If you specify the compaction during migration option, DFSMShsm
compacts each data set as it migrates. The first time the data set
migrates, DFSMShsm always compacts it. If a migrated data set is recalled
and is again a candidate for migration, DFSMShsm checks the compaction
history in the MCDS for that data set. If the compaction from the
earlier migration did not result in saving at least the percentage
of space that you specified with the COMPACTPERCENT parameter of the
SETSYS command, DFSMShsm does not perform compaction again.
Note: If
the data set is a SAM or VSAM compressed data set, DFSMShsm suspends
compaction during migration and records in the MCDS that the data
set is striped, compressed, or both.
If the data set is to migrate to an SDSP, the compaction size is
always estimated using an internal default of 50%. Previous compaction
history is not considered.
Alternately, if you specify zEDC compression instead
of compaction (via the ZCOMPRESS parameter of the SETSYS command),
zEDC compression will be done instead of compaction. If zEDC Services
are not available, DFSMShsm will use the SETSYS COMPACT settings
to determine what type of software compaction is used for migration.
See ZCOMPRESS: Specifying when compression with zEDC should be done for more information.