[linux-lvm] Snapshots of snapshots are not supported yet

[Zdenek Kabelac]

Dne 10.7.2013 05:45, markw at mohawksoft.com napsal(a):
> One more annoying question, if you have the patience.
>
> Suppose I create a thin provisioned volume, say disk0.
>
> I take a snapshot of disk0 and name it disk0_snap0
> After a number of changes I then take another snapshot, and call it
> disk0_snap1
>
> Is the thin provisioning stuff smart enough to realize that disk0_snap0
> should be rewired to reference disk0_snap1 as its origin? It would look
> like this:
>
> disk0 -> disk0_snap1 -> disk0_snap0
>
> What I would like to see is something like this:
>
> disk0 -> disk0_snap2 -> disk0_snap1 -> disk0_snap0
>
> Create disk0_snap3
>
> disk0 -> disk0_snap3 -> disk0_snap2 -> disk0_snap1 -> disk0_snap0
>
> Where all writes to disk0 result in CoW to disk0_snap3 and the remaining
> snaps remain unchanged.
>
> Here is the kicker, I want to perform a differential backup on disk0_snap3
> and disk0_snap2.  Suppose I already recreated disk0_snap2 on some server.
> I now want to update it to disk0_snap3. I need to get a block list from
> disk0_snap2 and disk0_snap3, then generate a list of blocks needed to
> permute snap2 to snap3.
>
> Any info?
>

Yes - differential snapshot will be supported through thin provisioning
target - where you will be able to make a simple diff just by reading
metadata - it will be essential piece of replication.

AFAIK Joe has this in plan for some time - and there was even some 
announcement from some third-party developer to support this.

There is not going to be any upstream support for doing this with
old-snaps in foreseeable future.

Also keep in mind your idea of using old-snap of snap of snap would be very 
slow and fragile to use.

Zdenek