[linux-lvm] Status of LVM snapshot merging

[Richard Shaw]

On Thu, May 13, 2010 at 12:02 PM, Mike Snitzer <snitzer at redhat.com> wrote:
> On Thu, May 13 2010 at 11:52am -0400,
> Richard Shaw <hobbes1069 at 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.

I only did google searching so I didn't get to the source level...

> 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.

That explains it for me. I'm running Fedora 12 so my current
kernel/lvm versions are 2.6.32 and 2.02.53 respectively. It looks like
Fedora 13 will have the required versions, but I actually would like
the merge option in order to upgrade to F13 beta with the ability to
roll back.

>> 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.

Thanks for the info! I may get brave and try to install the F13 kernel
and lvm2 package and see if I can get away with it. I will have to
update udev as well, right?

Thanks,
Richard