I have seen a few notes recently surrounding disk storage pools.  If you
are interested in keeping data on disk storage for restore purposes, you
can set cache to on.  Cache allows a copy of the data to stay in the
storage pool.  There are some performance hits, but have used it for
sysback pools and some data pools.  I have not done a comparison of backups
with it on and off to determine just how much degradation.

Bill Sherrill