Re: slow 'check'

["Raz Ben-Jehuda(caro)" <raziebe@gmail.com>]

On 2/10/07, Eyal Lebedinsky <eyal@eyal.emu.id.au> wrote:
> 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)
try dd with bs of 4x(5x256) = 5 M.

> 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.
Stripe cache size is less effective than previous versions
of raid5 since in some cases it is being bypassed.
Why do you check random access to the raid
and not sequential access.

> 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/>
>        attach .zip as .dat
> -
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-- 
Raz