[PATCH 0/2] [RFC] blkdev: flush generation optimization

[Dmitry Monakhov <dmonakhov@openvz.org>]

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Some filesystems try to optimize barrier flushes by maintaining
fs-specific generation counters, but if we introduce generic
flush generation counter for block device filesystems may use
it for fdatasync(2) optimization. Optimization should works if
userspace performs mutli-threaded IO with a lot of fdatasync()
Here are graphs for a test where each task performs random buffered writes
to dedicated file and performs fdatasync(2) after each operation.

Axis: x=nr_tasks, y=write_iops

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# Chunk server simulation workload
# Files 'chunk.$NUM_JOB.0' should be precreated before the test
# 
[global]
bs=4k
ioengine=psync
filesize=64M
size=8G
direct=0
runtime=30
directory=/mnt
fdatasync=1
group_reporting=1

[chunk]
overwrite=1
new_group=1
write_bw_log=bw.log
rw=randwrite
numjobs=${NUM_JOBS}
fsync=1
stonewall

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TOC:
0001 blkdev: add flush generation counter
0002 ext4: Add fdatasync scalability optimization