Andrew,
At a previous site, we had 3590 drives (10GB native / 30GB compressed) that
varied from 9.7GB to 80GB Full.  The variation was due to the type of data
existing on each tape, and if the client was performing compression or not.


When a tape was dedicated to a client doing compression, tapes were showing
a capacity near the low-end of the scale, but it was compressed data.

When a new oracle database (empty) was defined and backed up (client NOT
compressing), I would get figures toward the high end, as the drive
compressed the whale out of empty DB files.

As to the percentage of utilization, this can happen as data is expired on
the volume.  Do a "q v nnnnnn d=f" to get the reclamation value from that
volume.  Also, if the volume gets a write error, ADSM will mark the tape
READOnly and close it immediately.  This results in a tape only partially
used with utilization less than 100%.  (Write & Read error status can be
gleaned from the above command also.)

When you originally define the device in ADSM, you give the definition your
estimation of its capacity.   ADSM only uses this value for reporting pool
capacity & volume utilization percentage until the data on the volume has
exceeded your estimate, or the end of the volume has been reached.  Then the
volume is reported with the actual amount of data ADSM wrote to it.

As ADSM expires data on that volume, the utilization decreases directly as
the reclamation % increases, even if the volume is still filling.

I hope this helps
Randy