Right now, AFAIK, there's no way to influence how Tivoli makes decisions about which files to remove from the migrated file cache -- I think adding ways to influence those decisions would probably be better than to add a 'special' disk storage pool type.
The ability to say "Okay, if there's a shortage of space in the cached disk storage pool, prefer to keep files associated with management class A in the cache over files associated with class Y. Class Z should remain cached no matter what -- if that means run out of space, then run out of space." That would be much slicker, and provide more management options than just adding a whole different storage pool type :) It would also be neat to be able to copy files that have been removed from the cache back into the cache from the tape storage pools. Ah, the beauty of storage management. Kyle Sparger