You probably are not initiating your migration processes early enough...
might have to take it down to about 80%
also if you clients run with client compression, they preallocate based on
allocate size and then release any unused portions after the transfer is
complete... in other words, if you have 4 sessions sending 25 GB each but
they get a good 4/1 compression, you would still need 100 GB in your
diskpool to prevent any from going straight to tape.

Dwight