[Sheepdog] VDI locking, migration and qemu

[Chris Webb]

Hi!

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?

> Therefore, If crashed qemu can be detected by a local cluster daemon,
> we can monitor VMs properly.

Yes, I agree. This is the sort of thing I was suggesting with my 'heartbeat'
idea. It could also be done with (for example) a fcntl lock, or an open
pipe, or an open stream connection: anything that is guaranteed to go away
if the qemu process goes away.

Best wishes,

Chris.