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Date: Wed, 21 May 2008 02:04:33 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off
	(O_DIRECT)
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To: qemu-devel@nongnu.org
Cc: Blue Swirl <blauwirbel@gmail.com>, Laurent Vivier <Laurent.Vivier@bull.net>, Kevin Wolf <kwolf@suse.de>

Anthony Liguori wrote:
> Paul Brook wrote:
> >>When sector-aligned guest offsets are converted to sector-unaligned
> >>writes (e.g. due to qcow2 etc.), that property is no longer satisfied,
> >>and power failure of the host disk can cause more damage than the
> >>guest is designed to be resistant to.
> >>    
> >
> >Seems like the easiest solution would be to have qcow always align its 
> >writes.
> >We don't do on the fly compression, so it should be fairly easy to make 
> >this happen with minimal overhead.
> >  
> 
> That's not sufficient. O_DIRECT imposes not only offset alignment 
> requirements but also requirements on the buffer being read to. Most of 
> the code in QEMU does not properly align the read/write buffers.

The offset when reading is not so important.  And of course, can't be
guaranteed with qcow2 - as the sector being read may be compressed.

For writing, if the memory isn't suitably aligned for the
_transformed_ offset, you can either transform it differently, or copy
the memory to somewhere with the right offset first.

-- Jamie