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Improve too large physical ashift handling

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When iterating through children physical ashifts for vdev, prefer
ones above the maximum logical ashift, that we can actually use,
but within the administrator defined maximum.

When selecting top-level vdev ashift, do not set it to the defined
maximum in case physical ashift is even higher, but just ignore one.
Using the maximum does not prevent misaligned writes, but reduces
space efficiency.  Since ZFS tries to write data sequentially and
aggregates the writes, in many cases large misanigned writes may be
not as bad as the space penalty otherwise.

Allow internal physical ashifts for vdevs higher than SHIFT_MAX.
May be one day allocator or aggregation could benefit from that.

Reduce zfs_vdev_max_auto_ashift default from 16 (64KB) to 14 (16KB),
so that ZFS may still use bigger ashifts up to SHIFT_MAX (64KB),
but only if it really has to or explicitly told to, but not as an
"optimization".

There are some read-intensive NVMe SSDs that report Preferred Write
Alignment of 64KB, and attempt to build RAIDZ2 of those leads to a
space inefficiency that can't be justified.  Instead these changes
make ZFS fall back to logical ashift of 12 (4KB) by default and
only warn user that it may be suboptimal for performance.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #13798
This commit is contained in:
Alexander Motin
2022-09-08 13:30:53 -04:00
committed by Tony Hutter
parent ebbbe01e31
commit 44cec45f72
9 changed files with 69 additions and 13 deletions
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tests/zfs-tests/include/tunables.cfg
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@@ -81,7 +81,9 @@ TRIM_TXG_BATCH trim.txg_batch zfs_trim_txg_batch
TXG_HISTORY txg.history zfs_txg_history
TXG_TIMEOUT txg.timeout zfs_txg_timeout
UNLINK_SUSPEND_PROGRESS UNSUPPORTED zfs_unlink_suspend_progress
VDEV_FILE_LOGICAL_ASHIFT vdev.file.logical_ashift vdev_file_logical_ashift
VDEV_FILE_PHYSICAL_ASHIFT vdev.file.physical_ashift vdev_file_physical_ashift
VDEV_MAX_AUTO_ASHIFT vdev.max_auto_ashift zfs_vdev_max_auto_ashift
VDEV_MIN_MS_COUNT vdev.min_ms_count zfs_vdev_min_ms_count
VDEV_VALIDATE_SKIP vdev.validate_skip vdev_validate_skip
VOL_INHIBIT_DEV UNSUPPORTED zvol_inhibit_dev