Open Stack

This is in context to an ongoing review https://review.openstack.org/#/c/20333/   and comments on the review.

There are two cases which need to be considered for the topic of "Attached and Un-attached Volume migration".

Case 1: Is intended to be implemented by the above review . Libvirt api (migrateToURI)  takes care of correctly copying and syncing data between the original volumes (attached) on "cinder-node1"  and the new destination volumes on "cinder-node2", without taking the original volumes offline or detaching them before copying.

Case 2: Adding a separate command to migrate individual volumes that are un-attached could be useful in the following scenario:

"cinder-node1" contains 10 volumes , 2 volumes are attached to "instance1" on "compute-node1", To take down "cinder-node1", we can block migrate the "instance1" using this patch, which will migrate the 2 volumes to "cinder-node2", But the rest 8 volumes on "cinder-node1" need to be migrated too. Since taking down "cinder-node1" will render the remaining 8 volumes unusable. This is where individual volume migration can be used.
But , for all volumes on "cinder-node1" which are attached to "instance1", they cannot be simply live migrated individually or detached and then migrated.

There are two points to be observed here:
1) We cannot use LVM volume snapshots and recreate another volume on the new "cinder-node2", since LVM commands are specific to
the node.

2) We can use rsync to copy/sync between the original volume on "cinder-node1" and the destination volume on "cinder-node2".

3) The volume uuid will change, since we are creating a destination volume in which the source volume data will be copied, This might cause issue to users who are using the source volume uuid's in automated scripts etc.

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