The COMPRESS command compresses a PDS on a specified volume. Compression removes unused space between members in a partitioned data set. This recovered space is then available for reuse at the end of the data set. Depending on the filtering criteria you specify, you can compress all the partitioned data sets or only some of the data sets. This command is useful for compressing system partitioned data sets before applying maintenance (thus avoiding certain space-related abends). You must not compress the data sets that contain DFSMSdss or IEBCOPY executable code.

The actual PDS compression is done in place. To prevent loss of data if the system or the compression operation abnormally ends during processing, back up your volume or data sets that meet the filtering criteria before using this command.