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Date: Mon, 21 Mar 2011 14:23:19 +0200
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] block: Flush image after open
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org

On 03/09/2011 07:38 PM, Anthony Liguori wrote:
> On 03/09/2011 11:27 AM, Christoph Hellwig wrote:
>> On Wed, Mar 09, 2011 at 05:15:53PM +0100, Kevin Wolf wrote:
>>> Quoting the bug report:
>>>
>>>      qemu ensures that guest writes and qemu metadata writes hit the 
>>> disk
>>>      when necessary to prevent data corruption. However, if an image 
>>> was
>>>      in host pagecache prior to starting qemu, for example after 
>>> running
>>>      qemu-img convert, then nothing prevents writes from reaching the
>>>      disk out of order, potentially causing corruption.
>>>
>>> I'm not entirely sure if there is a realistic case where we would get
>>> corruption, but it's probably a case of better safe than sorry.
>> Except for SCSI with ordered tags (which we don't support) there are not
>> ordering guarantees in the storage protocols, and as such the above 
>> explanation
>> doesn't make any sense at all.
>
> Even if there was, a guest shouldn't be relying on the ordering of a 
> write that comes from a non-guest.
>
> I don't understand the failure scenario here.

   $ cp x.img y.img
   $ qemu -drive file=y.img,cache=writeback
<read something from disk, send it over the network>
<no guest flushes>
<host crash>

The guest may expect that any or none of its writes hit the disk, but 
that anything that it read from the disk, stays there.

-- 
error compiling committee.c: too many arguments to function