Sounds like you've got a handle on the issues... you will help performance
of concurrent sessions by defining the disk pool volumes so they're the
lined up with the physical volumes, and match or exceed the number of
concurrent sessions you plan to run.  You might just use 4 volumes, one each
for the 4 physical drives;  or, you might want to create two or more disk
volume files for each of the physical volumes.  You may want to define
multiple disk pools, since you may want to segregate some copy pool data,
for whatever reasons... such as collocation on for some, off for others.

If there's no need to migrate disk-to-tape, you'd want to ensure the
copypool jobs are run as soon as practicable after the normal nightly
backups.  Also, you might want to consider two copypool copies, one for
onsite and one for offsite.

I don't think you'll need to be concerned with disk pool storage
fragmentation, but you could eliminate it if you suspect fragmentation is
affecting performance; use "move data" to (effectively) remove such dead
spots from one volume (consolidating the unexpired data onto other volumes).
BTW, I don't think raw volumes are supported for disk pools;  you'll need to
create file system volumes.

Hope this helps.

Don France
Technical Architect - P.A.C.E.
Bus-Ph:  (408) 257-3037
mailto:dfrance AT pacbell DOT net
http://www.pacepros.com