openstack team mailing list archive

[Avishay Traeger <AVISHAY@xxxxxxxxxx>]

John Griffith <john.griffith@xxxxxxxxxxxxx> wrote on 11/11/2012 17:01:37:
> Hey Avishay,
>
> >>I guess I'm still confused about what a snapshot is
> >>in OpenStack.  Currently you can't really do anything with them via
> >>OpenStack.
> Sure you can, you can use them to create a new volume.  They make a
> good backup mechanism IMO.

Right - you can either make a new volume (=clone) or figure out somehow
what name Cinder gave to the snapshot and make a backup on your own.

[ Off topic question: How can a user determine the name of a
volume/snapshot on the back-end for a given volume/snapshot? ]

> I guess my question to you in your definition then is 'what's the
> difference between a snapshot and a clone'?

IMHO, a clone is a full, independent copy of the volume.  It would copy all
data (either in foreground or background) and leave the user with a volume
- as if he created a new volume and used dd to copy all the data.  A
snapshot is something that depends on the original volume and in most
implementations would contain only changed blocks.

"Clone" definitely has other meanings - what's your definition?

> Also, it Seems to me if we go with the idea of adding snapshot-
> restore, and a true "clone" feature you get everything that you're
> asking for here and more... so I'm not sure of the problem?  Maybe
> you could help me understand why defining snapshot and clone in the
> manner described doesn't seem appropriate?

So as I stated above, clone would give me a full copy, which can be
wasteful.  I should be able to read and write to snapshots within the
context of Cinder.

> FWIW, I think a R/O attach is something that would be good to have
> as an option regardless.

It's a necessity IMO.  R/W is less common, but also very important.
It's obvious that Cinder can't support features found in some storage
controllers, but even LVM2 supports R/W snapshots (released almost 10 years
ago).  Don't get me wrong - LVM is awesome - but if ubiquitous,
freely-available software supports a feature that controllers also
generally support, I think Cinder should support it too.

> Thanks,
> John

Thank you,
Avishay