In many installations it's a close to meaningless number. For empty tapes,
it's the estimated capacity of the tape multiplied by maxscratch. Then as
tapes become full, the actual number of bytes written will modify the
estimated capacity value. The actual bytes written depends on
compressibility of the data.  The reason why in most of my installations it
is a meaningless number is that tape libraries are managed by the number of
scratch tapes available. I generally suggest that there should be about 10%
scratch tapes in a tape library or at least five (five is also the threshold
value for attention grabbers in the TSM v5 Operational Reporting Facility).
Tape storage pools are then not restricted by the number of scratch tapes
that may be assigned. Very often maxscratch is set to 10000 or some such
number. You will often find large maxscratch values also in server-to-server
virtual volume environments where the maximum capacity in most cases is set
to be significantly smaller than the capacity of, for example LTO3 or LTO4
tapes.

Note that for device class disk storage pools the estimated capacity is the
actual capacity.

Joerg Pohlmann
250-585-3711

-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Mehdi Salehi
Sent: Friday, July 09, 2010 08:06
To: ADSM-L AT VM.MARIST DOT EDU
Subject: [ADSM-L] tape stg capacity

Hi,
How is tape storage pool "estimated capacity" calculated? I see different
values for similar storage pools of the same devclass and maxscratch.

Thanks