Re: Win2003 disk corruption with kvm-1.0. and virtio

[Sylvain Bauza <sylvain.bauza@digimind.com>]

Long lasting bug and huge update, but I think I got the root cause.
FYI, Windows 2003 is having a write cache enabled by default on disk 
drivers. Even with virtio (see driver details, policies).

As a consequence, any DLL which is open could be corrupted if we try a 
simple 'qemu-img convert' against the VM.
The proper way to do a live snapshot is to disable the writecache (and 
goodbye good perfs!) and do the convert.
The other way is to stop the VM, perform a 'qemu-img snapshot', then 
convert the snapshot.

Hope it can help other people.
-Sylvain


Le 14/02/2013 11:11, Sylvain Bauza a écrit :
> Interesting point you mention. Even if qcow2 is read only, the image 
> is changing (especially, I'm running IIS with ASP support and VB DLLs) 
> while the snapshot is taken.
>
> As asked in a second post, I'm running with latest Windows virtio 
> drivers, but I only apply a virtio driver update *after* running an 
> instance, not before taking the snapshot.
>
> What I'll try : run an instance, update the driver, stop the instance, 
> do a qemu-img convert once the instance is stopped.
>
>
> Le 14/02/2013 09:15, Stefan Hajnoczi a écrit :
>> On Wed, Feb 13, 2013 at 10:53:14AM +0100, Sylvain Bauza wrote:
>>> As per documentation, Nova (Openstack Compute layer) is doing a
>>> 'qemu-img convert -s' against a running instance.
>>> http://docs.openstack.org/trunk/openstack-compute/admin/content/creating-images-from-running-instances.html 
>>>
>> That command will not corrupt the running instance because it opens the
>> image read-only.
>>
>> It is possible that the new image is corrupted since qemu-img is reading
>> from a qcow2 file that is changing underneath it.  However, the chance
>> is small as long as the snapshot isn't deleted while qemu-img convert is
>> running.
>>
>> So this doesn't sound like the cause of the problems you are seeing.
>>
>> Stefan
>