Remove limit on number of async zio_frees of non-dedup blocks #10000
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The module parameter zfs_async_block_max_blocks limits the number of blocks that can be freed by the background freeing of filesystems and snapshots (from "zfs destroy"), in one TXG. This is useful when freeing dedup blocks, becuase each zio_free() of a dedup block can require an i/o to read the relevant part of the dedup table (DDT), and will also dirty that block. zfs_async_block_max_blocks is set to 100,000 by default. For the more typical case where dedup is not used, this can have a negative performance impact on the rate of background freeing (from "zfs destroy"). For example, with recordsize=8k, and TXG's syncing once every 5 seconds, we can free only 160MB of data per second, which may be much less than the rate we can write data. This change increases zfs_async_block_max_blocks to be unlimited by default. To address the dedup freeing issue, a new tunable is introduced, zfs_max_async_dedup_frees, which limits the number of zio_free()'s of dedup blocks done by background destroys, per txg. The default is 100,000 free's (same as the old zfs_async_block_max_blocks default). Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
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Is there any point keeping |
Codecov Report
@@ Coverage Diff @@
## master #10000 +/- ##
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+ Coverage 79.10% 79.67% +0.57%
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Files 385 385
Lines 121934 121939 +5
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+ Hits 96451 97160 +709
+ Misses 25483 24779 -704
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pcd1193182
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Feb 13, 2020
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Status: Code Review Needed
behlendorf
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Feb 13, 2020
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@scineram I think that |
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Status: Accepted
jsai20
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Mar 30, 2021
The module parameter zfs_async_block_max_blocks limits the number of blocks that can be freed by the background freeing of filesystems and snapshots (from "zfs destroy"), in one TXG. This is useful when freeing dedup blocks, becuase each zio_free() of a dedup block can require an i/o to read the relevant part of the dedup table (DDT), and will also dirty that block. zfs_async_block_max_blocks is set to 100,000 by default. For the more typical case where dedup is not used, this can have a negative performance impact on the rate of background freeing (from "zfs destroy"). For example, with recordsize=8k, and TXG's syncing once every 5 seconds, we can free only 160MB of data per second, which may be much less than the rate we can write data. This change increases zfs_async_block_max_blocks to be unlimited by default. To address the dedup freeing issue, a new tunable is introduced, zfs_max_async_dedup_frees, which limits the number of zio_free()'s of dedup blocks done by background destroys, per txg. The default is 100,000 free's (same as the old zfs_async_block_max_blocks default). Reviewed-by: Paul Dagnelie <pcd@delphix.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> Closes openzfs#10000
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Motivation and Context
The module parameter zfs_async_block_max_blocks limits the number of
blocks that can be freed by the background freeing of filesystems and
snapshots (from "zfs destroy"), in one TXG. This is useful when freeing
dedup blocks, becuase each zio_free() of a dedup block can require an
i/o to read the relevant part of the dedup table (DDT), and will also
dirty that block.
zfs_async_block_max_blocks is set to 100,000 by default. For the more
typical case where dedup is not used, this can have a negative
performance impact on the rate of background freeing (from "zfs
destroy"). For example, with recordsize=8k, and TXG's syncing once
every 5 seconds, we can free only 160MB of data per second, which may be
much less than the rate we can write data.
Description
This change increases zfs_async_block_max_blocks to be unlimited by
default. To address the dedup freeing issue, a new tunable is
introduced, zfs_max_async_dedup_frees, which limits the number of
zio_free()'s of dedup blocks done by background destroys, per txg. The
default is 100,000 free's (same as the old zfs_async_block_max_blocks
default).
How Has This Been Tested?
Without this change, deleting a filesystem with lots of blocks (non-dedup in this case, but behavior is the same for dedup):
With this change, deleting a filesystem with lots of dedup blocks (same behavior as without this change):
With this change, deleting a filesystem without dedup:
Types of changes
Checklist:
Signed-off-by.