Same question here - I have a running production VM, and I would like to try an OS upgrade by cloning the whole VM (without shutting it down, obviously) and trying the upgrade on a newly created throwaway clone.
So far I was able to click on “Save as” on all the disks of that production VM (which itself is not atomic, Save As takes a lot of time, so the inter-dependencies between the disks are obviously broken). And then create a new VM on top of these disks.
I think ONe should be able to use snapshots for this - AFAIK CEPH supports writable snapshots, and the snapshot can be created almost instantly. Can I have a disk which is USED_PERS by another VM, and attach its snapshot to a different VM?