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.

For me two of the attractions of a virtual tape library are being able
to do lanfree backups without using Sanergy, and having the VTL do
compression. 


H. Milton Johnson