FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/module/zfs/spa_checkpoint.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) 2017 by Delphix. All rights reserved.
     */
    
    /*
     * Storage Pool Checkpoint
     *
     * A storage pool checkpoint can be thought of as a pool-wide snapshot or
     * a stable version of extreme rewind that guarantees no blocks from the
     * checkpointed state will have been overwritten. It remembers the entire
     * state of the storage pool (e.g. snapshots, dataset names, etc..) from the
     * point that it was taken and the user can rewind back to that point even if
     * they applied destructive operations on their datasets or even enabled new
     * zpool on-disk features. If a pool has a checkpoint that is no longer
     * needed, the user can discard it.
     *
     * == On disk data structures used ==
     *
     * - The pool has a new feature flag and a new entry in the MOS. The feature
     *   flag is set to active when we create the checkpoint and remains active
     *   until the checkpoint is fully discarded. The entry in the MOS config
     *   (DMU_POOL_ZPOOL_CHECKPOINT) is populated with the uberblock that
     *   references the state of the pool when we take the checkpoint. The entry
     *   remains populated until we start discarding the checkpoint or we rewind
     *   back to it.
     *
     * - Each vdev contains a vdev-wide space map while the pool has a checkpoint,
     *   which persists until the checkpoint is fully discarded. The space map
     *   contains entries that have been freed in the current state of the pool
     *   but we want to keep around in case we decide to rewind to the checkpoint.
     *   [see vdev_checkpoint_sm]
     *
     * - Each metaslab's ms_sm space map behaves the same as without the
     *   checkpoint, with the only exception being the scenario when we free
     *   blocks that belong to the checkpoint. In this case, these blocks remain
     *   ALLOCATED in the metaslab's space map and they are added as FREE in the
     *   vdev's checkpoint space map.
     *
     * - Each uberblock has a field (ub_checkpoint_txg) which holds the txg that
     *   the uberblock was checkpointed. For normal uberblocks this field is 0.
     *
     * == Overview of operations ==
     *
     * - To create a checkpoint, we first wait for the current TXG to be synced,
     *   so we can use the most recently synced uberblock (spa_ubsync) as the
     *   checkpointed uberblock. Then we use an early synctask to place that
     *   uberblock in MOS config, increment the feature flag for the checkpoint
     *   (marking it active), and setting spa_checkpoint_txg (see its use below)
     *   to the TXG of the checkpointed uberblock. We use an early synctask for
     *   the aforementioned operations to ensure that no blocks were dirtied
     *   between the current TXG and the TXG of the checkpointed uberblock
     *   (e.g the previous txg).
     *
     * - When a checkpoint exists, we need to ensure that the blocks that
     *   belong to the checkpoint are freed but never reused. This means that
     *   these blocks should never end up in the ms_allocatable or the ms_freeing
     *   trees of a metaslab. Therefore, whenever there is a checkpoint the new
     *   ms_checkpointing tree is used in addition to the aforementioned ones.
     *
     *   Whenever a block is freed and we find out that it is referenced by the
     *   checkpoint (we find out by comparing its birth to spa_checkpoint_txg),
     *   we place it in the ms_checkpointing tree instead of the ms_freeingtree.
     *   This way, we divide the blocks that are being freed into checkpointed
     *   and not-checkpointed blocks.
     *
     *   In order to persist these frees, we write the extents from the
     *   ms_freeingtree to the ms_sm as usual, and the extents from the
     *   ms_checkpointing tree to the vdev_checkpoint_sm. This way, these
     *   checkpointed extents will remain allocated in the metaslab's ms_sm space
     *   map, and therefore won't be reused [see metaslab_sync()]. In addition,
     *   when we discard the checkpoint, we can find the entries that have
     *   actually been freed in vdev_checkpoint_sm.
     *   [see spa_checkpoint_discard_thread_sync()]
     *
     * - To discard the checkpoint we use an early synctask to delete the
     *   checkpointed uberblock from the MOS config, set spa_checkpoint_txg to 0,
     *   and wakeup the discarding zthr thread (an open-context async thread).
     *   We use an early synctask to ensure that the operation happens before any
    *   new data end up in the checkpoint's data structures.
    *
    *   Once the synctask is done and the discarding zthr is awake, we discard
    *   the checkpointed data over multiple TXGs by having the zthr prefetching
    *   entries from vdev_checkpoint_sm and then starting a synctask that places
    *   them as free blocks into their respective ms_allocatable and ms_sm
    *   structures.
    *   [see spa_checkpoint_discard_thread()]
    *
    *   When there are no entries left in the vdev_checkpoint_sm of all
    *   top-level vdevs, a final synctask runs that decrements the feature flag.
    *
    * - To rewind to the checkpoint, we first use the current uberblock and
    *   open the MOS so we can access the checkpointed uberblock from the MOS
    *   config. After we retrieve the checkpointed uberblock, we use it as the
    *   current uberblock for the pool by writing it to disk with an updated
    *   TXG, opening its version of the MOS, and moving on as usual from there.
    *   [see spa_ld_checkpoint_rewind()]
    *
    *   An important note on rewinding to the checkpoint has to do with how we
    *   handle ZIL blocks. In the scenario of a rewind, we clear out any ZIL
    *   blocks that have not been claimed by the time we took the checkpoint
    *   as they should no longer be valid.
    *   [see comment in zil_claim()]
    *
    * == Miscellaneous information ==
    *
    * - In the hypothetical event that we take a checkpoint, remove a vdev,
    *   and attempt to rewind, the rewind would fail as the checkpointed
    *   uberblock would reference data in the removed device. For this reason
    *   and others of similar nature, we disallow the following operations that
    *   can change the config:
    *      vdev removal and attach/detach, mirror splitting, and pool reguid.
    *
    * - As most of the checkpoint logic is implemented in the SPA and doesn't
    *   distinguish datasets when it comes to space accounting, having a
    *   checkpoint can potentially break the boundaries set by dataset
    *   reservations.
    */
   
   #include <sys/dmu_tx.h>
   #include <sys/dsl_dir.h>
   #include <sys/dsl_synctask.h>
   #include <sys/metaslab_impl.h>
   #include <sys/spa.h>
   #include <sys/spa_impl.h>
   #include <sys/spa_checkpoint.h>
   #include <sys/vdev_impl.h>
   #include <sys/zap.h>
   #include <sys/zfeature.h>
   
   /*
    * The following parameter limits the amount of memory to be used for the
    * prefetching of the checkpoint space map done on each vdev while
    * discarding the checkpoint.
    *
    * The reason it exists is because top-level vdevs with long checkpoint
    * space maps can potentially take up a lot of memory depending on the
    * amount of checkpointed data that has been freed within them while
    * the pool had a checkpoint.
    */
   static uint64_t zfs_spa_discard_memory_limit = 16 * 1024 * 1024;
   
   int
   spa_checkpoint_get_stats(spa_t *spa, pool_checkpoint_stat_t *pcs)
   {
           if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
                   return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT));
   
           memset(pcs, 0, sizeof (pool_checkpoint_stat_t));
   
           int error = zap_contains(spa_meta_objset(spa),
               DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT);
           ASSERT(error == 0 || error == ENOENT);
   
           if (error == ENOENT)
                   pcs->pcs_state = CS_CHECKPOINT_DISCARDING;
           else
                   pcs->pcs_state = CS_CHECKPOINT_EXISTS;
   
           pcs->pcs_space = spa->spa_checkpoint_info.sci_dspace;
           pcs->pcs_start_time = spa->spa_checkpoint_info.sci_timestamp;
   
           return (0);
   }
   
   static void
   spa_checkpoint_discard_complete_sync(void *arg, dmu_tx_t *tx)
   {
           spa_t *spa = arg;
   
           spa->spa_checkpoint_info.sci_timestamp = 0;
   
           spa_feature_decr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx);
           spa_notify_waiters(spa);
   
           spa_history_log_internal(spa, "spa discard checkpoint", tx,
               "finished discarding checkpointed state from the pool");
   }
   
   typedef struct spa_checkpoint_discard_sync_callback_arg {
           vdev_t *sdc_vd;
           uint64_t sdc_txg;
           uint64_t sdc_entry_limit;
   } spa_checkpoint_discard_sync_callback_arg_t;
   
   static int
   spa_checkpoint_discard_sync_callback(space_map_entry_t *sme, void *arg)
   {
           spa_checkpoint_discard_sync_callback_arg_t *sdc = arg;
           vdev_t *vd = sdc->sdc_vd;
           metaslab_t *ms = vd->vdev_ms[sme->sme_offset >> vd->vdev_ms_shift];
           uint64_t end = sme->sme_offset + sme->sme_run;
   
           if (sdc->sdc_entry_limit == 0)
                   return (SET_ERROR(EINTR));
   
           /*
            * Since the space map is not condensed, we know that
            * none of its entries is crossing the boundaries of
            * its respective metaslab.
            *
            * That said, there is no fundamental requirement that
            * the checkpoint's space map entries should not cross
            * metaslab boundaries. So if needed we could add code
            * that handles metaslab-crossing segments in the future.
            */
           VERIFY3U(sme->sme_type, ==, SM_FREE);
           VERIFY3U(sme->sme_offset, >=, ms->ms_start);
           VERIFY3U(end, <=, ms->ms_start + ms->ms_size);
   
           /*
            * At this point we should not be processing any
            * other frees concurrently, so the lock is technically
            * unnecessary. We use the lock anyway though to
            * potentially save ourselves from future headaches.
            */
           mutex_enter(&ms->ms_lock);
           if (range_tree_is_empty(ms->ms_freeing))
                   vdev_dirty(vd, VDD_METASLAB, ms, sdc->sdc_txg);
           range_tree_add(ms->ms_freeing, sme->sme_offset, sme->sme_run);
           mutex_exit(&ms->ms_lock);
   
           ASSERT3U(vd->vdev_spa->spa_checkpoint_info.sci_dspace, >=,
               sme->sme_run);
           ASSERT3U(vd->vdev_stat.vs_checkpoint_space, >=, sme->sme_run);
   
           vd->vdev_spa->spa_checkpoint_info.sci_dspace -= sme->sme_run;
           vd->vdev_stat.vs_checkpoint_space -= sme->sme_run;
           sdc->sdc_entry_limit--;
   
           return (0);
   }
   
   #ifdef ZFS_DEBUG
   static void
   spa_checkpoint_accounting_verify(spa_t *spa)
   {
           vdev_t *rvd = spa->spa_root_vdev;
           uint64_t ckpoint_sm_space_sum = 0;
           uint64_t vs_ckpoint_space_sum = 0;
   
           for (uint64_t c = 0; c < rvd->vdev_children; c++) {
                   vdev_t *vd = rvd->vdev_child[c];
   
                   if (vd->vdev_checkpoint_sm != NULL) {
                           ckpoint_sm_space_sum +=
                               -space_map_allocated(vd->vdev_checkpoint_sm);
                           vs_ckpoint_space_sum +=
                               vd->vdev_stat.vs_checkpoint_space;
                           ASSERT3U(ckpoint_sm_space_sum, ==,
                               vs_ckpoint_space_sum);
                   } else {
                           ASSERT0(vd->vdev_stat.vs_checkpoint_space);
                   }
           }
           ASSERT3U(spa->spa_checkpoint_info.sci_dspace, ==, ckpoint_sm_space_sum);
   }
   #endif
   
   static void
   spa_checkpoint_discard_thread_sync(void *arg, dmu_tx_t *tx)
   {
           vdev_t *vd = arg;
           int error;
   
           /*
            * The space map callback is applied only to non-debug entries.
            * Because the number of debug entries is less or equal to the
            * number of non-debug entries, we want to ensure that we only
            * read what we prefetched from open-context.
            *
            * Thus, we set the maximum entries that the space map callback
            * will be applied to be half the entries that could fit in the
            * imposed memory limit.
            *
            * Note that since this is a conservative estimate we also
            * assume the worst case scenario in our computation where each
            * entry is two-word.
            */
           uint64_t max_entry_limit =
               (zfs_spa_discard_memory_limit / (2 * sizeof (uint64_t))) >> 1;
   
           /*
            * Iterate from the end of the space map towards the beginning,
            * placing its entries on ms_freeing and removing them from the
            * space map. The iteration stops if one of the following
            * conditions is true:
            *
            * 1] We reached the beginning of the space map. At this point
            *    the space map should be completely empty and
            *    space_map_incremental_destroy should have returned 0.
            *    The next step would be to free and close the space map
            *    and remove its entry from its vdev's top zap. This allows
            *    spa_checkpoint_discard_thread() to move on to the next vdev.
            *
            * 2] We reached the memory limit (amount of memory used to hold
            *    space map entries in memory) and space_map_incremental_destroy
            *    returned EINTR. This means that there are entries remaining
            *    in the space map that will be cleared in a future invocation
            *    of this function by spa_checkpoint_discard_thread().
            */
           spa_checkpoint_discard_sync_callback_arg_t sdc;
           sdc.sdc_vd = vd;
           sdc.sdc_txg = tx->tx_txg;
           sdc.sdc_entry_limit = max_entry_limit;
   
           uint64_t words_before =
               space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t);
   
           error = space_map_incremental_destroy(vd->vdev_checkpoint_sm,
               spa_checkpoint_discard_sync_callback, &sdc, tx);
   
           uint64_t words_after =
               space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t);
   
   #ifdef ZFS_DEBUG
           spa_checkpoint_accounting_verify(vd->vdev_spa);
   #endif
   
           zfs_dbgmsg("discarding checkpoint: txg %llu, vdev id %lld, "
               "deleted %llu words - %llu words are left",
               (u_longlong_t)tx->tx_txg, (longlong_t)vd->vdev_id,
               (u_longlong_t)(words_before - words_after),
               (u_longlong_t)words_after);
   
           if (error != EINTR) {
                   if (error != 0) {
                           zfs_panic_recover("zfs: error %lld was returned "
                               "while incrementally destroying the checkpoint "
                               "space map of vdev %llu\n",
                               (longlong_t)error, vd->vdev_id);
                   }
                   ASSERT0(words_after);
                   ASSERT0(space_map_allocated(vd->vdev_checkpoint_sm));
                   ASSERT0(space_map_length(vd->vdev_checkpoint_sm));
   
                   space_map_free(vd->vdev_checkpoint_sm, tx);
                   space_map_close(vd->vdev_checkpoint_sm);
                   vd->vdev_checkpoint_sm = NULL;
   
                   VERIFY0(zap_remove(spa_meta_objset(vd->vdev_spa),
                       vd->vdev_top_zap, VDEV_TOP_ZAP_POOL_CHECKPOINT_SM, tx));
           }
   }
   
   static boolean_t
   spa_checkpoint_discard_is_done(spa_t *spa)
   {
           vdev_t *rvd = spa->spa_root_vdev;
   
           ASSERT(!spa_has_checkpoint(spa));
           ASSERT(spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT));
   
           for (uint64_t c = 0; c < rvd->vdev_children; c++) {
                   if (rvd->vdev_child[c]->vdev_checkpoint_sm != NULL)
                           return (B_FALSE);
                   ASSERT0(rvd->vdev_child[c]->vdev_stat.vs_checkpoint_space);
           }
   
           return (B_TRUE);
   }
   
   boolean_t
   spa_checkpoint_discard_thread_check(void *arg, zthr_t *zthr)
   {
           (void) zthr;
           spa_t *spa = arg;
   
           if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
                   return (B_FALSE);
   
           if (spa_has_checkpoint(spa))
                   return (B_FALSE);
   
           return (B_TRUE);
   }
   
   void
   spa_checkpoint_discard_thread(void *arg, zthr_t *zthr)
   {
           spa_t *spa = arg;
           vdev_t *rvd = spa->spa_root_vdev;
   
           for (uint64_t c = 0; c < rvd->vdev_children; c++) {
                   vdev_t *vd = rvd->vdev_child[c];
   
                   while (vd->vdev_checkpoint_sm != NULL) {
                           space_map_t *checkpoint_sm = vd->vdev_checkpoint_sm;
                           int numbufs;
                           dmu_buf_t **dbp;
   
                           if (zthr_iscancelled(zthr))
                                   return;
   
                           ASSERT3P(vd->vdev_ops, !=, &vdev_indirect_ops);
   
                           uint64_t size = MIN(space_map_length(checkpoint_sm),
                               zfs_spa_discard_memory_limit);
                           uint64_t offset =
                               space_map_length(checkpoint_sm) - size;
   
                           /*
                            * Ensure that the part of the space map that will
                            * be destroyed by the synctask, is prefetched in
                            * memory before the synctask runs.
                            */
                           int error = dmu_buf_hold_array_by_bonus(
                               checkpoint_sm->sm_dbuf, offset, size,
                               B_TRUE, FTAG, &numbufs, &dbp);
                           if (error != 0) {
                                   zfs_panic_recover("zfs: error %d was returned "
                                       "while prefetching checkpoint space map "
                                       "entries of vdev %llu\n",
                                       error, vd->vdev_id);
                           }
   
                           VERIFY0(dsl_sync_task(spa->spa_name, NULL,
                               spa_checkpoint_discard_thread_sync, vd,
                               0, ZFS_SPACE_CHECK_NONE));
   
                           dmu_buf_rele_array(dbp, numbufs, FTAG);
                   }
           }
   
           VERIFY(spa_checkpoint_discard_is_done(spa));
           VERIFY0(spa->spa_checkpoint_info.sci_dspace);
           VERIFY0(dsl_sync_task(spa->spa_name, NULL,
               spa_checkpoint_discard_complete_sync, spa,
               0, ZFS_SPACE_CHECK_NONE));
   }
   
   
   static int
   spa_checkpoint_check(void *arg, dmu_tx_t *tx)
   {
           (void) arg;
           spa_t *spa = dmu_tx_pool(tx)->dp_spa;
   
           if (!spa_feature_is_enabled(spa, SPA_FEATURE_POOL_CHECKPOINT))
                   return (SET_ERROR(ENOTSUP));
   
           if (!spa_top_vdevs_spacemap_addressable(spa))
                   return (SET_ERROR(ZFS_ERR_VDEV_TOO_BIG));
   
           if (spa->spa_removing_phys.sr_state == DSS_SCANNING)
                   return (SET_ERROR(ZFS_ERR_DEVRM_IN_PROGRESS));
   
           if (spa->spa_checkpoint_txg != 0)
                   return (SET_ERROR(ZFS_ERR_CHECKPOINT_EXISTS));
   
           if (spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
                   return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT));
   
           return (0);
   }
   
   static void
   spa_checkpoint_sync(void *arg, dmu_tx_t *tx)
   {
           (void) arg;
           dsl_pool_t *dp = dmu_tx_pool(tx);
           spa_t *spa = dp->dp_spa;
           uberblock_t checkpoint = spa->spa_ubsync;
   
           /*
            * At this point, there should not be a checkpoint in the MOS.
            */
           ASSERT3U(zap_contains(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
               DMU_POOL_ZPOOL_CHECKPOINT), ==, ENOENT);
   
           ASSERT0(spa->spa_checkpoint_info.sci_timestamp);
           ASSERT0(spa->spa_checkpoint_info.sci_dspace);
   
           /*
            * Since the checkpointed uberblock is the one that just got synced
            * (we use spa_ubsync), its txg must be equal to the txg number of
            * the txg we are syncing, minus 1.
            */
           ASSERT3U(checkpoint.ub_txg, ==, spa->spa_syncing_txg - 1);
   
           /*
            * Once the checkpoint is in place, we need to ensure that none of
            * its blocks will be marked for reuse after it has been freed.
            * When there is a checkpoint and a block is freed, we compare its
            * birth txg to the txg of the checkpointed uberblock to see if the
            * block is part of the checkpoint or not. Therefore, we have to set
            * spa_checkpoint_txg before any frees happen in this txg (which is
            * why this is done as an early_synctask as explained in the comment
            * in spa_checkpoint()).
            */
           spa->spa_checkpoint_txg = checkpoint.ub_txg;
           spa->spa_checkpoint_info.sci_timestamp = checkpoint.ub_timestamp;
   
           checkpoint.ub_checkpoint_txg = checkpoint.ub_txg;
           VERIFY0(zap_add(spa->spa_dsl_pool->dp_meta_objset,
               DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT,
               sizeof (uint64_t), sizeof (uberblock_t) / sizeof (uint64_t),
               &checkpoint, tx));
   
           /*
            * Increment the feature refcount and thus activate the feature.
            * Note that the feature will be deactivated when we've
            * completely discarded all checkpointed state (both vdev
            * space maps and uberblock).
            */
           spa_feature_incr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx);
   
           spa_history_log_internal(spa, "spa checkpoint", tx,
               "checkpointed uberblock txg=%llu", (u_longlong_t)checkpoint.ub_txg);
   }
   
   /*
    * Create a checkpoint for the pool.
    */
   int
   spa_checkpoint(const char *pool)
   {
           int error;
           spa_t *spa;
   
           error = spa_open(pool, &spa, FTAG);
           if (error != 0)
                   return (error);
   
           mutex_enter(&spa->spa_vdev_top_lock);
   
           /*
            * Wait for current syncing txg to finish so the latest synced
            * uberblock (spa_ubsync) has all the changes that we expect
            * to see if we were to revert later to the checkpoint. In other
            * words we want the checkpointed uberblock to include/reference
            * all the changes that were pending at the time that we issued
            * the checkpoint command.
            */
           txg_wait_synced(spa_get_dsl(spa), 0);
   
           /*
            * As the checkpointed uberblock references blocks from the previous
            * txg (spa_ubsync) we want to ensure that are not freeing any of
            * these blocks in the same txg that the following synctask will
            * run. Thus, we run it as an early synctask, so the dirty changes
            * that are synced to disk afterwards during zios and other synctasks
            * do not reuse checkpointed blocks.
            */
           error = dsl_early_sync_task(pool, spa_checkpoint_check,
               spa_checkpoint_sync, NULL, 0, ZFS_SPACE_CHECK_NORMAL);
   
           mutex_exit(&spa->spa_vdev_top_lock);
   
           spa_close(spa, FTAG);
           return (error);
   }
   
   static int
   spa_checkpoint_discard_check(void *arg, dmu_tx_t *tx)
   {
           (void) arg;
           spa_t *spa = dmu_tx_pool(tx)->dp_spa;
   
           if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT))
                   return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT));
   
           if (spa->spa_checkpoint_txg == 0)
                   return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT));
   
           VERIFY0(zap_contains(spa_meta_objset(spa),
               DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT));
   
           return (0);
   }
   
   static void
   spa_checkpoint_discard_sync(void *arg, dmu_tx_t *tx)
   {
           (void) arg;
           spa_t *spa = dmu_tx_pool(tx)->dp_spa;
   
           VERIFY0(zap_remove(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT,
               DMU_POOL_ZPOOL_CHECKPOINT, tx));
   
           spa->spa_checkpoint_txg = 0;
   
           zthr_wakeup(spa->spa_checkpoint_discard_zthr);
   
           spa_history_log_internal(spa, "spa discard checkpoint", tx,
               "started discarding checkpointed state from the pool");
   }
   
   /*
    * Discard the checkpoint from a pool.
    */
   int
   spa_checkpoint_discard(const char *pool)
   {
           /*
            * Similarly to spa_checkpoint(), we want our synctask to run
            * before any pending dirty data are written to disk so they
            * won't end up in the checkpoint's data structures (e.g.
            * ms_checkpointing and vdev_checkpoint_sm) and re-create any
            * space maps that the discarding open-context thread has
            * deleted.
            * [see spa_discard_checkpoint_sync and spa_discard_checkpoint_thread]
            */
           return (dsl_early_sync_task(pool, spa_checkpoint_discard_check,
               spa_checkpoint_discard_sync, NULL, 0,
               ZFS_SPACE_CHECK_DISCARD_CHECKPOINT));
   }
   
   EXPORT_SYMBOL(spa_checkpoint_get_stats);
   EXPORT_SYMBOL(spa_checkpoint_discard_thread);
   EXPORT_SYMBOL(spa_checkpoint_discard_thread_check);
   
   /* BEGIN CSTYLED */
   ZFS_MODULE_PARAM(zfs_spa, zfs_spa_, discard_memory_limit, U64, ZMOD_RW,
           "Limit for memory used in prefetching the checkpoint space map done "
           "on each vdev while discarding the checkpoint");
   /* END CSTYLED */

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