27  * Storage Pool Checkpoint
29  * A storage pool checkpoint can be thought of as a pool-wide snapshot or
35  * zpool on-disk features. If a pool has a checkpoint that is no longer
41  *   flag is set to active when we create the checkpoint and remains active
42  *   until the checkpoint is fully discarded. The entry in the MOS config
44  *   references the state of the pool when we take the checkpoint. The entry
45  *   remains populated until we start discarding the checkpoint or we rewind
48  * - Each vdev contains a vdev-wide space map while the pool has a checkpoint,
49  *   which persists until the checkpoint is fully discarded. The space map
51  *   but we want to keep around in case we decide to rewind to the checkpoint.
55  *   checkpoint, with the only exception being the scenario when we free
56  *   blocks that belong to the checkpoint. In this case, these blocks remain
58  *   vdev's checkpoint space map.
65  * - To create a checkpoint, we first wait for the current TXG to be synced,
68  *   uberblock in MOS config, increment the feature flag for the checkpoint
75  * - When a checkpoint exists, we need to ensure that the blocks that
76  *   belong to the checkpoint are freed but never reused. This means that
78  *   trees of a metaslab. Therefore, whenever there is a checkpoint the new
82  *   checkpoint (we find out by comparing its birth to spa_checkpoint_txg),
92  *   when we discard the checkpoint, we can find the entries that have
96  * - To discard the checkpoint we use an early synctask to delete the
100  *   new data end up in the checkpoint's data structures.
112  * - To rewind to the checkpoint, we first use the current uberblock and
119  *   An important note on rewinding to the checkpoint has to do with how we
121  *   blocks that have not been claimed by the time we took the checkpoint
127  * - In the hypothetical event that we take a checkpoint, remove a vdev,
134  * - As most of the checkpoint logic is implemented in the SPA and doesn't
136  *   checkpoint can potentially break the boundaries set by dataset
153  * prefetching of the checkpoint space map done on each vdev while
154  * discarding the checkpoint.
156  * The reason it exists is because top-level vdevs with long checkpoint
159  * the pool had a checkpoint.
196 	spa_history_log_internal(spa, "spa discard checkpoint", tx,  in spa_checkpoint_discard_complete_sync()
223 	 * the checkpoint's space map entries should not cross  in spa_checkpoint_discard_sync_callback()
340 	zfs_dbgmsg("discarding checkpoint: txg %llu, vdev id %lld, "  in spa_checkpoint_discard_thread_sync()
349 			    "while incrementally destroying the checkpoint "  in spa_checkpoint_discard_thread_sync()
432 				    "while prefetching checkpoint space map "  in spa_checkpoint_discard_thread()
486 	uberblock_t checkpoint = spa->spa_ubsync;  in spa_checkpoint_sync()  local
489 	 * At this point, there should not be a checkpoint in the MOS.  in spa_checkpoint_sync()
502 	ASSERT3U(checkpoint.ub_txg, ==, spa->spa_syncing_txg - 1);  in spa_checkpoint_sync()
505 	 * Once the checkpoint is in place, we need to ensure that none of  in spa_checkpoint_sync()
507 	 * When there is a checkpoint and a block is freed, we compare its  in spa_checkpoint_sync()
509 	 * block is part of the checkpoint or not. Therefore, we have to set  in spa_checkpoint_sync()
514 	spa->spa_checkpoint_txg = checkpoint.ub_txg;  in spa_checkpoint_sync()
515 	spa->spa_checkpoint_info.sci_timestamp = checkpoint.ub_timestamp;  in spa_checkpoint_sync()
517 	checkpoint.ub_checkpoint_txg = checkpoint.ub_txg;  in spa_checkpoint_sync()
521 	    &checkpoint, tx));  in spa_checkpoint_sync()
531 	spa_history_log_internal(spa, "spa checkpoint", tx,  in spa_checkpoint_sync()
532 	    "checkpointed uberblock txg=%llu", (u_longlong_t)checkpoint.ub_txg);  in spa_checkpoint_sync()
536  * Create a checkpoint for the pool.
553 	 * to see if we were to revert later to the checkpoint. In other  in spa_checkpoint()
556 	 * the checkpoint command.  in spa_checkpoint()
608 	spa_history_log_internal(spa, "spa discard checkpoint", tx,  in spa_checkpoint_discard_sync()
613  * Discard the checkpoint from a pool.
621 	 * won't end up in the checkpoint's data structures (e.g.  in spa_checkpoint_discard()
637 	"Limit for memory used in prefetching the checkpoint space map done "
638 	"on each vdev while discarding the checkpoint");