Ideally, TSM's provided DRM regimens would be followed to provide the 
discipline to properly handle tapes going out and coming back.  Alternately, 
you can do your own handling, providing that proper steps are followed...  A 
tape going offsite (out of the library) should have its access mode marked 
Offsite (not Unavailable).  Tapes marked Offsite can be surrogate-reclaimed, 
but that takes a lot longer than a direct reclamation.

Your 3584 cells should be occupied by mostly onsite volumes, with a reasonable 
number of cells held back for rotating some offsite tapes back in, as well as 
dbbackup circulation.  Onsite tapes typically consist of primary storage pool 
volumes; maybe also an onsite copy pool of the data, for more assured recovery 
of data, quickly.  If onsite tape reclamation is running to completion and yet 
the library continues to fill, it's probably growth in client activity, in 
conjunction with retention policies: use Query OCCupancy and accounting data 
reviews to pinpoint consumption.  Higher density drives and tapes (along with 
proper Devclass Format specs) are a salvation in such cases, as technology 
permits.  Also look for abandoned filespaces and no-longer-used copy storage 
pools as space wasters.  Make sure there are no excessive Reusedelay values.  
Make sure tapes are not going readonly due to drive issues.

    Richard Sims