Re: [Qemu-devel] Re: [PATCH v3 5/5] qed: Consistency check support

[Kevin Wolf]

Am 28.10.2010 12:15, schrieb Stefan Hajnoczi:
> On Wed, Oct 27, 2010 at 4:52 PM, Kevin Wolf <address@hidden> wrote:
>> Am 22.10.2010 16:56, schrieb Stefan Hajnoczi:
>>> This patch adds support for the qemu-img check command.  It also
>>> introduces a dirty bit in the qed header to mark modified images as
>>> needing a check.  This bit is cleared when the image file is closed
>>> cleanly.
>>>
>>> If an image file is opened and it has the dirty bit set, a consistency
>>> check will run and try to fix corrupted table offsets.  These
>>> corruptions may occur if there is power loss while an allocating write
>>> is performed.  Once the image is fixed it opens as normal again.
>>>
>>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>>
>> Hm, do I understand right that you fix the image and reset the dirty
>> flag in the header during bdrv_open? So how does this work with
>> migration, when the destination host opens the QED file before the
>> source closes it? Doesn't the destination destroy the image by "fixing" it?
>>
>> And even if that wasn't the case, clearing the flag means that the
>> source might do new writes and thinks that the flag is still set. If the
>> source crashes now, we may need a consistency check, but the dirty flag
>> isn't set any more.
>>
>> Am I missing some detail?
> 
> You're right, migration is not supported.  This is also true for the
> other image formats which cache metadata in memory though.

Is this still broken in upstream? This is a shame.

I think the problem was that Anthony was opposed to reopening the image
because we might lose it on that occasion? Instead of possibly crashing
a VM in the worst case we regularly corrupt images now... *sigh*

> I am actually looking into migration next together with Adam Litke who
> has already started.  We'll have to defer accessing the file until the
> source has flushed/closed it.

Sounds to me like an ugly workaround for the fundamental problem that
the image file is opened twice at the same time. Ugly because it's not
even in a central place, but must be done for each image format separately.

Opening the file read-only first and reopening it read-write when the
migration has completed would be much better. No image format driver
would have to be changed for that.

Kevin