After checking the source code for determining if Compute has a shared storage ( https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L3810 ), I doubt that in your case the 'instances_path' in nova.conf for the Compute hosts are different (assuming that the shared storage is correctly configured). If the 'instances_path' are the same, a file created on that path from source Compute must be available at same path on the destination Compute; in which case, 'mv' would be used instead of an 'rsync'.

Hope it helps!