Open Stack

> 
> The solution is conceptually simple.  We add a new API microversion in
> Cinder that adds and optional parameter called "generic_keep_source"
> (defaults to False) to both migrate and retype operations.
> 
> This means that if the driver optimized migration cannot do the
> migration and the generic migration code is the one doing the migration,
> then, instead of our final step being to swap the volume id's and
> deleting the source volume, what we would do is to swap the volume id's
> and move all the snapshots to reference the new volume.  Then we would
> create a user message with the new ID of the volume.
> 

How would you propose to "move all the snapshots to reference the new volume"?
Most storage does not allow a snapshot to be moved from one volume to another.
really the only way a migration of a snapshot can work across all storage types
would be to incrementally copy the data from a source to a destination up to
the point of the oldest snapshot, create a new snapshot on the new volume, then
proceed through until all snapshots have been rebuilt on the new volume.