Snapshot - Commit and Rollback

Group,
I am using LVM2 for my volume management and its working very fine.
Last day i try to take backup of the volumes using snapshot feature available. I have following queries regarding the snapshot working. After the snapshot is taken is there any way to update the contents (COMMIT) of the same snapshot with the changed contents in the share in a future point of time.
Also is there any feature like ROLLBACK to revert back the volume content with the contents of the nth snapshot.
As per now i have to store multiple snapshots for every changes happening in the volume, which is almost like wasting a hell lot of space.
If anyone put light on these and let me know where to refer to get more details on this will be highly appreciable.

Thanks in advance.

regards,
Rahul

Hi Group,
I have posted this mail regarding the SNAPSHOT operations in LVM2.
If anyone has any clue or comments please put forward. I am deliberately waiting for a reply from this group.

regards,
Rahul

COMMIT and ROLLBACK features are not currently implemented. They are certainly possible using device-mapper.

Here is an idea about ROLLBACK. Assume a primary LV 'P' and a snapshot 'S'. Any writes to 'P' copies the cluster to 'S' first. Any reads from 'S' get the data from 'P' unless it was copied to 'S'.

Now, for a rollback, create a new snapshot 'R' (in case we change our minds!). In addition, reads from 'P' fetch clusters from 'S' that were copied there, until that cluster is written on 'P'. 'P' now depends on 'S'.

To eventually get rid of 'S', we also have reads from 'P' that are mapped to 'S' also copy the cluster from 'S' back to 'P' (after copying from 'P' to 'R' of course). So simply by reading all of 'P', the clusters mapped to 'S' (the percentage displayed in lvs) can be reduced to 0, and 'S' can then be deleted.

This works even if 'S' is rw, but isn't a true rollback, of course, unless 'S' is ro. If 'S' is rw, we can call the above operation a 'COMMIT', and if we change our minds and revert to 'R', *that* is the 'ROLLBACK'.

	      Stuart D. Gathman <[email protected]>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
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