linux-kernel - RAID0 & diskstats.

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Message-ID: <20150304210904.GA26981@codemonkey.org.uk>
Date:	Wed, 4 Mar 2015 16:09:04 -0500
From:	Dave Jones <davej@...emonkey.org.uk>
To:	Linux Kernel <linux-kernel@...r.kernel.org>
Cc:	Neil Brown <neilb@...e.de>
Subject: RAID0 & diskstats.

Hi Neil,
   According to Documentation/iostats.txt, the 9th column of
/proc/diskstats (and its modern replacement in sysfs) should go to 0
as IO completes.

I assembled a RAID0 stripe using two SSD's, and saw this..

# mdadm --assemble /dev/md0
mdadm: /dev/md0 has been started with 2 drives.
# cat /sys/block/md0/stat
     167        0     5656        0        5        0     4096        0     172     3408   582825
# cat /sys/block/md0/stat
     167        0     5656        0        5        0     4096        0     172   231469 39809317

The 10th & 11th fields constantly increase, as field 9 remains non-zero.
If I mount and umount a filesystem on that volume, it works as expected,
but the 9th 'IOs inflight' field continues to rise and never decreases even
though the IO has obviously completed.

# umount /mnt/ssd
# cat /sys/block/md0/stat
     167        0     5656        0        9        0     4225        0     176   571384 98278615

The underlying disks have their respective stats entries behaving as
expected, it only seems to affect the upper md layer.

Some missing accounting somewhere in md ?

(Only tested on 4.0rc2 so far, and only on RAID0)

	Dave

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