Another consideration is that tape and disk sales people cleverly use
base 10, where 1K=1000, instead of what us geeks use where 1K=1024. So a
"800GB native capacity" LTO4 tape holds only 800,000,000,000 bytes,
which is less than 800GB. You lose 2.4% right there in salesman's tricky
fast talking.

Every time we get a new tape technology, I let TSM fill up a few tapes,
and then readjust the Estimated Capacity to a rough approximation of how
much actual data will fit. For LTO4 and our data, that is 1.2TB, though
I have "full" tapes ranging from 800GB to 2.4TB, depending upon the
nature of the data as others have said. With it set the way you have it,
I bet you're getting some surprises when a tape that was "70% full",
suddenly fills up.

When a tape physically fills up, TSM resets the Estimated Capacity to
how much data was actually written to the tape. This is essential for
doing reclamation arithmetic correctly. TSM is not controlling
compression other than just setting a bit in the driver that tells the
tape drive hardware to do its own compression. The tape drive's only
communication back to TSM as to how compression is going, is when it
reports back that a volume has filled. Then TSM refers to its database
to tabulate how much data it wrote to that volume. For all TSM knows,
you've got variable length tapes.

It sounds like everything is OK. Compression is working fine. It's just
that your data does not compress very much. I'd recommend reseting the
Estimated Capacity to 1TB in TSM, to make the arithmetic about things
like storage pool capacity less misleading. When you reset it, tapes
that are already "filling" will not have their capacity reset until
they fill up, are reclaimed, and returned to scratch status.

Roger Deschner      University of Illinois at Chicago     rogerd AT uic DOT edu
======I have not lost my mind -- it is backed up on tape somewhere.=====


On Thu, 9 Dec 2010, Mehdi Salehi wrote:

>Hi,
>Drives are LTO4, devclass is ULTRIUM4C, but all "Full" volumes are between
>800GB to 1TB. Is it normal? It seems TSM does not use compression.
>
>Mehdi
>