Question : Problem: dds4 tape has a capacity of 20 / 40 GB...  how do you know how well compressed the data is?

how do you tell how well the data is compressed on a DDS4 20 / 40 GB tape?  in some cases 25 GB won't fit and 30 GB will, depending on what you are putting on the tape.  but I don't see where you can see how maxed out the tape is

Answer : Problem: dds4 tape has a capacity of 20 / 40 GB...  how do you know how well compressed the data is?

This should show up in the backup logs of any decent backup program.

I hope this helps !