
   static uint64_t zfs_unflushed_max_mem_ppm = 1000;
   
   /*
    * Specific hard-limit in memory that ZFS allows to be used for
    * unflushed changes.
    */
   static uint64_t zfs_unflushed_max_mem_amt = 1ULL << 30;
   
   /*
    * The following tunable determines the number of blocks that can be used for
    * the log space maps. It is expressed as a percentage of the total number of
    * metaslabs in the pool (i.e. the default of 400 means that the number of log
    * blocks is capped at 4 times the number of metaslabs).
    *
    * This value exists to tune our flushing algorithm, with higher values
    * flushing metaslabs less often (doing less I/Os) per TXG versus lower values
    * flushing metaslabs more aggressively with the upside of saving overheads
    * when loading the pool. Another factor in this tradeoff is that flushing
    * less often can potentially lead to better utilization of the metaslab space
    * map's block size as we accumulate more changes per flush.
    *
    * Given that this tunable indirectly controls the flush rate (metaslabs
    * flushed per txg) and that's why making it a percentage in terms of the
    * number of metaslabs in the pool makes sense here.
    *
    * As a rule of thumb we default this tunable to 400% based on the following:
    *
    * 1] Assuming a constant flush rate and a constant incoming rate of log blocks
    *    it is reasonable to expect that the amount of obsolete entries changes
    *    linearly from txg to txg (e.g. the oldest log should have the most
    *    obsolete entries, and the most recent one the least). With this we could
    *    say that, at any given time, about half of the entries in the whole space
    *    map log are obsolete. Thus for every two entries for a metaslab in the
    *    log space map, only one of them is valid and actually makes it to the
    *    metaslab's space map.
    *    [factor of 2]
    * 2] Each entry in the log space map is guaranteed to be two words while
    *    entries in metaslab space maps are generally single-word.
    *    [an extra factor of 2 - 400% overall]
    * 3] Even if [1] and [2] are slightly less than 2 each, we haven't taken into
    *    account any consolidation of segments from the log space map to the
    *    unflushed range trees nor their history (e.g. a segment being allocated,
    *    then freed, then allocated again means 3 log space map entries but 0
    *    metaslab space map entries). Depending on the workload, we've seen ~1.8
    *    non-obsolete log space map entries per metaslab entry, for a total of
    *    ~600%. Since most of these estimates though are workload dependent, we
    *    default on 400% to be conservative.
    *
    *    Thus we could say that even in the worst
    *    case of [1] and [2], the factor should end up being 4.
    *
    * That said, regardless of the number of metaslabs in the pool we need to
    * provide upper and lower bounds for the log block limit.
    * [see zfs_unflushed_log_block_{min,max}]
    */
   static uint_t zfs_unflushed_log_block_pct = 400;
   
   /*
    * If the number of metaslabs is small and our incoming rate is high, we could
    * get into a situation that we are flushing all our metaslabs every TXG. Thus
    * we always allow at least this many log blocks.
    */
   static uint64_t zfs_unflushed_log_block_min = 1000;
   
   /*
    * If the log becomes too big, the import time of the pool can take a hit in
    * terms of performance. Thus we have a hard limit in the size of the log in
    * terms of blocks.
    */
   static uint64_t zfs_unflushed_log_block_max = (1ULL << 17);
   
   /*
    * Also we have a hard limit in the size of the log in terms of dirty TXGs.
    */
   static uint64_t zfs_unflushed_log_txg_max = 1000;
   
   /*
    * Max # of rows allowed for the log_summary. The tradeoff here is accuracy and
    * stability of the flushing algorithm (longer summary) vs its runtime overhead
    * (smaller summary is faster to traverse).
    */
   static uint64_t zfs_max_logsm_summary_length = 10;
   
   /*
    * Tunable that sets the lower bound on the metaslabs to flush every TXG.
    *
    * Setting this to 0 has no effect since if the pool is idle we won't even be
    * creating log space maps and therefore we won't be flushing. On the other
    * hand if the pool has any incoming workload our block heuristic will start
    * flushing metaslabs anyway.
    *
    * The point of this tunable is to be used in extreme cases where we really
    * want to flush more metaslabs than our adaptable heuristic plans to flush.
    */
   static uint64_t zfs_min_metaslabs_to_flush = 1;
   
   /*
    * Tunable that specifies how far in the past do we want to look when trying to
    * estimate the incoming log blocks for the current TXG.
    *
    * Setting this too high may not only increase runtime but also minimize the
    * effect of the incoming rates from the most recent TXGs as we take the
    * average over all the blocks that we walk
    * [see spa_estimate_incoming_log_blocks].
    */
   static uint64_t zfs_max_log_walking = 5;
   
   /*
    * This tunable exists solely for testing purposes. It ensures that the log
    * spacemaps are not flushed and destroyed during export in order for the
    * relevant log spacemap import code paths to be tested (effectively simulating
    * a crash).
    */
   int zfs_keep_log_spacemaps_at_export = 0;
   
   static uint64_t
   spa_estimate_incoming_log_blocks(spa_t *spa)
   {
           ASSERT3U(spa_sync_pass(spa), ==, 1);
           uint64_t steps = 0, sum = 0;
           for (spa_log_sm_t *sls = avl_last(&spa->spa_sm_logs_by_txg);
               sls != NULL && steps < zfs_max_log_walking;
               sls = AVL_PREV(&spa->spa_sm_logs_by_txg, sls)) {
                   if (sls->sls_txg == spa_syncing_txg(spa)) {
                           /*
                            * skip the log created in this TXG as this would
                            * make our estimations inaccurate.
                            */
                           continue;
                   }
                   sum += sls->sls_nblocks;
                   steps++;
           }
           return ((steps > 0) ? DIV_ROUND_UP(sum, steps) : 0);
   }
   
   uint64_t
   spa_log_sm_blocklimit(spa_t *spa)
   {
           return (spa->spa_unflushed_stats.sus_blocklimit);
   }
   
   void
   spa_log_sm_set_blocklimit(spa_t *spa)
   {
           if (!spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP)) {
                   ASSERT0(spa_log_sm_blocklimit(spa));
                   return;
           }
   
           uint64_t msdcount = 0;
           for (log_summary_entry_t *e = list_head(&spa->spa_log_summary);
               e; e = list_next(&spa->spa_log_summary, e))
                   msdcount += e->lse_msdcount;
   
           uint64_t limit = msdcount * zfs_unflushed_log_block_pct / 100;
           spa->spa_unflushed_stats.sus_blocklimit = MIN(MAX(limit,
               zfs_unflushed_log_block_min), zfs_unflushed_log_block_max);
   }
   
   uint64_t
   spa_log_sm_nblocks(spa_t *spa)
   {
           return (spa->spa_unflushed_stats.sus_nblocks);
   }
   
   /*
    * Ensure that the in-memory log space map structures and the summary
    * have the same block and metaslab counts.
    */
   static void
   spa_log_summary_verify_counts(spa_t *spa)
   {
           ASSERT(spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP));
   
           if ((zfs_flags & ZFS_DEBUG_LOG_SPACEMAP) == 0)
                   return;
   
           uint64_t ms_in_avl = avl_numnodes(&spa->spa_metaslabs_by_flushed);
   
           uint64_t ms_in_summary = 0, blk_in_summary = 0;
           for (log_summary_entry_t *e = list_head(&spa->spa_log_summary);
               e; e = list_next(&spa->spa_log_summary, e)) {
                   ms_in_summary += e->lse_mscount;
                   blk_in_summary += e->lse_blkcount;
           }
   
           uint64_t ms_in_logs = 0, blk_in_logs = 0;
           for (spa_log_sm_t *sls = avl_first(&spa->spa_sm_logs_by_txg);
               sls; sls = AVL_NEXT(&spa->spa_sm_logs_by_txg, sls)) {
                   ms_in_logs += sls->sls_mscount;
                   blk_in_logs += sls->sls_nblocks;
           }
   
           VERIFY3U(ms_in_logs, ==, ms_in_summary);
           VERIFY3U(ms_in_logs, ==, ms_in_avl);
           VERIFY3U(blk_in_logs, ==, blk_in_summary);
           VERIFY3U(blk_in_logs, ==, spa_log_sm_nblocks(spa));
   }
   
   static boolean_t
   summary_entry_is_full(spa_t *spa, log_summary_entry_t *e, uint64_t txg)
   {
           if (e->lse_end == txg)
                   return (0);
           if (e->lse_txgcount >= DIV_ROUND_UP(zfs_unflushed_log_txg_max,
               zfs_max_logsm_summary_length))
                   return (1);
           uint64_t blocks_per_row = MAX(1,
               DIV_ROUND_UP(spa_log_sm_blocklimit(spa),
               zfs_max_logsm_summary_length));
           return (blocks_per_row <= e->lse_blkcount);
   }
   
   /*
    * Update the log summary information to reflect the fact that a metaslab
    * was flushed or destroyed (e.g due to device removal or pool export/destroy).
    *
    * We typically flush the oldest flushed metaslab so the first (and oldest)
    * entry of the summary is updated. However if that metaslab is getting loaded
    * we may flush the second oldest one which may be part of an entry later in
    * the summary. Moreover, if we call into this function from metaslab_fini()
    * the metaslabs probably won't be ordered by ms_unflushed_txg. Thus we ask
    * for a txg as an argument so we can locate the appropriate summary entry for
    * the metaslab.
    */
   void
   spa_log_summary_decrement_mscount(spa_t *spa, uint64_t txg, boolean_t dirty)
   {
           /*
            * We don't track summary data for read-only pools and this function
            * can be called from metaslab_fini(). In that case return immediately.
            */
           if (!spa_writeable(spa))
                   return;
   
           log_summary_entry_t *target = NULL;
           for (log_summary_entry_t *e = list_head(&spa->spa_log_summary);
               e != NULL; e = list_next(&spa->spa_log_summary, e)) {
                   if (e->lse_start > txg)
                           break;
                   target = e;
           }
   
           if (target == NULL || target->lse_mscount == 0) {
                   /*
                    * We didn't find a summary entry for this metaslab. We must be
                    * at the teardown of a spa_load() attempt that got an error
                    * while reading the log space maps.
                    */
                   VERIFY3S(spa_load_state(spa), ==, SPA_LOAD_ERROR);
                   return;
           }
   
           target->lse_mscount--;
           if (dirty)
                   target->lse_msdcount--;
   }
   
   /*
    * Update the log summary information to reflect the fact that we destroyed
    * old log space maps. Since we can only destroy the oldest log space maps,
    * we decrement the block count of the oldest summary entry and potentially
    * destroy it when that count hits 0.
    *
    * This function is called after a metaslab is flushed and typically that
    * metaslab is the oldest flushed, which means that this function will
    * typically decrement the block count of the first entry of the summary and
    * potentially free it if the block count gets to zero (its metaslab count
    * should be zero too at that point).
    *
    * There are certain scenarios though that don't work exactly like that so we
    * need to account for them:
    *
    * Scenario [1]: It is possible that after we flushed the oldest flushed
    * metaslab and we destroyed the oldest log space map, more recent logs had 0
    * metaslabs pointing to them so we got rid of them too. This can happen due
    * to metaslabs being destroyed through device removal, or because the oldest
    * flushed metaslab was loading but we kept flushing more recently flushed
    * metaslabs due to the memory pressure of unflushed changes. Because of that,
    * we always iterate from the beginning of the summary and if blocks_gone is
    * bigger than the block_count of the current entry we free that entry (we
    * expect its metaslab count to be zero), we decrement blocks_gone and on to
    * the next entry repeating this procedure until blocks_gone gets decremented
    * to 0. Doing this also works for the typical case mentioned above.
    *
    * Scenario [2]: The oldest flushed metaslab isn't necessarily accounted by
    * the first (and oldest) entry in the summary. If the first few entries of
    * the summary were only accounting metaslabs from a device that was just
    * removed, then the current oldest flushed metaslab could be accounted by an
    * entry somewhere in the middle of the summary. Moreover flushing that
    * metaslab will destroy all the log space maps older than its ms_unflushed_txg
    * because they became obsolete after the removal. Thus, iterating as we did
    * for scenario [1] works out for this case too.
    *
    * Scenario [3]: At times we decide to flush all the metaslabs in the pool
    * in one TXG (either because we are exporting the pool or because our flushing
    * heuristics decided to do so). When that happens all the log space maps get
    * destroyed except the one created for the current TXG which doesn't have
    * any log blocks yet. As log space maps get destroyed with every metaslab that
    * we flush, entries in the summary are also destroyed. This brings a weird
    * corner-case when we flush the last metaslab and the log space map of the
    * current TXG is in the same summary entry with other log space maps that
    * are older. When that happens we are eventually left with this one last
    * summary entry whose blocks are gone (blocks_gone equals the entry's block
    * count) but its metaslab count is non-zero (because it accounts all the
    * metaslabs in the pool as they all got flushed). Under this scenario we can't
    * free this last summary entry as it's referencing all the metaslabs in the
    * pool and its block count will get incremented at the end of this sync (when
    * we close the syncing log space map). Thus we just decrement its current
    * block count and leave it alone. In the case that the pool gets exported,
    * its metaslab count will be decremented over time as we call metaslab_fini()
    * for all the metaslabs in the pool and the entry will be freed at
    * spa_unload_log_sm_metadata().
    */
   void
   spa_log_summary_decrement_blkcount(spa_t *spa, uint64_t blocks_gone)
   {
           log_summary_entry_t *e = list_head(&spa->spa_log_summary);
           ASSERT3P(e, !=, NULL);
           if (e->lse_txgcount > 0)
                   e->lse_txgcount--;
           for (; e != NULL; e = list_head(&spa->spa_log_summary)) {
                   if (e->lse_blkcount > blocks_gone) {
                           e->lse_blkcount -= blocks_gone;
                           blocks_gone = 0;
                           break;
                   } else if (e->lse_mscount == 0) {
                           /* remove obsolete entry */
                           blocks_gone -= e->lse_blkcount;
                           list_remove(&spa->spa_log_summary, e);
                           kmem_free(e, sizeof (log_summary_entry_t));
                   } else {
                           /* Verify that this is scenario [3] mentioned above. */
                           VERIFY3U(blocks_gone, ==, e->lse_blkcount);
   
                           /*
                            * Assert that this is scenario [3] further by ensuring
                            * that this is the only entry in the summary.
                            */
                           VERIFY3P(e, ==, list_tail(&spa->spa_log_summary));
                           ASSERT3P(e, ==, list_head(&spa->spa_log_summary));
   
                           blocks_gone = e->lse_blkcount = 0;
                           break;
                   }
           }
   
           /*
            * Ensure that there is no way we are trying to remove more blocks
            * than the # of blocks in the summary.
            */
           ASSERT0(blocks_gone);
   }
   
   void
   spa_log_sm_decrement_mscount(spa_t *spa, uint64_t txg)
   {
           spa_log_sm_t target = { .sls_txg = txg };
           spa_log_sm_t *sls = avl_find(&spa->spa_sm_logs_by_txg,
               &target, NULL);
   
           if (sls == NULL) {
                   /*
                    * We must be at the teardown of a spa_load() attempt that
                    * got an error while reading the log space maps.
                    */
                   VERIFY3S(spa_load_state(spa), ==, SPA_LOAD_ERROR);
                   return;
           }
   
           ASSERT(sls->sls_mscount > 0);
           sls->sls_mscount--;
   }
   
   void
   spa_log_sm_increment_current_mscount(spa_t *spa)
   {
           spa_log_sm_t *last_sls = avl_last(&spa->spa_sm_logs_by_txg);
           ASSERT3U(last_sls->sls_txg, ==, spa_syncing_txg(spa));
           last_sls->sls_mscount++;
   }
   
   static void
   summary_add_data(spa_t *spa, uint64_t txg, uint64_t metaslabs_flushed,
       uint64_t metaslabs_dirty, uint64_t nblocks)
   {
           log_summary_entry_t *e = list_tail(&spa->spa_log_summary);
   
           if (e == NULL || summary_entry_is_full(spa, e, txg)) {
                   e = kmem_zalloc(sizeof (log_summary_entry_t), KM_SLEEP);
                   e->lse_start = e->lse_end = txg;
                   e->lse_txgcount = 1;
                   list_insert_tail(&spa->spa_log_summary, e);
           }
   
           ASSERT3U(e->lse_start, <=, txg);
           if (e->lse_end < txg) {
                   e->lse_end = txg;
                   e->lse_txgcount++;
           }
           e->lse_mscount += metaslabs_flushed;
           e->lse_msdcount += metaslabs_dirty;
           e->lse_blkcount += nblocks;
   }
   
   static void
   spa_log_summary_add_incoming_blocks(spa_t *spa, uint64_t nblocks)
   {
           summary_add_data(spa, spa_syncing_txg(spa), 0, 0, nblocks);
   }
   
   void
   spa_log_summary_add_flushed_metaslab(spa_t *spa, boolean_t dirty)
   {
           summary_add_data(spa, spa_syncing_txg(spa), 1, dirty ? 1 : 0, 0);
   }
   
   void
   spa_log_summary_dirty_flushed_metaslab(spa_t *spa, uint64_t txg)
   {
           log_summary_entry_t *target = NULL;
           for (log_summary_entry_t *e = list_head(&spa->spa_log_summary);
               e != NULL; e = list_next(&spa->spa_log_summary, e)) {
                   if (e->lse_start > txg)
                           break;
                   target = e;
           }
           ASSERT3P(target, !=, NULL);
           ASSERT3U(target->lse_mscount, !=, 0);
           target->lse_msdcount++;
   }
   
   /*
    * This function attempts to estimate how many metaslabs should
    * we flush to satisfy our block heuristic for the log spacemap
    * for the upcoming TXGs.
    *
    * Specifically, it first tries to estimate the number of incoming
    * blocks in this TXG. Then by projecting that incoming rate to
    * future TXGs and using the log summary, it figures out how many
    * flushes we would need to do for future TXGs individually to
    * stay below our block limit and returns the maximum number of
    * flushes from those estimates.
    */
   static uint64_t
   spa_estimate_metaslabs_to_flush(spa_t *spa)
   {
           ASSERT(spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP));
           ASSERT3U(spa_sync_pass(spa), ==, 1);
           ASSERT(spa_log_sm_blocklimit(spa) != 0);
   
           /*
            * This variable contains the incoming rate that will be projected
            * and used for our flushing estimates in the future.
            */
           uint64_t incoming = spa_estimate_incoming_log_blocks(spa);
   
           /*
            * At any point in time this variable tells us how many
            * TXGs in the future we are so we can make our estimations.
            */
           uint64_t txgs_in_future = 1;
   
           /*
            * This variable tells us how much room do we have until we hit
            * our limit. When it goes negative, it means that we've exceeded
            * our limit and we need to flush.
            *
            * Note that since we start at the first TXG in the future (i.e.
            * txgs_in_future starts from 1) we already decrement this
            * variable by the incoming rate.
            */
           int64_t available_blocks =
               spa_log_sm_blocklimit(spa) - spa_log_sm_nblocks(spa) - incoming;
   
           int64_t available_txgs = zfs_unflushed_log_txg_max;
           for (log_summary_entry_t *e = list_head(&spa->spa_log_summary);
               e; e = list_next(&spa->spa_log_summary, e))
                   available_txgs -= e->lse_txgcount;
   
           /*
            * This variable tells us the total number of flushes needed to
            * keep the log size within the limit when we reach txgs_in_future.
            */
           uint64_t total_flushes = 0;
   
           /* Holds the current maximum of our estimates so far. */
           uint64_t max_flushes_pertxg = zfs_min_metaslabs_to_flush;
   
           /*
            * For our estimations we only look as far in the future
            * as the summary allows us.
            */
           for (log_summary_entry_t *e = list_head(&spa->spa_log_summary);
               e; e = list_next(&spa->spa_log_summary, e)) {
   
                   /*
                    * If there is still room before we exceed our limit
                    * then keep skipping TXGs accumulating more blocks
                    * based on the incoming rate until we exceed it.
                    */
                   if (available_blocks >= 0 && available_txgs >= 0) {
                           uint64_t skip_txgs = (incoming == 0) ?
                               available_txgs + 1 : MIN(available_txgs + 1,
                               (available_blocks / incoming) + 1);
                           available_blocks -= (skip_txgs * incoming);
                           available_txgs -= skip_txgs;
                           txgs_in_future += skip_txgs;
                           ASSERT3S(available_blocks, >=, -incoming);
                           ASSERT3S(available_txgs, >=, -1);
                   }
   
                   /*
                    * At this point we're far enough into the future where
                    * the limit was just exceeded and we flush metaslabs
                    * based on the current entry in the summary, updating
                    * our available_blocks.
                    */
                   ASSERT(available_blocks < 0 || available_txgs < 0);
                   available_blocks += e->lse_blkcount;
                   available_txgs += e->lse_txgcount;
                   total_flushes += e->lse_msdcount;
   
                   /*
                    * Keep the running maximum of the total_flushes that
                    * we've done so far over the number of TXGs in the
                    * future that we are. The idea here is to estimate
                    * the average number of flushes that we should do
                    * every TXG so that when we are that many TXGs in the
                    * future we stay under the limit.
                    */
                   max_flushes_pertxg = MAX(max_flushes_pertxg,
                       DIV_ROUND_UP(total_flushes, txgs_in_future));
           }
           return (max_flushes_pertxg);
   }
   
   uint64_t
   spa_log_sm_memused(spa_t *spa)
   {
           return (spa->spa_unflushed_stats.sus_memused);
   }
   
   static boolean_t
   spa_log_exceeds_memlimit(spa_t *spa)
   {
           if (spa_log_sm_memused(spa) > zfs_unflushed_max_mem_amt)
                   return (B_TRUE);
   
           uint64_t system_mem_allowed = ((physmem * PAGESIZE) *
               zfs_unflushed_max_mem_ppm) / 1000000;
           if (spa_log_sm_memused(spa) > system_mem_allowed)
                   return (B_TRUE);
   
           return (B_FALSE);
   }
   
   boolean_t
   spa_flush_all_logs_requested(spa_t *spa)
   {
           return (spa->spa_log_flushall_txg != 0);
   }
   
   void
   spa_flush_metaslabs(spa_t *spa, dmu_tx_t *tx)
   {
           uint64_t txg = dmu_tx_get_txg(tx);
   
           if (spa_sync_pass(spa) != 1)
                   return;
   
           if (!spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP))
                   return;
   
           /*
            * If we don't have any metaslabs with unflushed changes
            * return immediately.
            */
           if (avl_numnodes(&spa->spa_metaslabs_by_flushed) == 0)
                   return;
   
           /*
            * During SPA export we leave a few empty TXGs to go by [see
            * spa_final_dirty_txg() to understand why]. For this specific
            * case, it is important to not flush any metaslabs as that
            * would dirty this TXG.
            *
            * That said, during one of these dirty TXGs that is less or
            * equal to spa_final_dirty(), spa_unload() will request that
            * we try to flush all the metaslabs for that TXG before
            * exporting the pool, thus we ensure that we didn't get a
            * request of flushing everything before we attempt to return
            * immediately.
            */
           if (spa->spa_uberblock.ub_rootbp.blk_birth < txg &&
               !dmu_objset_is_dirty(spa_meta_objset(spa), txg) &&
               !spa_flush_all_logs_requested(spa))
                   return;
   
           /*
            * We need to generate a log space map before flushing because this
            * will set up the in-memory data (i.e. node in spa_sm_logs_by_txg)
            * for this TXG's flushed metaslab count (aka sls_mscount which is
            * manipulated in many ways down the metaslab_flush() codepath).
            *
            * That is not to say that we may generate a log space map when we
            * don't need it. If we are flushing metaslabs, that means that we
            * were going to write changes to disk anyway, so even if we were
            * not flushing, a log space map would have been created anyway in
            * metaslab_sync().
            */
           spa_generate_syncing_log_sm(spa, tx);
   
           /*
            * This variable tells us how many metaslabs we want to flush based
            * on the block-heuristic of our flushing algorithm (see block comment
            * of log space map feature). We also decrement this as we flush
            * metaslabs and attempt to destroy old log space maps.
            */
           uint64_t want_to_flush;
           if (spa_flush_all_logs_requested(spa)) {
                   ASSERT3S(spa_state(spa), ==, POOL_STATE_EXPORTED);
                   want_to_flush = UINT64_MAX;
           } else {
                   want_to_flush = spa_estimate_metaslabs_to_flush(spa);
           }
   
           /* Used purely for verification purposes */
           uint64_t visited = 0;
   
           /*
            * Ideally we would only iterate through spa_metaslabs_by_flushed
            * using only one variable (curr). We can't do that because
            * metaslab_flush() mutates position of curr in the AVL when
            * it flushes that metaslab by moving it to the end of the tree.
            * Thus we always keep track of the original next node of the
            * current node (curr) in another variable (next).
            */
           metaslab_t *next = NULL;
           for (metaslab_t *curr = avl_first(&spa->spa_metaslabs_by_flushed);
               curr != NULL; curr = next) {
                   next = AVL_NEXT(&spa->spa_metaslabs_by_flushed, curr);
   
                   /*
                    * If this metaslab has been flushed this txg then we've done
                    * a full circle over the metaslabs.
                    */
                   if (metaslab_unflushed_txg(curr) == txg)
                           break;
   
                   /*
                    * If we are done flushing for the block heuristic and the
                    * unflushed changes don't exceed the memory limit just stop.
                    */
                   if (want_to_flush == 0 && !spa_log_exceeds_memlimit(spa))
                           break;
   
                   if (metaslab_unflushed_dirty(curr)) {
                           mutex_enter(&curr->ms_sync_lock);
                           mutex_enter(&curr->ms_lock);
                           metaslab_flush(curr, tx);
                           mutex_exit(&curr->ms_lock);
                           mutex_exit(&curr->ms_sync_lock);
                           if (want_to_flush > 0)
                                   want_to_flush--;
                   } else
                           metaslab_unflushed_bump(curr, tx, B_FALSE);
   
                   visited++;
           }
           ASSERT3U(avl_numnodes(&spa->spa_metaslabs_by_flushed), >=, visited);
   
           spa_log_sm_set_blocklimit(spa);
   }
   
   /*
    * Close the log space map for this TXG and update the block counts
    * for the log's in-memory structure and the summary.
    */
   void
   spa_sync_close_syncing_log_sm(spa_t *spa)
   {
           if (spa_syncing_log_sm(spa) == NULL)
                   return;
           ASSERT(spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP));
   
           spa_log_sm_t *sls = avl_last(&spa->spa_sm_logs_by_txg);
           ASSERT3U(sls->sls_txg, ==, spa_syncing_txg(spa));
   
           sls->sls_nblocks = space_map_nblocks(spa_syncing_log_sm(spa));
           spa->spa_unflushed_stats.sus_nblocks += sls->sls_nblocks;
   
           /*
            * Note that we can't assert that sls_mscount is not 0,
            * because there is the case where the first metaslab
            * in spa_metaslabs_by_flushed is loading and we were
            * not able to flush any metaslabs the current TXG.
            */
           ASSERT(sls->sls_nblocks != 0);
   
           spa_log_summary_add_incoming_blocks(spa, sls->sls_nblocks);
           spa_log_summary_verify_counts(spa);
   
           space_map_close(spa->spa_syncing_log_sm);
           spa->spa_syncing_log_sm = NULL;
   
           /*
            * At this point we tried to flush as many metaslabs as we
            * can as the pool is getting exported. Reset the "flush all"
            * so the last few TXGs before closing the pool can be empty
            * (e.g. not dirty).
            */
           if (spa_flush_all_logs_requested(spa)) {
                   ASSERT3S(spa_state(spa), ==, POOL_STATE_EXPORTED);
                   spa->spa_log_flushall_txg = 0;
           }
   }
   
   void
   spa_cleanup_old_sm_logs(spa_t *spa, dmu_tx_t *tx)
   {
           objset_t *mos = spa_meta_objset(spa);
   
           uint64_t spacemap_zap;
           int error = zap_lookup(mos, DMU_POOL_DIRECTORY_OBJECT,
               DMU_POOL_LOG_SPACEMAP_ZAP, sizeof (spacemap_zap), 1, &spacemap_zap);
           if (error == ENOENT) {
                   ASSERT(avl_is_empty(&spa->spa_sm_logs_by_txg));
                   return;
           }
           VERIFY0(error);
   
           metaslab_t *oldest = avl_first(&spa->spa_metaslabs_by_flushed);
           uint64_t oldest_flushed_txg = metaslab_unflushed_txg(oldest);
   
           /* Free all log space maps older than the oldest_flushed_txg. */
           for (spa_log_sm_t *sls = avl_first(&spa->spa_sm_logs_by_txg);
               sls && sls->sls_txg < oldest_flushed_txg;
               sls = avl_first(&spa->spa_sm_logs_by_txg)) {
                   ASSERT0(sls->sls_mscount);
                   avl_remove(&spa->spa_sm_logs_by_txg, sls);
                   space_map_free_obj(mos, sls->sls_sm_obj, tx);
                   VERIFY0(zap_remove_int(mos, spacemap_zap, sls->sls_txg, tx));
                   spa_log_summary_decrement_blkcount(spa, sls->sls_nblocks);
                   spa->spa_unflushed_stats.sus_nblocks -= sls->sls_nblocks;
                   kmem_free(sls, sizeof (spa_log_sm_t));
           }
   }
   
   static spa_log_sm_t *
   spa_log_sm_alloc(uint64_t sm_obj, uint64_t txg)
   {
           spa_log_sm_t *sls = kmem_zalloc(sizeof (*sls), KM_SLEEP);
           sls->sls_sm_obj = sm_obj;
           sls->sls_txg = txg;
           return (sls);
   }
   
   void
   spa_generate_syncing_log_sm(spa_t *spa, dmu_tx_t *tx)
   {
           uint64_t txg = dmu_tx_get_txg(tx);
           objset_t *mos = spa_meta_objset(spa);
   
           if (spa_syncing_log_sm(spa) != NULL)
                   return;
   
           if (!spa_feature_is_enabled(spa, SPA_FEATURE_LOG_SPACEMAP))
                   return;
   
           uint64_t spacemap_zap;
           int error = zap_lookup(mos, DMU_POOL_DIRECTORY_OBJECT,
               DMU_POOL_LOG_SPACEMAP_ZAP, sizeof (spacemap_zap), 1, &spacemap_zap);
           if (error == ENOENT) {
                   ASSERT(avl_is_empty(&spa->spa_sm_logs_by_txg));
   
                   error = 0;
                   spacemap_zap = zap_create(mos,
                       DMU_OTN_ZAP_METADATA, DMU_OT_NONE, 0, tx);
                   VERIFY0(zap_add(mos, DMU_POOL_DIRECTORY_OBJECT,
                       DMU_POOL_LOG_SPACEMAP_ZAP, sizeof (spacemap_zap), 1,
                       &spacemap_zap, tx));
                   spa_feature_incr(spa, SPA_FEATURE_LOG_SPACEMAP, tx);
           }
           VERIFY0(error);
   
           uint64_t sm_obj;
           ASSERT3U(zap_lookup_int_key(mos, spacemap_zap, txg, &sm_obj),
               ==, ENOENT);
           sm_obj = space_map_alloc(mos, zfs_log_sm_blksz, tx);
           VERIFY0(zap_add_int_key(mos, spacemap_zap, txg, sm_obj, tx));
           avl_add(&spa->spa_sm_logs_by_txg, spa_log_sm_alloc(sm_obj, txg));
   
           /*
            * We pass UINT64_MAX as the space map's representation size
            * and SPA_MINBLOCKSHIFT as the shift, to make the space map
            * accept any sorts of segments since there's no real advantage
            * to being more restrictive (given that we're already going
            * to be using 2-word entries).
            */
           VERIFY0(space_map_open(&spa->spa_syncing_log_sm, mos, sm_obj,
               0, UINT64_MAX, SPA_MINBLOCKSHIFT));
   
           spa_log_sm_set_blocklimit(spa);
   }
   
   /*
    * Find all the log space maps stored in the space map ZAP and sort
   * them by their TXG in spa_sm_logs_by_txg.
   */
  static int
  spa_ld_log_sm_metadata(spa_t *spa)
  {
          int error;
          uint64_t spacemap_zap;
  
          ASSERT(avl_is_empty(&spa->spa_sm_logs_by_txg));
  
          error = zap_lookup(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
              DMU_POOL_LOG_SPACEMAP_ZAP, sizeof (spacemap_zap), 1, &spacemap_zap);
          if (error == ENOENT) {
                  /* the space map ZAP doesn't exist yet */
                  return (0);
          } else if (error != 0) {
                  spa_load_failed(spa, "spa_ld_log_sm_metadata(): failed at "
                      "zap_lookup(DMU_POOL_DIRECTORY_OBJECT) [error %d]",
                      error);
                  return (error);
          }
  
          zap_cursor_t zc;
          zap_attribute_t za;
          for (zap_cursor_init(&zc, spa_meta_objset(spa), spacemap_zap);
              (error = zap_cursor_retrieve(&zc, &za)) == 0;
              zap_cursor_advance(&zc)) {
                  uint64_t log_txg = zfs_strtonum(za.za_name, NULL);
                  spa_log_sm_t *sls =
                      spa_log_sm_alloc(za.za_first_integer, log_txg);
                  avl_add(&spa->spa_sm_logs_by_txg, sls);
          }
          zap_cursor_fini(&zc);
          if (error != ENOENT) {
                  spa_load_failed(spa, "spa_ld_log_sm_metadata(): failed at "
                      "zap_cursor_retrieve(spacemap_zap) [error %d]",
                      error);
                  return (error);
          }
  
          for (metaslab_t *m = avl_first(&spa->spa_metaslabs_by_flushed);
              m; m = AVL_NEXT(&spa->spa_metaslabs_by_flushed, m)) {
                  spa_log_sm_t target = { .sls_txg = metaslab_unflushed_txg(m) };
                  spa_log_sm_t *sls = avl_find(&spa->spa_sm_logs_by_txg,
                      &target, NULL);
  
                  /*
                   * At this point if sls is zero it means that a bug occurred
                   * in ZFS the last time the pool was open or earlier in the
                   * import code path. In general, we would have placed a
                   * VERIFY() here or in this case just let the kernel panic
                   * with NULL pointer dereference when incrementing sls_mscount,
                   * but since this is the import code path we can be a bit more
                   * lenient. Thus, for DEBUG bits we always cause a panic, while
                   * in production we log the error and just fail the import.
                   */
                  ASSERT(sls != NULL);
                  if (sls == NULL) {
                          spa_load_failed(spa, "spa_ld_log_sm_metadata(): bug "
                              "encountered: could not find log spacemap for "
                              "TXG %llu [error %d]",
                              (u_longlong_t)metaslab_unflushed_txg(m), ENOENT);
                          return (ENOENT);
                  }
                  sls->sls_mscount++;
          }
  
          return (0);
  }
  
  typedef struct spa_ld_log_sm_arg {
          spa_t *slls_spa;
          uint64_t slls_txg;
  } spa_ld_log_sm_arg_t;
  
  static int
  spa_ld_log_sm_cb(space_map_entry_t *sme, void *arg)
  {
          uint64_t offset = sme->sme_offset;
          uint64_t size = sme->sme_run;
          uint32_t vdev_id = sme->sme_vdev;
  
          spa_ld_log_sm_arg_t *slls = arg;
          spa_t *spa = slls->slls_spa;
  
          vdev_t *vd = vdev_lookup_top(spa, vdev_id);
  
          /*
           * If the vdev has been removed (i.e. it is indirect or a hole)
           * skip this entry. The contents of this vdev have already moved
           * elsewhere.
           */
          if (!vdev_is_concrete(vd))
                  return (0);
  
          metaslab_t *ms = vd->vdev_ms[offset >> vd->vdev_ms_shift];
          ASSERT(!ms->ms_loaded);
  
          /*
           * If we have already flushed entries for this TXG to this
           * metaslab's space map, then ignore it. Note that we flush
           * before processing any allocations/frees for that TXG, so
           * the metaslab's space map only has entries from *before*
           * the unflushed TXG.
           */
          if (slls->slls_txg < metaslab_unflushed_txg(ms))
                  return (0);
  
          switch (sme->sme_type) {
          case SM_ALLOC:
                  range_tree_remove_xor_add_segment(offset, offset + size,
                      ms->ms_unflushed_frees, ms->ms_unflushed_allocs);
                  break;
          case SM_FREE:
                  range_tree_remove_xor_add_segment(offset, offset + size,
                      ms->ms_unflushed_allocs, ms->ms_unflushed_frees);
                  break;
          default:
                  panic("invalid maptype_t");
                  break;
          }
          if (!metaslab_unflushed_dirty(ms)) {
                  metaslab_set_unflushed_dirty(ms, B_TRUE);
                  spa_log_summary_dirty_flushed_metaslab(spa,
                      metaslab_unflushed_txg(ms));
          }
          return (0);
  }
  
  static int
  spa_ld_log_sm_data(spa_t *spa)
  {
          spa_log_sm_t *sls, *psls;
          int error = 0;
  
          /*
           * If we are not going to do any writes there is no need
           * to read the log space maps.
           */
          if (!spa_writeable(spa))
                  return (0);
  
          ASSERT0(spa->spa_unflushed_stats.sus_nblocks);
          ASSERT0(spa->spa_unflushed_stats.sus_memused);
  
          hrtime_t read_logs_starttime = gethrtime();
  
          /* Prefetch log spacemaps dnodes. */
          for (sls = avl_first(&spa->spa_sm_logs_by_txg); sls;
              sls = AVL_NEXT(&spa->spa_sm_logs_by_txg, sls)) {
                  dmu_prefetch(spa_meta_objset(spa), sls->sls_sm_obj,
                      0, 0, 0, ZIO_PRIORITY_SYNC_READ);
          }
  
          uint_t pn = 0;
          uint64_t ps = 0;
          psls = sls = avl_first(&spa->spa_sm_logs_by_txg);
          while (sls != NULL) {
                  /* Prefetch log spacemaps up to 16 TXGs or MBs ahead. */
                  if (psls != NULL && pn < 16 &&
                      (pn < 2 || ps < 2 * dmu_prefetch_max)) {
                          error = space_map_open(&psls->sls_sm,
                              spa_meta_objset(spa), psls->sls_sm_obj, 0,
                              UINT64_MAX, SPA_MINBLOCKSHIFT);
                          if (error != 0) {
                                  spa_load_failed(spa, "spa_ld_log_sm_data(): "
                                      "failed at space_map_open(obj=%llu) "
                                      "[error %d]",
                                      (u_longlong_t)sls->sls_sm_obj, error);
                                  goto out;
                          }
                          dmu_prefetch(spa_meta_objset(spa), psls->sls_sm_obj,
                              0, 0, space_map_length(psls->sls_sm),
                              ZIO_PRIORITY_ASYNC_READ);
                          pn++;
                          ps += space_map_length(psls->sls_sm);
                          psls = AVL_NEXT(&spa->spa_sm_logs_by_txg, psls);
                          continue;
                  }
  
                  /* Load TXG log spacemap into ms_unflushed_allocs/frees. */
                  kpreempt(KPREEMPT_SYNC);
                  ASSERT0(sls->sls_nblocks);
                  sls->sls_nblocks = space_map_nblocks(sls->sls_sm);
                  spa->spa_unflushed_stats.sus_nblocks += sls->sls_nblocks;
                  summary_add_data(spa, sls->sls_txg,
                      sls->sls_mscount, 0, sls->sls_nblocks);
  
                  struct spa_ld_log_sm_arg vla = {
                          .slls_spa = spa,
                          .slls_txg = sls->sls_txg
                  };
                  error = space_map_iterate(sls->sls_sm,
                      space_map_length(sls->sls_sm), spa_ld_log_sm_cb, &vla);
                  if (error != 0) {
                          spa_load_failed(spa, "spa_ld_log_sm_data(): failed "
                              "at space_map_iterate(obj=%llu) [error %d]",
                              (u_longlong_t)sls->sls_sm_obj, error);
                          goto out;
                  }
  
                  pn--;
                  ps -= space_map_length(sls->sls_sm);
                  space_map_close(sls->sls_sm);
                  sls->sls_sm = NULL;
                  sls = AVL_NEXT(&spa->spa_sm_logs_by_txg, sls);
  
                  /* Update log block limits considering just loaded. */
                  spa_log_sm_set_blocklimit(spa);
          }
  
          hrtime_t read_logs_endtime = gethrtime();
          spa_load_note(spa,
              "read %llu log space maps (%llu total blocks - blksz = %llu bytes) "
              "in %lld ms", (u_longlong_t)avl_numnodes(&spa->spa_sm_logs_by_txg),
              (u_longlong_t)spa_log_sm_nblocks(spa),
              (u_longlong_t)zfs_log_sm_blksz,
              (longlong_t)((read_logs_endtime - read_logs_starttime) / 1000000));
  
  out:
          if (error != 0) {
                  for (spa_log_sm_t *sls = avl_first(&spa->spa_sm_logs_by_txg);
                      sls; sls = AVL_NEXT(&spa->spa_sm_logs_by_txg, sls)) {
                          if (sls->sls_sm) {
                                  space_map_close(sls->sls_sm);
                                  sls->sls_sm = NULL;
                          }
                  }
          } else {
                  ASSERT0(pn);
                  ASSERT0(ps);
          }
          /*
           * Now that the metaslabs contain their unflushed changes:
           * [1] recalculate their actual allocated space
           * [2] recalculate their weights
           * [3] sum up the memory usage of their unflushed range trees
           * [4] optionally load them, if debug_load is set
           *
           * Note that even in the case where we get here because of an
           * error (e.g. error != 0), we still want to update the fields
           * below in order to have a proper teardown in spa_unload().
           */
          for (metaslab_t *m = avl_first(&spa->spa_metaslabs_by_flushed);
              m != NULL; m = AVL_NEXT(&spa->spa_metaslabs_by_flushed, m)) {
                  mutex_enter(&m->ms_lock);
                  m->ms_allocated_space = space_map_allocated(m->ms_sm) +
                      range_tree_space(m->ms_unflushed_allocs) -
                      range_tree_space(m->ms_unflushed_frees);
  
                  vdev_t *vd = m->ms_group->mg_vd;
                  metaslab_space_update(vd, m->ms_group->mg_class,
                      range_tree_space(m->ms_unflushed_allocs), 0, 0);
                  metaslab_space_update(vd, m->ms_group->mg_class,
                      -range_tree_space(m->ms_unflushed_frees), 0, 0);
  
                  ASSERT0(m->ms_weight & METASLAB_ACTIVE_MASK);
                  metaslab_recalculate_weight_and_sort(m);
  
                  spa->spa_unflushed_stats.sus_memused +=
                      metaslab_unflushed_changes_memused(m);
  
                  if (metaslab_debug_load && m->ms_sm != NULL) {
                          VERIFY0(metaslab_load(m));
                          metaslab_set_selected_txg(m, 0);
                  }
                  mutex_exit(&m->ms_lock);
          }
  
          return (error);
  }
  
  static int
  spa_ld_unflushed_txgs(vdev_t *vd)
  {
          spa_t *spa = vd->vdev_spa;
          objset_t *mos = spa_meta_objset(spa);
  
          if (vd->vdev_top_zap == 0)
                  return (0);
  
          uint64_t object = 0;
          int error = zap_lookup(mos, vd->vdev_top_zap,
              VDEV_TOP_ZAP_MS_UNFLUSHED_PHYS_TXGS,
              sizeof (uint64_t), 1, &object);
          if (error == ENOENT)
                  return (0);
          else if (error != 0) {
                  spa_load_failed(spa, "spa_ld_unflushed_txgs(): failed at "
                      "zap_lookup(vdev_top_zap=%llu) [error %d]",
                      (u_longlong_t)vd->vdev_top_zap, error);
                  return (error);
          }
  
          for (uint64_t m = 0; m < vd->vdev_ms_count; m++) {
                  metaslab_t *ms = vd->vdev_ms[m];
                  ASSERT(ms != NULL);
  
                  metaslab_unflushed_phys_t entry;
                  uint64_t entry_size = sizeof (entry);
                  uint64_t entry_offset = ms->ms_id * entry_size;
  
                  error = dmu_read(mos, object,
                      entry_offset, entry_size, &entry, 0);
                  if (error != 0) {
                          spa_load_failed(spa, "spa_ld_unflushed_txgs(): "
                              "failed at dmu_read(obj=%llu) [error %d]",
                              (u_longlong_t)object, error);
                          return (error);
                  }
  
                  ms->ms_unflushed_txg = entry.msp_unflushed_txg;
                  ms->ms_unflushed_dirty = B_FALSE;
                  ASSERT(range_tree_is_empty(ms->ms_unflushed_allocs));
                  ASSERT(range_tree_is_empty(ms->ms_unflushed_frees));
                  if (ms->ms_unflushed_txg != 0) {
                          mutex_enter(&spa->spa_flushed_ms_lock);
                          avl_add(&spa->spa_metaslabs_by_flushed, ms);
                          mutex_exit(&spa->spa_flushed_ms_lock);
                  }
          }
          return (0);
  }
  
  /*
   * Read all the log space map entries into their respective
   * metaslab unflushed trees and keep them sorted by TXG in the
   * SPA's metadata. In addition, setup all the metadata for the
   * memory and the block heuristics.
   */
  int
  spa_ld_log_spacemaps(spa_t *spa)
  {
          int error;
  
          spa_log_sm_set_blocklimit(spa);
  
          for (uint64_t c = 0; c < spa->spa_root_vdev->vdev_children; c++) {
                  vdev_t *vd = spa->spa_root_vdev->vdev_child[c];
                  error = spa_ld_unflushed_txgs(vd);
                  if (error != 0)
                          return (error);
          }
  
          error = spa_ld_log_sm_metadata(spa);
          if (error != 0)
                  return (error);
  
          /*
           * Note: we don't actually expect anything to change at this point
           * but we grab the config lock so we don't fail any assertions
           * when using vdev_lookup_top().
           */
          spa_config_enter(spa, SCL_CONFIG, FTAG, RW_READER);
          error = spa_ld_log_sm_data(spa);
          spa_config_exit(spa, SCL_CONFIG, FTAG);
  
          return (error);
  }
  
  /* BEGIN CSTYLED */
  ZFS_MODULE_PARAM(zfs, zfs_, unflushed_max_mem_amt, U64, ZMOD_RW,
          "Specific hard-limit in memory that ZFS allows to be used for "
          "unflushed changes");
  
  ZFS_MODULE_PARAM(zfs, zfs_, unflushed_max_mem_ppm, U64, ZMOD_RW,
          "Percentage of the overall system memory that ZFS allows to be "
          "used for unflushed changes (value is calculated over 1000000 for "
          "finer granularity)");
  
  ZFS_MODULE_PARAM(zfs, zfs_, unflushed_log_block_max, U64, ZMOD_RW,
          "Hard limit (upper-bound) in the size of the space map log "
          "in terms of blocks.");
  
  ZFS_MODULE_PARAM(zfs, zfs_, unflushed_log_block_min, U64, ZMOD_RW,
          "Lower-bound limit for the maximum amount of blocks allowed in "
          "log spacemap (see zfs_unflushed_log_block_max)");
  
  ZFS_MODULE_PARAM(zfs, zfs_, unflushed_log_txg_max, U64, ZMOD_RW,
      "Hard limit (upper-bound) in the size of the space map log "
      "in terms of dirty TXGs.");
  
  ZFS_MODULE_PARAM(zfs, zfs_, unflushed_log_block_pct, UINT, ZMOD_RW,
          "Tunable used to determine the number of blocks that can be used for "
          "the spacemap log, expressed as a percentage of the total number of "
          "metaslabs in the pool (e.g. 400 means the number of log blocks is "
          "capped at 4 times the number of metaslabs)");
  
  ZFS_MODULE_PARAM(zfs, zfs_, max_log_walking, U64, ZMOD_RW,
          "The number of past TXGs that the flushing algorithm of the log "
          "spacemap feature uses to estimate incoming log blocks");
  
  ZFS_MODULE_PARAM(zfs, zfs_, keep_log_spacemaps_at_export, INT, ZMOD_RW,
          "Prevent the log spacemaps from being flushed and destroyed "
          "during pool export/destroy");
  /* END CSTYLED */
  
  ZFS_MODULE_PARAM(zfs, zfs_, max_logsm_summary_length, U64, ZMOD_RW,
          "Maximum number of rows allowed in the summary of the spacemap log");
  
  ZFS_MODULE_PARAM(zfs, zfs_, min_metaslabs_to_flush, U64, ZMOD_RW,
          "Minimum number of metaslabs to flush per dirty TXG");

Cache object: e26b816ace87f5289f34773b21a96b04