Hi there,
The original post was old so I moved this into its own discussion.
Compression is a best-effort process dependent on a number of things like files being backed up, hardware type (does it support compression or not) etc.
You wouldn't get any compression of already compressed files for instance, and might see them expand in size (something that does happen if you read the forums). These would be zip files, images etc. Also, the impression created by tape manufacturers that say a 400GB tape can be compressed to store 800GB of data is a bit of a fallacy in the real-world and you are unlikely to get that. You'd see ratios from 1:1 to maybe 1.7:1, but very rarely 2:1.
You get software compression (driven by your server) and hardware compression (driven by your tape device).
The TN below explains why you are unlikely to get 2:1 compression as well as how to set it up:
http://www.symantec.com/business/support/index?page=content&id=TECH6076
Thanks!
