Comment 1 for bug 1734838

The reason may be that, rbd image can not be deleted if it is in use by an instance, because it is a rbd snap, and an instance system disk is a cloned volume from the snap in rbd pool, if we call glance to delete the snap, "HTTPConflict: 409 Conflict: Image 9a993e71-71ca-490e-b3cb-0b4dca2e574c could not be deleted because it is in use: The image cannot be deleted because it is in use through the backend store outside of Glance. (HTTP 409)" will be returned.