Comment 5 for bug 1532884

If we use option "--migration-policy on-demand" we'll get:

$cinder retype --migration-policy on-demand 5643803e-ed2e-4ac6-ba1f-c5e666d013da ceph2
where 5643803e-ed2e-4ac6-ba1f-c5e666d013da is volume' id created on host with volume_backend_name =ceph

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/os_brick/initiator/linuxrbd.py", line 94, in __init__
    try:
  File "/usr/lib/python2.7/dist-packages/rbd.py", line 372, in __init__
    raise make_ex(ret, 'error opening image %s at snapshot %s' % (name, snapshot))
ImageNotFound: error opening image volume-f9b00cba-cbdb-4665-9a1b-2eb7616b59a3 at snapshot None

Failed to copy volume 5643803e-ed2e-4ac6-ba1f-c5e666d013da to f9b00cba-cbdb-4665-9a1b-2eb7616b59a3

And volume get migration_status=Error

All of the above is true with regard to patch fix https://review.openstack.org/#/c/266180/