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Different Snapshot Time, Persistent vs Ephemeral Booted


On our OpenStack + Ceph infrastructure, we find that the snapshot time between VMs which are booted from volume (persistent) are so fast due to it is just to do Ceph snapshot. But, for VMs which are booted from ephemeral are so long, they take some minutes, depends on the size of ephemeral disk. As long as I know, ephemeral booted VMs are snapshots from base images on Ceph. Why those kinds of VM have different snapshot time? Is it about different method of snapshot? Why the ephemeral booted VMs take uploading time where persistent booted VMs don't do that? It seem that ephemeral booted VMs snapshot is done on compute nodes but persistent booted VMs snapshot is done on Ceph itself, isn't it?

Best regards,