Table 1 shows the compaction status assigned to an empty tape when it is first selected for output.

As you can see, the TAPEHARDWARECOMPACT|NOTAPEHARDWARECOMPACT parameters of the SETSYS command apply only to 3480X devices; 3480 devices cannot compact data and 3490 and 3590 devices always compact data when it is written by DFSMShsm unless overridden by a data class option.

For hardware compaction of data, it is recommended that you turn off DFSMShsm software compaction when the hardware compaction algorithm function is available and when only one step is involved in moving a data set from level 0 directly to tape.

If two steps are involved, such as migration to ML1 DASD followed by movement to tape, the decision is more involved. You need to determine the space-saving benefit on level 1 volumes and the time the data resides there, versus the processing-unit usage (CPU time) to do the compaction. You might consider specifying compaction but use the ARCMDEXT installation exit to not compact data sets that are either small or moved quickly on to migration level 2. Using compaction algorithms on data already compacted by DFSMShsm does not, in general, significantly help or hinder. For more information about using the ARCMDEXT exit, refer to z/OS DFSMS Installation Exits.