Greetings oh wise and varied *SM'ers!

I have an internals question.

I have a primary disk pool BACKUP_DISK, with a collocated primary tape pool
BACKUP_TAPE that BACKUP_DISK spools to when it gets full (or at 16:30,
whichever comes first :). I have enough disk that I seldom need to migrate
data out of BACKUP_DISK during the night, but occasionally we have a really
heavy day, and BACKUP_DISK goes over its threshold and migrates before its
schedule. For disaster recovery I have a non-collocated tape copypool
BACKUP_OFF. During the day our DRM script does a "backup stg BACKUP_DISK
BACKUP_OFF" then "backup stg BACKUP_TAPE BACKUP_OFF", in case we had a
migration in the night. So far that is fairly straightforward, but now comes
the tricky part....

If a long-running backup pushes BACKUP_DISK over its threshold after some of
the files are sent from BACKUP_DISK to BACKUP_OFF, but before the backup
from BACKUP_TAPE to BACKUP_OFF starts, then the files are there to be backed
up to BACKUP_OFF *again* from BACKUP_TAPE. So the question is: Does TSM
recognize that the files are already in BACKUP_OFF, or does it make another
copy?

The reason I am asking is that we are using LOTS of tapes, and management
wants me to see if I can cut back on the burn rate. I know that tape to tape
copy would be slower because I would have multiple input mounts per output
tape, but I don't do the disk to offsite backup, but instead migrate, then
tape to offsite backup then I know that the offsite pool only has one copy
of all the files.

Thanks for any insights.

- Kai.