Comment 1 for bug 1665272

I've looked into this and it seems more involved than I had first thought. The requirement to use enabled_backends causes volumes to be created with a different os-vol-host-attr:host. If a cloud is upgraded to Ocata any existing volumes will have an invalid os-vol-host-attr:host. The existing volumes need to have that attribute updated via cinder-manage *1

*1 https://docs.openstack.org/admin-guide/blockstorage-multi-backend.html#enable-multiple-storage-back-ends