You shouldn't estimate a capacity.  Let the drive figure it out.  ADSM will
assume the capacity to be the native capacity.  For DLT4000, that's 20 GB
per tape.  Up until 100% utilized, you will see the data on the tape track
with the percentage of 20 GB that has been consumed.  After you reach 20 GB
on the tape, you will see the utilization at 100%, but data can still go to
the tape.  Makes sense since ADSM has no way of knowing what sort of
compression you may get.  The only real way to know what compression ratio
you did achieve is to wait until the tape says full.  Then you know.

No, already compressed data won't compress anymore.  Hardware compression is
almost always better than software compression.  If you don't have slow
networks, don't let the client compress the data.  Client compression is
almost always slower.  Overall elapsed backup time will be longer if the
client is compressing the data.

Kelly J. Lipp
Storage Solutions Specialists, Inc.
PO Box 51313
Colorado Springs CO 80949
719-531-5926
www.storsol.com
lipp AT storsol DOT com