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Date: Thu, 27 Aug 2009 00:17:22 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] Re: Notes on block I/O data integrity
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To: Jamie Lokier <jamie@shareable.org>
Cc: rusty@rustcorp.com.au, Javier Guerra <javier@guerrag.com>, Christoph Hellwig <hch@lst.de>, kvm@vger.kernel.org, qemu-devel@nongnu.org

On Wed, Aug 26, 2009 at 07:57:55PM +0100, Jamie Lokier wrote:
> Christoph Hellwig wrote:
> > > what about LVM? iv'e read somewhere that it used to just eat barriers
> > > used by XFS, making it less safe than simple partitions.
> > 
> > Oh, any additional layers open another by cans of worms.  On Linux until
> > very recently using LVM or software raid means only disabled
> > write caches are safe.
> 
> I believe that's still true except if there's more than one backing
> drive, so software RAID still isn't safe.  Did that change?

Yes, it did change.  That beeing said with the amount of bugs in
filesystems realted to write barriers my expectation for the RAID
and device mapper code is not too high.  I will recommend to keep
doing what people caring for their data have been doing since these
volatile write caches came up:  turn them off.