> Hi
>
> I'd like announce beta version of snapshot merging. The patches can be
> downloaded at
http://people.redhat.com/mpatocka/patches/ The patches are
> against kernel 2.6.26, device-mapper.1.02.27 and LVM2.2.02.39.
>
> The snapshot merging allows you to merge snapshot content back into the
> original device. The most useful use for this feature is the possibility
> to rollback state of the whole computer after failed package upgrade,
> administrator's error or so.
>
> Merging of a snapshot is initiated with command "lvconvert -M
> vg/lv_snapshot". lvconvert polls for the termination of merging and then
> automatically removes the merging snapshot. If the computer crashes, the
> merging resumes after a crash and background polling is restarted too
> (from lvchange -ay or vgchange -ay command).
>
> While the merging is active, any accesses to the origin device are
> dicrected the snapshot that is being merged. When the merging finishes,
> the origin target is seamlessly reloaded and the merging snapshot is
> dropped. The filesystem can stay mounted during this time.
>
> There are three types of merging:
>
> --nameorigin (default) - the resulting logical volume will have name,
> minor number and UUID of the original origin volume. When this mode is
> used, neither snapshot nor the origin can be mounted when the merging
> starts. (but you can mount the origin immediatelly after you start
> merging, you don't have to wait until it finishes)
>
> --namesnapshot - the resulting logical volume will have name, minor number
> and UUID of the snapshot to be merged. When the merging starts, the
> snapshot can be mounted and the origin cannot be mounted.
>
> --onactivate - marks the snapshot for merge on next activate (when
> lvchange -ay or vgchange -ay changes the state from inactiva to active),
> but doesn't do actual merging. This option is useful if you need to merge
> over a filesystem that cannot be unmounted (for example root) --- use
> lvconvert -M --onactivate and reboot the computer. On next reboot, the
> merge will start and the system will run with root filesystem
> corresponding to the snapshot that is being merged.
>
> You can have multiple snapshots before you start merging, any existing
> snapshots are maintained stable (i.e. new exceptions to them are allocated
> if the merging modifies the origin). The exception is --namesnapshot
> option which requires that you have only one snapshot. While the merging
> is in progress, new ssnapshots cannot be created.
> For example:
> - take snapshot before system-wide package update
> - do update
> - now suppose that you don't like the result of the update:
> - take another snapshot
> - initiate -M --onactivate with the first snapshot
> - reboot
> - after reboot you are running with root filesystem in the state before
> the update and you can examine the result of the update on the second
> snapshot. If you decide that you like the update anyway, you can merge it
> back again (with reboot).
> (don't forget to regenerate lvm on your initramdisk if you are going to
> try this on root filesystem).
>
>
> This implementation of snapshot merging is meant to be stable, report any
> possible bugs to me.
