openstack team mailing list archive

[Clay Gerrard <clay.gerrard@xxxxxxxxxxxxx>]

> Andiabes
> Sent: Thursday, July 21, 2011 5:57 AM
> 
> I think vish pointed out the main differences between the 2 entities,
> and maybe that can lead to name disambiguation...
> 
> Backup is a full copy, and usable without the original object being
> available in any state ( original or modified). It's expensive, since
> it's a full copy. Main use cases are dr and recovery.
> 
> Snapshot represents a point in time state of the object. It's
> relatively cheap ( with the expectation that some copy-on-write or
> differencing technique is used). Only usable if the reference point of
> the snapshot is available (could be thought of as an incremental
> backup); what that reference point is depends on the underlying
> implementation technology. Main use case is rewinding to so a historic
> state some time in the future.
> 

I think this is a pretty good description of the fundamental differences in the characteristics of what have classically been called backups and snapshots.  IMO, EC2/EBS muddied the waters on the distinction between backups vs. snapshot in the cloud, and since I don't really *know* what they do in the background internally - I feel like OpenStack has an opportunity to do better.  There's a difference between a fast copy-on-write-ish volume "snapshot" and an archived disk-image-ish dump of a volume to an object store "backup".

> That said, with the prereqs met, both can probably be used to mount a
> new volume.

Exactly.  

> Reasonable?
>

So should the distinction be purely and implementation detail or will the OSAPI support a way to expose the difference to the consumer?  Naming aside, there's uses cases where one may be better suited to the task-at-hand than the other.

Clay Gerrard



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