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Re: [Qemu-devel] Re: Notes on block I/O data integrity

From: Jamie Lokier
Subject: Re: [Qemu-devel] Re: Notes on block I/O data integrity
Date: Wed, 26 Aug 2009 19:57:55 +0100
User-agent: Mutt/1.5.13 (2006-08-11)

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

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