DLT tapes contain a calibration track.  This is used to store info
about the format of the tape (e.g. 7000 vs 8000, compressed vs not).
The first time you write from the beginning of the tape (BOT), the
calibration track is written, and that's the format that will be used
forever (sort of).  

I'm not familiar with your library, but my point is that it's going to
read what's on the tape regardless of how you have it set.

Most DLT drives have lights that you can view to see, among other
things, if compression is on.  BTW, when no tape is mounted, the
compression light is off.  Perhaps your library does the same.

If you want to reformat a DLT, you need a drive that has the density
override button.  Insert a tape, use the override button to force the
density (including compression) you want, and write anything to the
tape (still at BOT).  Of course, you've just lost anything on the
tape.  

You'll also need to set up your OS to use compression (or at least
attempt to, depends on the drive setting).  On unix, this is done by
choosing the right device file.

L

P.S.  The reformatting stuff may not be necessary to just add/remove
compression.  Perhaps any write from BOT will do it.  I really don't
remember.  The method above will work though.

Lee Anne Pedersen writes:
> After following this thread, I'm curious to experiment with h/w instead of 
> s/w compression. I have a Compaq TL891DLX library with two DLT8000 drives. 
> I've tried to turn on compression for each drive via the system menu, but 
> the settings are never saved; the drive status always says Compression: Off.
> Does anyone have experience with this hardware with which they could 
> enlighten me?
> 
> Thanks,
> Lee Anne

-- 
Larry Kingery 
           Proofread carefully to see if you any words out.