DMU: Do not pre-read holes during write #15371
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dmu_tx_check_ioerr() pre-reads blocks that are going to be dirtied as part of transaction to both prefetch them and check for errors. But it makes no sense to do it for holes, since there are no disk reads to prefetch and there can be no errors. On the other side those blocks are anonymous, and they are freed immediately by the dbuf_rele() without even being put into dbuf cache, so we just burn CPU time on decompression and overheads and get absolutely no result at the end. Use of dbuf_hold_impl() with fail_sparse parameter allows to skip the extra work, and on my tests with sequential 8KB writes to empty ZVOL with 32KB blocks shows throughput increase from 1.7 to 2GB/s. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
behlendorf
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bwatkinson
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Status: Accepted
amotin
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Type: Performance
amotin
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Nov 6, 2023
dmu_tx_check_ioerr() pre-reads blocks that are going to be dirtied as part of transaction to both prefetch them and check for errors. But it makes no sense to do it for holes, since there are no disk reads to prefetch and there can be no errors. On the other side those blocks are anonymous, and they are freed immediately by the dbuf_rele() without even being put into dbuf cache, so we just burn CPU time on decompression and overheads and get absolutely no result at the end. Use of dbuf_hold_impl() with fail_sparse parameter allows to skip the extra work, and on my tests with sequential 8KB writes to empty ZVOL with 32KB blocks shows throughput increase from 1.7 to 2GB/s. Reviewed-by: Brian Atkinson <[email protected]> Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15371
defaziogiancarlo
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Nov 17, 2023
dmu_tx_check_ioerr() pre-reads blocks that are going to be dirtied as part of transaction to both prefetch them and check for errors. But it makes no sense to do it for holes, since there are no disk reads to prefetch and there can be no errors. On the other side those blocks are anonymous, and they are freed immediately by the dbuf_rele() without even being put into dbuf cache, so we just burn CPU time on decompression and overheads and get absolutely no result at the end. Use of dbuf_hold_impl() with fail_sparse parameter allows to skip the extra work, and on my tests with sequential 8KB writes to empty ZVOL with 32KB blocks shows throughput increase from 1.7 to 2GB/s. Reviewed-by: Brian Atkinson <[email protected]> Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15371
ixhamza
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Nov 20, 2023
dmu_tx_check_ioerr() pre-reads blocks that are going to be dirtied as part of transaction to both prefetch them and check for errors. But it makes no sense to do it for holes, since there are no disk reads to prefetch and there can be no errors. On the other side those blocks are anonymous, and they are freed immediately by the dbuf_rele() without even being put into dbuf cache, so we just burn CPU time on decompression and overheads and get absolutely no result at the end. Use of dbuf_hold_impl() with fail_sparse parameter allows to skip the extra work, and on my tests with sequential 8KB writes to empty ZVOL with 32KB blocks shows throughput increase from 1.7 to 2GB/s. Reviewed-by: Brian Atkinson <[email protected]> Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15371
lundman
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Dec 12, 2023
dmu_tx_check_ioerr() pre-reads blocks that are going to be dirtied as part of transaction to both prefetch them and check for errors. But it makes no sense to do it for holes, since there are no disk reads to prefetch and there can be no errors. On the other side those blocks are anonymous, and they are freed immediately by the dbuf_rele() without even being put into dbuf cache, so we just burn CPU time on decompression and overheads and get absolutely no result at the end. Use of dbuf_hold_impl() with fail_sparse parameter allows to skip the extra work, and on my tests with sequential 8KB writes to empty ZVOL with 32KB blocks shows throughput increase from 1.7 to 2GB/s. Reviewed-by: Brian Atkinson <[email protected]> Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15371
defaziogiancarlo
pushed a commit
to LLNL/zfs
that referenced
this pull request
Apr 18, 2024
dmu_tx_check_ioerr() pre-reads blocks that are going to be dirtied as part of transaction to both prefetch them and check for errors. But it makes no sense to do it for holes, since there are no disk reads to prefetch and there can be no errors. On the other side those blocks are anonymous, and they are freed immediately by the dbuf_rele() without even being put into dbuf cache, so we just burn CPU time on decompression and overheads and get absolutely no result at the end. Use of dbuf_hold_impl() with fail_sparse parameter allows to skip the extra work, and on my tests with sequential 8KB writes to empty ZVOL with 32KB blocks shows throughput increase from 1.7 to 2GB/s. Reviewed-by: Brian Atkinson <[email protected]> Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15371
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dmu_tx_check_ioerr() pre-reads blocks that are going to be dirtied as part of transaction to both prefetch them and check for errors. But it makes no sense to do it for holes, since there are no disk reads to prefetch and there can be no errors. On the other side those blocks are anonymous, and they are freed immediately by the dbuf_rele() without even being put into dbuf cache, so we just burn CPU time on decompression and overheads and get absolutely no result at the end.
Use of dbuf_hold_impl() with fail_sparse parameter allows to skip the extra work, and on my tests with sequential 8KB writes to empty ZVOL with 32KB blocks shows throughput increase from 1.7 to 2GB/s.
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