Re: [Qemu-devel] [PATCH] block: Flush image after open

[Kevin Wolf]

Am 21.03.2011 13:23, schrieb Avi Kivity:
> 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.

Is it true for real hardware? Consider a reboot, you could still have
some data in a volatile disk write cache if the OS that ran before the
reboot hasn't flushed it.

Kevin