The way we have found to do this is to limit the number of scratch tapes in
the collocated pool.  Say you have 100 clients, each with 20 GB.  Further
assume large capacity tapes, say 40GB or more.  Then it might make some
sense to "double up" your clients on tapes.  You can do this by setting
maxscratch on the pool to 50.  The first 50 clients will get there own
tapes.  The 51st client will piggyback onto the least used tape of the
first 50 and on it goes until all 100 clients are sharing the 50 tapes.
 You can reduce maxscratch on the fly.

Obviously, by limiting scratch you can find yourself in a situation where
your pool is full.  Set up another tape pool for emergencies.  Have the
first pool's migration target this emergency pool.  If the first pool
should fill, you'll be in good shape.  To rectify the problem, add more
scratch volumes and issue a move data from the cartridges in the emergency
pool.

All of this only makes sense (if it ever makes sense) in an environment
where client data storage is small relative to tape cartridge size.

Kelly Lipp
SSSI