linux-kernel - Re: RAID extremely slow

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Date:	Thu, 26 Jul 2012 15:36:57 -0700
From:	Kevin Ross <kevin@...ilyross.net>
To:	Phil Turmel <philip@...mel.org>
CC:	linux-kernel@...r.kernel.org,
	linux-raid <linux-raid@...r.kernel.org>
Subject: Re: RAID extremely slow

On 07/25/2012 10:00 PM, Kevin Ross wrote:
>
>>>
>>>> unused devices:<none>
>>>>
>>>> # cat /proc/sys/dev/raid/speed_limit_min
>>>> 10000
>>> MD is unable to reach its minimum rebuild rate while other system
>>> activity is ongoing.  You might want to lower this number to see if 
>>> that
>>> gets you out of the stalls.
>>>
>>> Or temporarily shut down mythtv.
>>
>> I will try lowering those numbers next time this happens, which will 
>> probably be within the next day or two.  That's about how often this 
>> happens.
>
> Unfortunately, it has happened again, with speeds at near zero.
>
> # cat /proc/mdstat
> Personalities : [raid6] [raid5] [raid4]
> md0 : active raid6 sdh1[0] sdd1[9] sde1[10] sdb1[6] sdi1[7] sdc1[4] 
> sdf1[3] sdg1[8] sdj1[1]
>       6837311488 blocks super 1.2 level 6, 512k chunk, algorithm 2 
> [9/9] [UUUUUUUUU]
>       [=>...................]  resync =  8.3% (81251712/976758784) 
> finish=1057826.4min speed=14K/sec
>
> unused devices: <none>
>
> atop doesn't show ANY activity on the raid device or the individual 
> drives.
> http://img687.imageshack.us/img687/2913/screenshotfrom201207252.png
>
> Also, I tried writing to a test file with the following command, and 
> it hangs.  I let it go for about 30 minutes, with no change.
>
> # dd if=/dev/zero of=test bs=1M count=1
>
> dmesg only reports hung tasks.  It doesn't report any other problems.  
> Here's my dmesg output:
> http://pastebin.ca/2174778
>
> I'm going to try rebooting into single user mode, and see if the 
> rebuild succeeds without stalling.
>
> -- Kevin

It rebuilt fine in single user mode, with speeds usually around 
50MB/sec.  But after exiting single user mode, and allowing MythTV and 
other programs to start, within 30 minutes I had the problem again.  
Basically a hung filesystem.  I couldn't even "cat /proc/mdstat", that 
just hung.  Lots of hung task warnings in dmesg.

Because Phil suggested that fsync calls might cause stalls, I commented 
out the fsync in MythTV.  I'll run with that for awhile, and see how 
things work out.  So far it isn't adversely affecting MythTV.

Thanks!
-- Kevin

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