Knowledgebase: Using disk drives
Running two disks as parallel Virtual Libraries
Posted by Sven Koester on 09 March 2010 15:54
This article describes the configuration to use two hard disks as parallelized Virtual Jukeboxes

When to parallelize multiple jukeboxes:
Please follow this article in case you want to backup to multiple hard disk volumes that remain connected to the host. The reason for this setup (aside of its speed advantage) is a technical limitation in the PresSTORE jobs: Once started, a job cannot dynamically switch to another drive. So when writing to jukebox 1 and all media are full, PresSTORE will not continue on its own on the other jukebox, even if there is plenty of space left there.

In case you want to use alternating hard disks, please refer to the following KB article instead: Virtual jukeboxes and removable disks

Parallelizing multiple disks:
  • increases the size of the virtual jukebox, so more data can be saved.
  • may speed up the backup, namely when the hard disks are slow (like external USB disks) and make a performance bottleneck 
  • allows further use of smaller (often older) hard disks
How to parallelize:
  • Setup the virtual jukebox(es) normally.
  • Label the virtual tapes equally on both jukeboxes. If the tapes are already labeled, just keep them. There is no need to relabel tapes and delete the data. [Note: In case you plan to use multiple media pools, one for each jukebox, no special setup is required, just run the jobs that write to either jukebox in parallel].
  • Then please enter the Media Pool Manager and edit the Media pool settings. On the General Setup page, please enter in the Drives area at Use listed drives only the first Drive of each jukebox. Usually these are named drive_1.1and drive_2.1.
  • In the parallelizing options, change the number of drives per stream from the default value 1 to 2. This allows PresSTORE to use two drives at a time, and the previous step limited that to one drive on each jukebox.
That's all.

When a job is started that is intended to write to the media pool, it will attempt to find empty tapes in both jukeboxes and attempt to write with both drives. In case there are writable tapes in one jukebox only,  PresSTORE will write only on that one. 

PresSTORE's virtual jukeboxes contain two virtual drives, so by default they allow reading and writing the data at the same time. Parallelizing reduces the usable drives to one per virtual jukebox. In some cases, e.g. when creating Synthetic backups, multiple drives are required. In that case disks cannot be parallelized as described here.
Another limitation that may arise is that when one jukebox becomes full, PresSTORE will wait for new tapes in that Jukebox, but it cannot switch the files already sent to that jukebox to the other one. So for a flawlessly running backup we recommend to use two equal sized jukeboxes and to fill them parallel only, so the occasion where one of the two jukeboxes become full remain rare. In case both jukeboxes become full.... you still could add a third one.

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