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Different Snapshot Time, Persistent vs Ephemeral Booted

asked 2018-10-03 14:28:59 -0500

Mrxlazuardin gravatar image


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,

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answered 2018-10-03 18:50:06 -0500

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.

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Hi Bernd,

I just following How to make that ephemeral storage use RBD snapshot for doing instance snapshot like on persistent storage (volume) and use RBD snapshot as new item on image store without having to do long time copying.

Best regard

Mrxlazuardin gravatar imageMrxlazuardin ( 2018-10-06 16:22:12 -0500 )edit

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Asked: 2018-10-03 14:28:59 -0500

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Last updated: Oct 03 '18