Hi!

> define tapetype DDS-4-40{
>  comment "DDS-4 PowerVault with compression"
> DDS-4-20
>  length 40000 mbytes
> }

This length number is probably too optimistic. This assumes that your 
data compresses with a 2:1 ratio, which is rarely seen in real life. 
FWIW, here's my tapetype with DDS4 drive using hardware compression:

define tapetype DDS4 {
   comment "DDS4 using hardware compression"
   length 25000 Mbytes
   filemark 0 Kbytes
   speed 2816 Kbytes
}

The length number is really just a guess (I think I underestimate the 
compression a little, but since I only have 10 GB of data this is not 
an issue for me right now). The speed number is taken from "avg tp 
write rate" as reported by Amanda's nightly run.
--
Toomas Aas | toomas.aas AT raad.tartu DOT ee | http://www.raad.tartu.ee/~toomas/
* One way to be happy ever after is not to be after too much.