I think with ADSM you could define different device classes for the same
tape drive, one compressed and one not compressed. I guess if you got real
creative you could setup storage pools for compressed data and storage pools
for un-compressed data and have the client backups go to the appropriate
storage pools. It would be easier to just turn it off all together. I have
seen migration's where ADSM is copying data from one tape drive to another
take all day on data that was already compressed with tape drive compression
turned on. That's why I turned it off on the client and just let the drives
handle it.

Mark