Cinder Multi-Backend GOTCHA

When making puppet changes to handle OpenStack Cinder with multi-backends, we created a really painful problem. The multi-backend basically moves our Cinder (with ceph backend) from basically a default name of our cluster (like cluster01) to a backend specific name (cluster0@ceph) so that we can add additional backends (like cephssd and solidfire and 3par).

Unfortunately, this had the really bad side-effect of dropping the "host" that was providing just "cluster01" service. All attached volumes continued to work fine. However, it became impossible to launch new instances with these volumes, terminate (properly) instances with these volumes, or delete these older volumes.

The fix (once you understand the problem) is very straightforward:
cinder-manage volume update-host --currenthost cluster01 --newhost cluster01@ceph

NOTE: Don't do this unless these are really the same services. Presumably safe if you have a single cinder backend and are prepping for multi-backend.