Re: xfs and raid5 - "Structure needs cleaning for directory open"

From: Doug Ledford <dledford@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Rainer Fuegenstein <rfu@kaneda.iguw.tuwien.ac.at>,
	xfs@oss.sgi.com, linux-raid@vger.kernel.org
Subject: Re: xfs and raid5 - "Structure needs cleaning for directory open"
Date: Mon, 17 May 2010 18:18:28 -0400	[thread overview]
Message-ID: <4BF1C0B4.5090009@redhat.com> (raw)
In-Reply-To: <20100517214532.GL8120@dastard>

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On 05/17/2010 05:45 PM, Dave Chinner wrote:
> On Mon, May 17, 2010 at 05:28:30PM -0400, Doug Ledford wrote:
>> On 05/09/2010 10:20 PM, Dave Chinner wrote:
>>> On Sun, May 09, 2010 at 08:48:00PM +0200, Rainer Fuegenstein wrote:
>>>>
>>>> today in the morning some daemon processes terminated because of
>>>> errors in the xfs file system on top of a software raid5, consisting
>>>> of 4*1.5TB WD caviar green SATA disks.
>>>
>>> Reminds me of a recent(-ish) md/dm readahead cancellation fix - that
>>> would fit the symptoms of (btree corruption showing up under heavy IO
>>> load but no corruption on disk. However, I can't seem to find any
>>> references to it at the moment (can't remember the bug title), but
>>> perhaps your distro doesn't have the fix in it?
>>>
>>> Cheers,
>>>
>>> Dave.
>>
>> That sounds plausible, as does hardware error.  A memory bit flip under
>> heavy load would cause the in memory data to be corrupt while the on
>> disk data is good.
> 
> The data dumps from the bad blocks weren't wrong by a single bit -
> they were unrecogniѕable garbage - so that it very unlikely to be
> a memory erro causing the problem.

Not true.  It can still be a single bit error but a single bit error
higher up in the chain.  Aka a single bit error in the scsi command to
read various sectors, then you read in all sorts of wrong data and
everything from there is totally whacked.

>> By waiting to check it until later, the bad memory
>> was flushed at some point and when the data was reloaded it came in ok
>> this time.
> 
> Yup - XFS needs to do a better job of catching this case - the
> prototype metadata checksumming patch caught most of these cases...
> 
> Cheers,
> 
> Dave.

-- 
Doug Ledford <dledford@redhat.com>
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	      http://people.redhat.com/dledford

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