Steve,

You can check your disk performance.  When you're backing up to diskpool,
check your CPU's Wait I/O using
vmstat 1 10

If you're getting significant Wait I/O, you can do an
iostat 1 10
to check which volumes are getting hit the hardest.  If you don't have
significant Wait I/O, I would suggest Disk is not your bottleneck.

Alex Paschal
Storage Administrator
Freightliner, LLC
(503) 745-6850 phone/vmail