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[Andiabes <andi.abes@xxxxxxxxx>]

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. 

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

On Jul 20, 2011, at 5:27 PM, Chuck Thier <cthier@xxxxxxxxx> wrote:

> Yeah, I think you are illustrating how this generates much confusion :)
> 
> To try to be more specific, the base functionality should be:
> 
> 1. Create a point in time backup of a volume
> 2. Create a new volume from a backup (I guess it seems reasonable to
> call this a clone)
> 
> This emulates the behavior of what EC2/EBS provide with volume
> snapshots.  In this scenario, a "restore" is create a new volume from
> the backup, and delete the old volume.
> 
> In the Storage world, much more can generally be done with snapshots.
> For example in most storage system snapshots are treated just like a
> normal volume and can be mounted directly.  A snapshot is often used
> when creating a backup to ensure that you have a consistent point in
> time backup, which I think most of the confusion comes from.
> 
> What we finally call it doesn't matter as much to me, as long as we
> paint a consistent story that isn't confusing, and that we get it in
> the Openstack API.
> 
> --
> Chuck
> 
> On Wed, Jul 20, 2011 at 3:33 PM, Vishvananda Ishaya
> <vishvananda@xxxxxxxxx> wrote:
>> In rereading this i'm noticing that you are actually suggesting alternative usage:
>> 
>> backup/clone
>> 
>> snapshot/restore
>> 
>> Correct?
>> 
>> It seems like backup and snapshot are kind of interchangable.  This is quite confusing, perhaps we should refer to them as:
>> 
>> partial-snapshot
>> 
>> whole-snapshot
>> 
>> or something along those lines that conveys that one is a differencing image and one is a copy of the entire object?
>> 
>> On Jul 20, 2011, at 12:01 PM, Chuck Thier wrote:
>> 
>>> At the last developers summit, it was noted by many, that the idea of
>>> a volume snaphsot in the cloud is highly overloaded.  EBS uses the
>>> notion of snapshots for making point in time backups of a volume that
>>> can be used to create a new volume from.  These are not true snapshots
>>> though from a storage world view.  Because of this I would like to
>>> make the following proposal:
>>> 
>>> Add a backup API to the Openstack API for Nova Volume.  This is to
>>> provide EBS style snapshot functionality in the Openstack API.  I'm
>>> proposing to name it backup instead of snapshot as that seems to
>>> better describe what is happening.  It also allows room for other
>>> storage backends to expose real snapshot capabilities down the road.
>>> 
>>> In the case of Lunr, we would be making backups of volumes to swift
>>> (possibly abstracted through glance in the future).
>>> 
>>> I have started a blueprint and spec at:
>>> 
>>> https://blueprints.launchpad.net/nova/+spec/backups-api
>>> http://etherpad.openstack.org/volume-backup
>>> 
>>> Please feel free to comment and contribute.
>>> 
>>> --
>>> Chuck
>>> 
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>> 
>> 
> 
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