On Fri, Sep 09, 2005 at 09:39:31AM +0100, Chuck Amadi Systems Administrator 
wrote:
> Hi I have managed to config the hard disk partitions in my disklist but
> I am short of tape space to include the remainder of the hard disk
> partitions.
> 
> I had initially run the following commands is it possible to get
> compression functioning to enable to use 40GB as opposed to native 20GB.
> 
> * -  amtapetype -f /dev/nst0 # determine tape type takes along time
> * -  turn off datacompression run mt command that controls magnetic tape
> drive operation. * -  mt -f /dev/nst0 status and mt -f /dev/nst0
> datcompression 0 Compression off.
> 
> As I don't want to buy bigger tapes if possible.

Your tape's capacity is 20GB!  Not 40GB, never will be, no way, no how.

But compression (hardware or software) might be able to shrink a lot of
data down to 20GB.

The rub is that compression algorithms, whether applied by your computer
or by the computer in your tape drive, are only effective the first time
they are applied.  And as PB pointed out, when fed already compressed
data, the dumb algorithm used by the tape drive is likely to expand,
rather than shrink, the data!

An analogy, your new 250GB hard disk drive.  If you use it to store a
bunch of zip'ed files, does it become a 500GB hard disk drive?  No,
it is still a 250GB drive that happens to be storing compressed data.

Similarly, your 20GB tape stores 20GB of data.  Those data may be
20GB of plain old uncompressed information, or a lot more information
that is compressed by your computer before taping or by the drive
during taping -- but not both!.

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
 4455 Province Line Road        (609) 252-0159
 Princeton, NJ  08540-4322      (609) 683-7220 (fax)