slow 'check'

[Eyal Lebedinsky <eyal@eyal.emu.id.au>]

I have a six-disk RAID5 over sata. First two disks are on the mobo and last four
are on a Promise SATA-II-150-TX4. The sixth disk was added recently and I decided
to run a 'check' periodically, and started one manually to see how long it should
take. Vanilla 2.6.20.

A 'dd' test shows:

# dd if=/dev/md0 of=/dev/null bs=1024k count=10240
10240+0 records in
10240+0 records out
10737418240 bytes transferred in 84.449870 seconds (127145468 bytes/sec)

This is good for this setup. A check shows:

$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sda1[0] sdf1[5] sde1[4] sdd1[3] sdc1[2] sdb1[1]
      1562842880 blocks level 5, 256k chunk, algorithm 2 [6/6] [UUUUUU]
      [>....................]  check =  0.8% (2518144/312568576) finish=2298.3min speed=2246K/sec

unused devices: <none>

which is an order of magnitude slower (the speed is per-disk, call it 13MB/s
for the six). There is no activity on the RAID. Is this expected? I assume
that the simple dd does the same amount of work (don't we check parity on
read?).

I have these tweaked at bootup:
	echo 4096 >/sys/block/md0/md/stripe_cache_size
	blockdev --setra 32768 /dev/md0

Changing the above parameters seems to not have a significant effect.

The check logs the following:

md: data-check of RAID array md0
md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for data-check.
md: using 128k window, over a total of 312568576 blocks.

Does it need a larger window (whatever a window is)? If so, can it
be set dynamically?

TIA

-- 
Eyal Lebedinsky (eyal@eyal.emu.id.au) <http://samba.org/eyal/>
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