[Rdo-list] Creating nova instance without backing image

Hi, In our OpenStack installation (Havana, RDO), we use Ceph as storage as Nova, Glance and Cinder backend. On compute nodes, instance's disks are kept on Ceph but base images are copied from Glance (Ceph) to local disk. This works well with small images of OS's. However, if we upload a big image (e.g. migrating bare hardware system to the cloud) that is only used by one instance, the image becomes the backing file stored locally on the compute node. Here, the compute node hit disk space limit because we expect all the data to reside on Ceph. Is there any way to tell Nova to not use backing image for an instance? Maybe there exists a special image property to set that tells Nova to copy the whole image, which would put it entirely in Ceph without a backing file? I hope this use case makes sense for and I'd appreciate if you had any suggestions how to resolve this issue. Best, Daniel