On 3/8/22 11:42 PM, Song Liu wrote:

RAID arrays check/repair operations benefit a lot from merging requests.
If we only check the previous entry for merge attempt, many merge will be
missed. As a result, significant regression is observed for RAID check
and repair.

Fix this by checking more than just the previous entry when
plug->multiple_queues == true.

This improves the check/repair speed of a 20-HDD raid6 from 19 MB/s to
103 MB/s.

Do the underlying disks not have an IO scheduler attached? Curious why
the merges aren't being done there, would be trivial when the list is
flushed out. Because if the perf difference is that big, then other
workloads would be suffering they are that sensitive to being within a
plug worth of IO.

Between your two approaches, I do greatly prefer the first one though.

-- 
Jens Axboe