Compaction percentage

When compacting a data set during migration or backup, DFSMShsm keeps a running total of the number of bytes in each compacted block that is written to the migration or backup volume. DFSMShsm also keeps a running total of the number of bytes that were in the blocks before compaction. With these values, DFSMShsm determines the space savings value, expressed as a percentage.

                 Total Bytes            Total Bytes
                 Before Compaction  —  After Compaction
Space Savings = _________________________________________ x 100
                     Total Bytes Before Compaction

DFSMShsm uses the space savings percentage to determine if it should compact recalled or recovered data sets when they are subsequently backed up or migrated again. You specify this space saving percentage when you specify the SETSYS COMPACTPERCENT command.

At least one track on the DASD migration or backup volume must be saved, or the compacted data set is not eligible for compaction when it is subsequently migrated or backed up.

Note: For SDSP data sets, DFSMShsm considers only the space saving percentage because small-data-set packing is intended for small user data sets where the space savings is typically less than a track.

If you specify . . .	Then . . .
SETSYS COMPACT (DASDMIGRATE \| TAPEMIGRATE)	DFSMShsm compacts each record of a data set on a level 0 volume the first time it migrates the data set. During subsequent migrations from level 0 volumes (as a result of recall), DFSMShsm performs additional compaction of the data set only if the percentage of space savings (as indicated from the original backup or migration) exceeds the value specified with the SETSYS COMPACTPERCENT command.
SETSYS COMPACT (DASDBACKUP \| TAPEBACKUP)	DFSMShsm compacts each record of a data set on a level 0 volume the first time it backs up the data set. During subsequent backups (as a result of recovery), DFSMShsm performs additional compaction of the data set only if the percentage of space savings (as indicated by the original backup) exceeds the value specified with the SETSYS COMPACTPERCENT command.

DFSMShsm stores the space saving percentage in the MCDS data set (MCD record) or the BCDS data set (MCB record). If the MCD or MCB record is deleted (for example, during migration cleanup or expiration of backup versions), the previous savings by compaction is lost and cannot affect whether or not DFSMShsm compacts the data set during subsequent migrations or backups.