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03 Sep 2010

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	Understanding Virtual Jukeboxes

Solution When using virtual jukeboxes it may be helpful to understand how PresSTORE creates and handles jukeboxes on disks.

First of all, a physical jukebox can handle multiple tapes, each one with a (nearly) fixed size. Tapes are mapped to files on virtual jukeboxes. Same as with physical jukeboxes, this does not mean that the tapes are integrated part of the jukebox, they are just handled by the jukebox. From this point of view, one can understand a virtual jukebox as an organisational unit to handle files as tapes.

To approximate physical devices, the virtual jukebox also defines a number of slots (comparable to a number of files) and a size that the files will get when created. Often, these parameters do not fit on the first trial.

Resizing a jukebox

It is not possible to resize a jukebox, neither a physical nor a logical one. Because of that, there is no user interface to resize a jukebox. Note that in the jukebox setup

   Jukebox Manager -> Select Jukebox -> Edit -> Setup

there are two fields to define a first slot and a last slot. These fields do not change the size of the jukebox, but restrict the use of the slots for PresSTORE. This input is intended to restrict PresSTORE to a subset of slots on a physical jukebox, namely in case multiple applications share the same device. This is most probably not the case on a virtual jukebox.

In order to resize a jukebox, simply delete the jukebox and create a new one with the new size. This operation does not affect the files that have already been created in this jukebox. In case files - or virtual tapes - shall be taken over to the new jukebox, move them to the new location, in case the new jukebox has a different location to store files, and call the inventory function:

   Jukebox Manager -> Select Jukebox -> Edit -> Jukebox Inventory

This way, existing files can be used in the new jukebox. Note that these files will keep the old size.

Managing virtual tapes

The files used by a virtual jukebox are handled in the same way as physical tapes. To remove a tape in or out of the library, just move the file in or out of the jukeboxes directory.
Do not forget to call the inventory function in case you moved the tapes manually. This behaviour may be useful whenÂ the jukebox is located on a changeable hard disk:

   * Create the jukebox on an exchangeable hard disk
   * To extend the storage, remove the disk and mount another disk at the same location
   * Execute a jukebox inventory function

This way, one can manange multiple virtual tapes on the jukebox. Note that the directory configured a data directory in the jukebox setup in PresSTORE must be present in all drives.

Using software compression

The most physical drives support hardware compression to extend the capacity of tapes. PresSTOREs virtual jukeboxes can simulate that behaviour by using software compression. Note that software compression is time consuming but may enhance the available space.
PresSTORE 3 uses an optimized compression on Virtual Jukeboxes permanently.
In order to enable software compression in PresSTORE 2, add a small "c" to the Jukebox directory path:

   Jukebox Manager -> Double click you Jukebox -> OS Path

Note that the jukebox can handle compressed and uncompressed data at the same time. The compression takes place data block wise, so existing files are simply continued with compressed data. There is no data migration required.

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22 May 2007 03:12 PM