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Date: Wed, 26 Aug 2009 19:57:55 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: Notes on block I/O data integrity
Message-ID: <20090826185755.GF25726@shareable.org>
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To: Christoph Hellwig <hch@lst.de>
Cc: rusty@rustcorp.com.au, qemu-devel@nongnu.org, kvm@vger.kernel.org, Javier Guerra <javier@guerrag.com>

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?

But even with barriers, software RAID may have a consistency problem
if one stripe is updated and the system fails before the matching
parity stripe is updated.

I've been told that some hardware RAID implementations implement a
kind of journalling to deal with this, but Linux software RAID does not.

-- Jamie