If you are using hardware compression, there is no way to tell, since
compression algorithms vary so widely. Additionally, if you use
hardware compression, you are severely limited in what devices you can
use to restore data. Different brands, models and sometimes even
firmware levels of "the same" device can have different algorithms. If
you replace a drive under warrantee, there is no guarantee that you'll
be able to read tapes on the new drive. I have 6 backup servers ranging
from dual PPro 200s to a dual Xeon 400 and notice little to no
difference in throughput with hardware compression vs software
compression.

To your question - If you were using software or no compression open
the GUI, go to the reports tab, open saved and view "Media Sets" for
bytes written to each volume.

Shelia wrote:
>
> I am using ver. 8, hardware compression and DLTIV media, HP 718 DLT Library.
> How can I monitor the data capacity on each tape? I believe each tape is
> maxing out at approx 56GB data. My objective is to limit backup jobs to
> one tape. For instance: 1st monday differential equals 13GB, 2nd monday
> differential 24GB. The 3rd monday differential will span 2 tapes. I am
> trying to avoid this. Any ideas?
> Thanks