Hi!

> I've indicated 40Gb because my PowerVault is configured as yours, to
> compress data by hardware ( I think in a 2:1 ratio by default). 

Your drive is configured to do hardware compression, period. The data 
compression rate cannot be configured. The drive just does its best. 
Drive manufacturers like to advertise that their drives compress data 
by 2:1 ratio, but that's just what it is - advertising. The compression 
ratio that you actually get depends of the nature of your data. If the 
data is all text files and such, you might get even much better 
compression ratio than 2:1. If, on the other hand, you are backing up 
mostly .mp3, .jpg and .tar.gz files then you'll get no compression at 
all, because the data is already compressed. In fact, applying hardware 
compression to such files might even make the backup bigger than the 
original.

The compression ratio that people who back up entire servers with 
various types of files - some better compressible, some worse - is 
typically somewhere around 1.4:1.
--
Toomas Aas | toomas.aas AT raad.tartu DOT ee | http://www.raad.tartu.ee/~toomas/
* RUNTIME ERROR 6D at 417A:32CF : Incompetent user