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Date: Tue, 20 May 2008 19:58:34 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off
	(O_DIRECT)
References: <1211283126.4314.70.camel@frecb07144>	<48332AB9.3010707@codemonkey.ws>	<20080520223602.GE27853@shareable.org>
	<200805202352.17807.paul@codesourcery.com>
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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>

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.

Regards,

Anthony Liguori

> Paul
>
>
>