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Is ceph used as ephemeral storage backend aswell? Because there is a problem when snapshotting with ceph as backend. Instead of relying on ceph snapshot, nova instead calls qemu for snapshots. So it first downloads the instance image on local storage then uploads it into glance. It's a long and bandwidght intense process. I read somewhere that this is fixed in liberty but i cannot confirm it because we do not use ephemeral storaget for our instances, only cinder volumes.

For a workaround, you could use cinder volumes backed by ceph rbd for instance boot volume. When taking a snapshot of an instance the process is intant, because of cephs copy on write future. So you will see no downtime on instance!