If you have a large number of small clients then defining a pool with max
scratch set to some value (less than no of clients) but leaving colocation
on will try and keep client data on the same tape but will use no more than
this number of tapes hence stacking multiple clients on the same tape.

e.g. 50 clients with say 1GB of data using Magstar (10GB/cart as compressed)
is approximately 5 tapes worth of data. Given that data will expire and
tapes will not be full (60% approx assuming reclamation at 60%) gives 9
tapes. Add some for the filling tapes which could be empty in a worse case
 I would add 50% for this as running out of tapes will cause failures during
migration and I haven't been running this for long enough to be less
generous) gives a max scratch of 14 or so.

If you used colocation in a large pool you would have at worst 100 tapes
used by these clients (one full one filling) so it's a large saving, and
ADSM will try and keep the client data together.

If you want to monitor this then keep track of the number of filling tapes
in the pool and generate warning and error messages when this drops below
some value.

Hope this helps

Nick Clayton
Vertex Data Science Ltd