Ok, I have a 3575 library, but the kind of device does not matter at all
when it come to "software compression".

software compression.
  If enabled, you minimize the traffic on the network and the media
  seems to store less data, but of course this is because the
  compression is done at the source. ... and the decompression too!
  Certainly the backups are slower.

hardware compression.
  On unix systems it depends on the device file. So in my case, I tend
  to have enabled both software compression and hardware compression,
  but if the client is not powerfull enough I set the COMPRESSION=NO
  and the compression's left to the device.

How much the compression compress?
It only depends on the data and on the compression algorithm. Magstar
and/or ADSM seems to have a good compression algorithm but because is
also very fast I don't think it is the best. Probably 'gzip -9' has a
better compression rate ... but of course it takes longer to compress!

On 3570 I noticed:
COMPRESSION=NO, media are full with 10/12GB of data
COMRPESSION=YES, media are full at about 5GB which turns out to be the
  native capacity of a Magstar, so my guess is that hardware and
  software compression algorithms are pretty equal.

From my experience I noticed that:
DAT / DDS2 compress less than DLT and Magstar 3570
DLT and Magstar 3570 are equivalent.

I hope this helps

Ciao Mauro
> I'll ask a slightly different question but related.  Netware offers
compression,
>  correct?  When the client backs up files in a compressed netware file
system, i
> s the data uncompressed before it is sent?  I would think it is, but if
so, deco
> mpressing, and then compressing again with ADSM would seem to me a bit
slow.
>
> I haven't seen any situations where compression=on has worked well.
I'm an old
> hardware guy and I've got to think that the hardware compression will
always be
> better than software compression.
>
> Kelly Lipp
> SSSI
>
> -----Original Message-----
> From:   Tom Haddad [SMTP:thaddad AT COLUMBIAENERGYGROUP DOT COM]
> Sent:   Tuesday, January 12, 1999 1:47 PM
> To:     ADSM-L AT VM.MARIST DOT EDU
> Subject:        3570 Magstar device and compression
>
> I have 3570 devices (models B and C) running ADSM v3.1.
> They have been operational for a short time, but I have questions about
client
> compression.
> The drives are defined as FORMAT=DRIVE and DEVTYPE=3570.
>
> In regards to Netware (4.11) clients, how can I maximize my compression
gain??
> I attempted to leave COMP=NO at the server (for the node) and COMP OFF
in the
> OPT file and let the hardware take
> over with compression. Looks Iike I average about 7 or 9G max on a tape
> cartridge as they became full.
> Next I want to put COMP ON in the OPT file and see if I get more
compression..
> Any help?
>
> I think my NT clients gain an overall better compression rate than my
Netware
> clients..hmmm!!
>
> Looks like compression gain is driven by  the type of client(data),
> hardware...etc..