I am trying to implement a backup process where a complete set of file
copies are kept on disk in addition to tape.  Our current process is a
very traditional basic setup with backups going to a single disk storage
pool that then migrates to tape during the day.  After the migration
completes, the process then makes multiple tape copies for offsite and
onsite vault storage.

What is the best process to keep a disk based version of the most recent
file copy?  Is it enabling the cache for the disk storage pool and
increasing the storage pool size to hold the copies; or is it better to
go with a disk to tape (d2t)/disk to disk to tape (dd2t) dedicated
device?

Just for reference, here are the details of our TSM environment.

Current TSM Server OS:          AIX 5.1 ML 07
Current TSM Server Ver:         5.2.2.14
Disk Storage:                   DS4400 (FastT 700) SAN - Fibre Channel
Tape Storage:                   3583-L72 with 3 LTO2 fibre drives