We have a several hundred VMs which were built on cinder block devices as root drives which use a SAN backend. Now we want to change their backend from the SAN to Ceph.

We can shutdown the VMs but we will not destroy them. I am aware that there is a cinder migrate volume command to change a volume’s backend, but it requires that the volume be completely detached. Forcing a detached state on

that volume does let the volume migration take place, but the volume’s path in Nova block_device_mapping doesn’t update, for obvious reasons.

So, I am considering forcing the volumes to a detached status in Cinder and then manually updating the nova db block_device_mapping entry for each volume so that the VM can boot back up afterwards. 

However, before I start toying with the database and accidentally break stuff, has anyone else ever done something similar? Got any tips or hints on how best to proceed?