The data compaction option can save space on migration and backup volumes by encoding each block of each data set that DFSMShsm migrates or backs up. DFSMShsm compacts data with the Huffman Frequency Encoding compaction algorithm. The compacted output blocks can vary in size. An input block consisting of many least-used EBCDIC characters can be even longer after being encoded. If this occurs, DFSMShsm passes the original data block without compaction to the output routine.

Whether DFSMShsm compacts each block of data as the data is backed up or migrated from a level 0 volume is determined by the SETSYS COMPACT command. DFSMShsm compacts each block of data as the data set is backed up or migrated from a level 0 volume. Compaction or decompaction never occurs when a data set moves from one migration volume to another or from one backup volume to another. DFSMShsm does not compact data sets when they are migrated for extent reduction, are in compressed format, or during DASD conversion.

When DFSMShsm recalls or recovers a compacted data set, DFSMShsm automatically decodes and expands the data set. DFSMShsm decompacts encoded data even if you later run with SETSYS COMPACT(NONE).

If you do not want a specific data set to be compacted during volume migration or volume backup, invoke the data set migration exit (ARCMDEXT) or the data set backup exit (ARCBDEXT) to prevent compaction of that data set. For more information about the data set migration exit and the data set backup exit, refer to z/OS DFSMS Installation Exits.