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An instance snapshot is an image. When taking a snapshot, the disk is copied to an image that is uploaded to Glance's image store. You can then launch other instances from this saved image.

Ephemeral storage is normally implemented as a file on the compute node. If that is the case in your cloud, taking a snapshot from an instance with ephemeral storage requires copying the file to the Glance store. That can take a moment.

When you take a snapshot from an instance launched from a volume, this volume must also be copied to Glance. However, you can use Cinder as a Glance store, which allows creating an image with copy-on-write if the Cinder backend supports it.

From what you say, it seems that you use Ceph as Cinder backend and Cinder as Glance store. If not, I would be interested in your precise configuration, including your implementation of ephemeral storage.