The Cinder Volume REST API should, at the very least, catch VolumeInUse and return an HTTP 409 Conflict status code, which the cinderclient, called via nova-compute, can translate to a non-500 Internal Server Error exception.
The Cinder Volume REST API should, at the very least, catch VolumeInUse and return an HTTP 409 Conflict status code, which the cinderclient, called via nova-compute, can translate to a non-500 Internal Server Error exception.