From mboxrd@z Thu Jan  1 00:00:00 1970
From: Justin Piszcz <jpiszcz@lucidpixels.com>
Subject: Re: slow 'check'
Date: Sat, 10 Feb 2007 04:25:47 -0500 (EST)
Message-ID: <Pine.LNX.4.64.0702100425150.8446@p34.internal.lan>
References: <45CD5B26.5030707@eyal.emu.id.au>
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To: Eyal Lebedinsky <eyal@eyal.emu.id.au>
Cc: linux-raid list <linux-raid@vger.kernel.org>
List-Id: linux-raid.ids



On Sat, 10 Feb 2007, Eyal Lebedinsky 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)
>
> 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/>
> 	attach .zip as .dat
> -
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>

As you add disks onto the PCI bus it will get slower.  For 6 disks you 
should get faster than 2MB/s however..

You can try increasing the min speed of the raid rebuild.

Justin.