If you are using collocation then you may come across what I've seen.

I find setting the LOW migration to a very low number causes the system
to take an 'inverse proportional' amount of time to empty. (or whatever
...
(I like to call these things 'approaching the speed of light' problem.)

What I mean is the lower you get in the threshold 'floor' the more
fragmented and smaller the user amount of data to be dumped is.
You spend more time with the mount / demounts and not much time
with the writing on tapes.  I have 3590s.

If I'm desperate to empty the diskpool, like for a big change, then I
usually wait until my patience has run out (a technical parameter?)
and restart migrations with the collocation turned off.

... joe.f.

Joseph A Faracchio,  Systems Programmer, UC Berkeley
Private mail on any topic should be directed to :
         joef AT socrates.berkeley DOT edu
 (510)642-7638 (w)  (209)483-JOEF (M)
                             5633
163 days until retirement!!!!!!!!!!!!!!!!!!!!!!!!