[dm-devel] [RFC] Multiple Snapshots - Manageability problem

[Mike Snitzer]

On 1/12/07, Benjamin Marzinski <bmarzins at redhat.com> wrote:
> On Thu, Jan 11, 2007 at 11:18:13AM -0700, Vijai Babu Madhavan wrote:

> > I would like to receive some comments about what users, developers
> > and others think about this.
> >
>
> Have you taken a look at Daniel Phillips cluster snapshot work?
>
> http://sources.redhat.com/cluster/csnap/index.html
>
> The code is not complete, and am not sure if Daniel is doing any work on it at
> all, but it has a nice design to store the cow data, and that URL contains the
> design documents. In brief:
>
> There is one device that stores all cow data (the snapstore). It has three
> main parts, an allocation bitmap, a superblock that stores metadata, and
> an exception btree. The exception btree is indexed by the location of the data
> on the origin. For each chuck on the origin device that has cow data for one or
> more snapshots, there is an exception in the btree that lists the location of
> the cow data on the snapstore device, and the snapshots which are using that
> exception.  This list of snapshots is stored as a bitmask.
>
> This means that no matter now many snapshots you have, all you need to do to
> write to the origin is check the btree.

Daniel's work has become an integral part of the hotcakes project:
http://code.google.com/p/hotcakes/

There was/is talk of making the dm targets used for ddsnap work
locally as a replacement for dm-snapshot.  It might be wise to catch
up with the hotcakes people to see if you could leverage it as the
basis for a dm-snapshot++

Mike