Re: [linux-lvm] Status of LVM snapshot merging

[Mike Snitzer <snitzer@redhat.com>]

On Thu, May 13 2010 at 11:52am -0400,
Richard Shaw <hobbes1069@gmail.com> wrote:

> There does not seem to be much information available on the status of
> LVM snapshot merging available. I looked at the patches available from
> the original author and both my kernel and LVM versions are newer than
> what he provides patches for.

I'm not sure where you're looking for status but both the upstream
linux-2.6 and LVM2 sources are quite public.

The snapshot-merge target support was included in Linux v2.6.33.
The corresponding LVM2 support was fully baked with v2.02.62, there were
also 2 fixes in v2.02.64.

> In lieu of the feature being available, does anyone know if it would
> be possible to use rsync to effectivly merge a snapshot volume with
> its origin volume?
> 
> If so, my plan was to try an extensive upgrade of my system on the
> snapshot volume. If all goes well, I would then rsync the snapshot
> back to the origin, update my fstab, and remove the snapshot. If it
> does not go well, that's the easy part, just drop the snapshot and go
> back to the origin.

I'm not going to comment on using rsync as a poor-man's rollback
mechanism.  The snapshot-merge support is widely available so I think
it'd be great if you gave it a try.

So rsync aside, the problem with your proposed approach is two-fold:
1) having your package manager install into the snapshot LV.
2) getting your system to boot off the snapshot LV.  

These aren't insurmountable problems but they would need to be overcome
and things _could_ get dicey.

Generally the process is inverted: you make the changes to the system
(after having created a snapshot).  If you don't like the changes you
use 'lvconvert --merge ...' to merge the snapshot back to origin on the
next reboot (reboot needed for the root LV).  This gives you instant
rollback.

Regards,
Mike