OK... TSM will estimate the capacity of a device/media pair based on what
you set with the device class definition and if nothing is specified there,
it will be based on what the ~driver~ reports, THIS IS WHILE A VOLUME IS
FILLING.
Now what is actually shown as the capacity once the volume is FULL is what
has been written to the tape.
What does all of this mean ?
I'd say the 6500MB is close to your stated 7 GB native capacity... if you do
a "q dev f=d" and see "Est/Max Capacity (MB)" as blank, the 6500 is being
provided by the device driver and that is where it is coming from...
Now the 21 GB is an ~estimated at 3/1 compression" and if your full volumes
show only about 12-13 GB that means that either you have uncompressed
cliient that doesn't compress very well OR you have a mixture of compressed
& uncompressed (at the client) client data.
Basically when a tape's EndOfTape marker is reached, TSM assigns a capacity
equal to what it has written to that specific tape.
If you front end things with a diskpool, the amount reported on a tape is
what was read from the disk and put on the tape.
Client compressed data doesn't reduce at the tape drive with tape drive
compression SO if all your client data came into your tsm server as ~client
compressed~, I'd expect the capacity of FULL tapes to  be about 7 GB.
If all your data came in from your clients uncompressed AND it were all
Oracle DB file type data I'd expect your FULL volumes to be about 21-22 GB.
If any of the data is something like a .zip file or other type of already
compressed data outside tsm client compression, it won't compress at the
drive either and that can cause variations in what the capacity is reported
on FULL volumes.

whew, my explanation isn't very pretty but does it clear things up a bit ???

Dwight



-----Original Message-----
From: Elio Vannelli [mailto:elio.vannelli_external AT WARTSILA DOT COM]
Sent: Monday, January 13, 2003 10:06 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Why my library doesn't compress data?


HI all,

This is a newby question.

I have an IBM 3575 that could store 7GB native data per volume or
compress them till 21GB per volume.
My file server stores 250GB. When I have a full backup of my file server
(under Windows 2000) I have 40 volumes and every volume has
"EST_CAPACITY_MB" between 6200MB and 6500MB. Other servers can store
till 13200MB, but no more.

May you help me, please?

Thanks in advance.

Elio Vannelli