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In the Previous BP [1], support for iscsi backend is introduced into
glance. However, it was abandoned because of Cinder backend
replacement.

The reason is that all storage backend details should be hidden by
cinder, not exposed to other projects. However, with more and more
interest in "Converged Storage" like Ceph, it's necessary to expose
storage backend to glance as well as cinder.

An example  is that when transferring bits between volume and image,
we can utilize advanced storage offload capability like linked clone
to do very fast instant copy. Maybe we need a more general glance
backend location support not only with iscsi.



[1] https://blueprints.launchpad.net/glance/+spec/iscsi-backend-store