Generic volume migration was added to Cinder during the Liberty cycle.  This allows the RBD driver to participate in volume migration using the "generic" method where each block is read from the source volume and written to the target volume.  Although this gives customers the ability to migration volumes where previously they had no options, the block-for-block copy is more work than needed when both backends (source and target) reside on the same Ceph cluster.  In this case, an optimized migration methond can be implemented to take advantage of the underlying storage knowlege.  We should do this.