Knowledgebase: Volume handling
Handle clones
Posted by Sven Koester, Last modified by Andre Kuehnemund on 19 September 2012 14:21

Cloning is a mechanism to maintain a duplicate volume, thus increasing data availability. In case the original volume becomes unreadable, the data can still be recovered from its clone. PresSTORE supports pool cloning.

Pool cloning means that a second media is written in parallel at the same time. Pool cloning is set up via web interface in the page Media Pools->yourpool->General Setup. Check the box Clone data and select the drive(s) to use for the regular volume and for the cloned volume. Pool cloning is possible only in case you have at least two drives. Volumes generated by Pool cloning are identical, the same files are stored on both volumes. All volumes, the clone and the regular one belong to the same media pool.

Plan cloning was an older variant of the cloning capability that has been discontinued in PresSTORE version 3 and up.

Customers who only have a single tape drive may want to consider archiving the same data twice. The resulting tapes won't be clones. However, just as with cloning, the benefit would be to have to tapes (or sets of tapes) of the same archived data. If one tape or tape set gets lost or becomes unreadable, one would still have the other tape or tape set to recover data from. 

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Comments (9)
Jason Whetstone
28 March 2014 15:26
Can you tell me what happens if I turn on pool cloning after an archive pool has already been started? We have a pool of about 40 archive tapes. We want to enable cloning on the pool. I understand that it will do nothing as far as cloning previous tapes. What I would like to know is that if I have two partially written tapes already started in the archive pool, and turn on pool cloning now, how would P4 handle the cloning? Would it begin cloning immediately or once the next volume is started?
Andre Kuehnemund
07 April 2015 12:18
Cloning would start with the next new pair of tapes. Cloning is designed to create to create two identical tapes or sets of tapes - and therefore has to start with at least two empty tapes.
Mike Goslin
05 May 2015 21:30
how do I separate the clones from the primary tapes in a very large tape pool (i.e. not manually). The goal is to store the clones off-site.
Andre Kuehnemund
20 August 2015 10:32
One idea would be to partition a tape library into two separate libraries, then (in the pool settings) list the tape drive from the first library under 'Use listed drives only:' and the other tape drive under 'Drives for cloning:'.
Mike Goslin
25 September 2015 4:39
The two partition approach is a good one, but it does not work for our situation.

In addition to the backup job, there are several archive jobs. For the archive jobs, only the clone tape is removed and stored off site. The master tape remains in the library as long as we have space. Thus, over time, having two or more partitions will leave the tape library with lots of empty slots in one partition and very full in another partition. Because large tape libraries are licensed by the active slot, this approach would get very expensive, very fast. We needed a solution that optimized the capacity and drive usage of the tape library.

Our solution (for anyone facing this dilemma) was to make a copy of the database using the "Download support Data" feature. Then we run a SQL query against the database copy (never do this to your live database). The output of the query is then parsed by a shell script to find matching "copy of" records. Each pair of "copy of" records are split into two text files. Those text files form the "Master" and "Clone" tape lists. It is a bit of a process, but it gets the job done.
Camille Darley
05 November 2015 13:35
Is there a way to generate a clone afterwards? Or to declare a volume as a clone of another, and then use the re-generate function?
Andre Kuehnemund
20 January 2016 13:38
Camille: It is not possible to retroactively clone an existing tape. The reason being that there is at least the potential of the new "clone" tape to be able to hold less data than the original tape. If that were to happen, you would need a second "clone" tape in order to finish the cloning process. You would end up with two clone tapes for one original tape - and as a result you wouldn't have a one-on-one master/clone relationship.
When using cloning the way it is meant to be used, P5 will write to both tapes simultaneously. When the first tape is full (doesn't matter which one of the two), the other tape will be closed as well. That way, both tapes end up being identical clones.
Mikael Nordenström
21 March 2016 15:44
So, how would you know which tapes are clones for offsite storage?
Mike Goslin
12 July 2016 18:36
We had that problem as well. The solution is a bit messy and not officially approved, but it works. See the last paragraph of my comment from 24 September 2015 19:39. I can add detail if you are interested.
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