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From: Neil Brown <neilb@cse.unsw.edu.au>
To: Andrew Morton <akpm@digeo.com>
Date: Tue, 18 Mar 2003 16:59:53 +1100
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Cc: gilbertd@treblig.org, linux-kernel@vger.kernel.org, ext3-users@redhat.com
Subject: Re: 2.4.20: ext3/raid5 - allocating block in system zone/multiple 1
 requests for sector
In-Reply-To: message from Andrew Morton on Monday March 17
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On Monday March 17, akpm@digeo.com wrote:
> Neil Brown <neilb@cse.unsw.edu.au> wrote:
> >
> > These two symptoms strongly suggest a buffer aliasing problem.
> > i.e. you have two buffers (one for data and one for metadata)
> > that refer to the same location on disc.
> > One is part of a file that was recently deleted, but the buffer hasn't
> > been flushed yet.  The other is part of a new directory.
> > The old buffer and the new buffer both get written to disc at much the
> > same time (hence the "multiple 1 requests"), but the old buffer hits
> > the disc second and so corrupts the filesystem.
> 
> This aliasing can happen very easily with direct-io, and it is something
> which drivers should be able to cope with.
> 
> I hope RAID is not still assuming that all requests are unique in this way?

No.  RAID copes.  If raid5 sees a write request for a block that it
already has a pending write request for, it will print a warning and
delay the second until the first complete.

In the cas in question I don't think raid5 is contributing to the
problem.  It is just provide extra information which might help point
towards the problem - i.e. it is confirming that some sort of aliasing
is happening.

NeilBrown
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