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