Data sets sometimes exist on volumes in a format (compacted or uncompacted) that seems to conflict with the type of compaction specified with the SETSYS command. The following examples illustrate how this occurs.

DFSMShsm compacts data sets only when it copies them onto a DFSMShsm-owned volume from a level 0 volume.

Similarly, if you are not compacting data sets that migrate to DASD and you are compacting data sets that migrate directly to tape, both compacted and uncompacted data sets can migrate to level 2 tapes. The uncompacted data sets occur because the data sets are not compacted when they migrate to the migration level 1 DASD and the compaction is not changed when they later migrate to a migration level 2 tape. However, data sets migrating directly to tape are compacted.

You can also have mixed compacted and uncompacted backup data sets and they, too, can be on either DASD or tape.

If you specify compaction for data sets backed up to DASD but no compaction for migrated data sets, any data set that migrates when it needs to be backed up is uncompacted when it is backed up from the migration volume.

Similarly, if you specify compaction for migrated data sets but no compaction for backed up data sets, a data set that migrates when it needs to be backed up migrates in compacted form. When the data set is backed up from the migration volume, it is backed up in its compacted form even though you specified no compaction for backup.

Data sets that are backed up to DASD volumes retain their compaction characteristic when they are spilled to tape. Thus, if you are not compacting data sets backed up to tape but you are compacting data sets backed up to DASD, you can have both compacted and uncompacted data sets on the same tapes. Data sets that are compacted and backed up to tape, likewise, can share tapes with uncompacted data sets that were backed up to DASD.