OpenZFS 9166 - zfs storage pool checkpoint

Details about the motivation of this feature and its usage can
be found in this blogpost:

    https://sdimitro.github.io/post/zpool-checkpoint/

A lightning talk of this feature can be found here:
https://www.youtube.com/watch?v=fPQA8K40jAM

Implementation details can be found in big block comment of
spa_checkpoint.c

Side-changes that are relevant to this commit but not explained
elsewhere:

* renames members of "struct metaslab trees to be shorter without
  losing meaning

* space_map_{alloc,truncate}() accept a block size as a
  parameter. The reason is that in the current state all space
  maps that we allocate through the DMU use a global tunable
  (space_map_blksz) which defauls to 4KB. This is ok for metaslab
  space maps in terms of bandwirdth since they are scattered all
  over the disk. But for other space maps this default is probably
  not what we want. Examples are device removal's vdev_obsolete_sm
  or vdev_chedkpoint_sm from this review. Both of these have a
  1:1 relationship with each vdev and could benefit from a bigger
  block size.

Porting notes:

* The part of dsl_scan_sync() which handles async destroys has
  been moved into the new dsl_process_async_destroys() function.

* Remove "VERIFY(!(flags & FWRITE))" in "kernel.c" so zhack can write
  to block device backed pools.

* ZTS:
  * Fix get_txg() in zpool_sync_001_pos due to "checkpoint_txg".

  * Don't use large dd block sizes on /dev/urandom under Linux in
    checkpoint_capacity.

  * Adopt Delphix-OS's setting of 4 (spa_asize_inflation =
    SPA_DVAS_PER_BP + 1) for the checkpoint_capacity test to speed
    its attempts to fill the pool

  * Create the base and nested pools with sync=disabled to speed up
    the "setup" phase.

  * Clear labels in test pool between checkpoint tests to avoid
    duplicate pool issues.

  * The import_rewind_device_replaced test has been marked as "known
    to fail" for the reasons listed in its DISCLAIMER.

  * New module parameters:

      zfs_spa_discard_memory_limit,
      zfs_remove_max_bytes_pause (not documented - debugging only)
      vdev_max_ms_count (formerly metaslabs_per_vdev)
      vdev_min_ms_count

Authored by: Serapheim Dimitropoulos <[email protected]>
Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: John Kennedy <[email protected]>
Reviewed by: Dan Kimmel <[email protected]>
Reviewed by: Brian Behlendorf <[email protected]>
Approved by: Richard Lowe <[email protected]>
Ported-by: Tim Chase <[email protected]>
Signed-off-by: Tim Chase <[email protected]>

OpenZFS-issue: https://illumos.org/issues/9166
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/7159fdb8
Closes #7570

1 files changed, 23 insertions, 1 deletions

diff --git a/include/sys/uberblock_impl.h b/include/sys/uberblock_impl.h
index 08eeabdda..113df7c61 100644
--- a/include/sys/uberblock_impl.h
+++ b/include/sys/uberblock_impl.h
@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2017 by Delphix. All rights reserved.
+ * Copyright (c) 2016, 2017 by Delphix. All rights reserved.
  */
 
 #ifndef _SYS_UBERBLOCK_IMPL_H
@@ -61,6 +61,28 @@ struct uberblock {
 	uint64_t	ub_mmp_magic;	/* MMP_MAGIC			*/
 	uint64_t	ub_mmp_delay;	/* nanosec since last MMP write	*/
 	uint64_t	ub_mmp_seq;	/* reserved for sequence number	*/
+
+	/*
+	 * ub_checkpoint_txg indicates two things about the current uberblock:
+	 *
+	 * 1] If it is not zero then this uberblock is a checkpoint. If it is
+	 *    zero, then this uberblock is not a checkpoint.
+	 *
+	 * 2] On checkpointed uberblocks, the value of ub_checkpoint_txg is
+	 *    the ub_txg that the uberblock had at the time we moved it to
+	 *    the MOS config.
+	 *
+	 * The field is set when we checkpoint the uberblock and continues to
+	 * hold that value even after we've rewound (unlike the ub_txg that
+	 * is reset to a higher value).
+	 *
+	 * Besides checks used to determine whether we are reopening the
+	 * pool from a checkpointed uberblock [see spa_ld_select_uberblock()],
+	 * the value of the field is used to determine which ZIL blocks have
+	 * been allocated according to the ms_sm when we are rewinding to a
+	 * checkpoint. Specifically, if blk_birth > ub_checkpoint_txg, then
+	 * the ZIL block is not allocated [see uses of spa_min_claim_txg()].
+	 */
 	uint64_t	ub_checkpoint_txg;
 };