Either it isn't completing normally/fully or the data is very comressable.
With hardware compression and the right data you can get pretty amazing compression ratios, for example text files and log files will compress a ton.
We'll need further information to really give you a better answer. Like, what type of system, data, backup software, and hardware are you using?
The system this is happening on is a iseries Model 550 using V5R3, using BRMS, the tape drive is an LTO2 , to be specific it a 3582-L23 tape library. It's SCSI attached via FC5702. We use a single tape drive.
The majority of the data being saved are large multi-member physical files (with way too many logicals), but of course there are good number of log files and there is some text files although not that significant.
Doing the math with the LTO2 capacity and size of this system being saved (4406 GB with 80 used.) is just seems impossible to fit this much data onto 3 tapes, yet we get all objects saved using BRMS.
Thx - gambitt7
The direct answer is that you're not saving 1TB per tape. The LTO-2 tapes store 400GB (compressed). Assuming maximum efficiency, you're saving approx 1.2TB total on your three tapes.
Without seeing how your saves run, what your libraries and directories look like, and having an overview of your data characteristics, it's impossible to say what's happening.
Traditionally, AS/400 sites use their space extremely wastefully (rather than achieving unusually high "compressibility" except by compressing out the waste). Most likely, restructuring your databases could probably cut your "80% DASD used" down to 40% or less.