linux-kernel - Re: [OOPS] 2.6.21-rc6-git5 in cfq_dispatch

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Date:	Wed, 25 Apr 2007 13:54:48 +0400
From:	Brad Campbell <brad@...p.net.au>
To:	Neil Brown <neilb@...e.de>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [OOPS] 2.6.21-rc6-git5 in cfq_dispatch_insert

Neil Brown wrote:
> On Wednesday April 25, jens.axboe@...cle.com wrote:
>> BUT... That may explain while we are only seeing it on md. Would md
>> ever be issuing such requests that trigger this condition?
> 
> Can someone remind me which raid level(s) was/were involved?

Raid-5 gegraded here, But I've had it on my server on a fully functioning raid-5 also. Don't think 
I've seen it on any of my raid-6 yet.

> I guess it could happen with raid4/5/6.  A read request that was
> properly aligned (and we do encourage proper alignment) will be passed
> directly to the underlying device.  A concurrent write elsewhere could
> require the same block to be read into the stripe-cache to enable a
> parity calculation.  So you could get two reads at the same block
> address.

Except all the filesystems I've had this occur on are Ext3 and all are mounted noatime, with no 
processes ever writing to the filesystems. So here it occurs with reads only.

Brad
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