As long as you can migrate the data from disk to tape *faster* than the new
data is arriving in the disk pool, you'll be fine.  If your tape drives are
very slow to load and your pool is relatively small, one implication of this
is that you should not use the default of "90%" as the high migration
threshold, because you'll run out of space in the disk pool before you get
the tape spun up.

What appears to happen if the disk pool gets to 100% full and the backup is
still in progress is that the server realizes that data must now be backed
up straight to tape.  This is a higher priority than a migration, so the
migration is killed by the server and the backup continues, direct to tape.
This can be somewhat messy.  By leaving yourself enough room in the disk
pool with an appropriate upper migration threshold, you can avoid this
event.

John Sorensen
SSSI