block: avoid redundant flushes for O_DSYNC direct writes - #110
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added 2 commits
August 21, 2026 15:12
For an O_DIRECT | O_DSYNC write, dio_bio_write_op() adds REQ_FUA to the
bio, so the data is durable once the direct I/O returns. The
unconditional generic_write_sync() in blkdev_write_iter() then issues a
REQ_PREFLUSH that is redundant.
Skip it when the direct path already provided durability via FUA. A
need_sync flag, clear by default, is set only for buffered writes and
for the buffered fallback after a partial direct write.
Measured on a Seagate ST20000NM007D (20 TB, 7200 rpm, fua=1,
write_cache=write back), Linux v7.2.0-rc7, single-threaded pwrite()
loop opening the raw block device with O_WRONLY | O_DIRECT | O_DSYNC,
4 KiB writes for 60 s:
Sequential 4 KiB writes:
baseline patched
IOPS 119.7 7497.0
avg latency (us) 8357 133
p50 latency (us) 8346 127
p99 latency (us) 8368 395
p99.9 latency (us) 8728 569
Random 4 KiB writes (100 GiB span):
baseline patched
IOPS 156.1 666.4
avg latency (us) 6405 1500
p50 latency (us) 6186 1450
p99 latency (us) 16133 2285
p99.9 latency (us) 17250 9916
Signed-off-by: Zhenxian Ma <mzx199711@gmail.com>
Signed-off-by: Zhenxian Ma <mazhenxian@xiaohongshu.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
When a block device does not support FUA natively, the block layer emulates it by adding a cache flush to every write bio. An O_DSYNC direct write that spans N bios then costs N flushes, rather than the single generic_write_sync() issued after the write completes. Introduce blkdev_dio_fua() to decide when REQ_FUA is set. A synchronous write can rely on generic_write_sync() when the device lacks FUA, so it sets REQ_FUA only when bdev_fua() is true. An asynchronous write completes in blkdev_bio_end_io() and cannot call the blocking generic_write_sync(), so it keeps REQ_FUA (emulated when needed) to stay durable. Suggested-by: Christoph Hellwig <hch@infradead.org> Signed-off-by: Zhenxian Ma <mzx199711@gmail.com> Signed-off-by: Zhenxian Ma <mazhenxian@xiaohongshu.com> Reviewed-by: Christoph Hellwig <hch@lst.de>
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Pull request for series with
subject: block: avoid redundant flushes for O_DSYNC direct writes
version: 2
url: http://redsun45:8000/project/linux-block/list/?series=630