I am not quite sure what you meant by "with TSM you can use disk only for backups instead of onsite tape". I do not see any reason why you can not use FILE device types for both primary and copypool stg pools. Of course if you actually want to move a copy of the data offsite, then you would need some type of removable media such as tape.
With data on disk a simple copy can move it hundreds of miles in seconds or minutes. Can't a FILE device pool be used to recover data in an emergency?
Am I making this too simplistic?
Thanks,
Sal
