[Sheepdog] VDI locking, migration and qemu

[MORITA Kazutaka]

On 11/29/2009 12:45 AM, Chris Webb wrote:
> MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp> writes:
> 
>> Even read-only access is not allowed while another qemu is doing
>> write access. However, there is no problem about your patch, I guess.
>>
>> It is because Sheepdog allow us to clone images only from
>> snapshot images. It means a backing image is always read-only.
>> If users specify writable Sheepdog VDI as a backing image,
>> qemu-img returns error.
> 
> What about the case of (for example) a qcow2 file backed by a Sheepdog
> image? Is that also restricted to be a snapshot image (and therefore safe)
> or is there a potential problem there?

Yes, in this case, a backing Sheepdog image is restricted to a snapshot
image, too.

However, taking other formats into account, bdrv_claim may be required
even when opening as read-only. It is because qcow2 cannot prevent other
VMs write data to an opened backing image.
If so, bdrv_claim need more arguments such as open flags
to notify the opened image is read-only or not, right?

Regards,

MORITA Kazutaka