Space considerations

Using larger block sizes saves dump space and improves performance by minimizing the number of I/O operations performed during a dump operation.

The default block size for output records that are written to tape is the optimum block size for the output device (262 144 is the maximum). You can change this default to 32 760 by using the installation options exit routine. Refer to the discussion of system-determined block size in z/OS DFSMS Using Data Sets for a description of the optimum block sizes of the supported tape devices.

For more information about the installation options exit routine, refer to z/OS DFSMS Installation Exits.

For output records that are written to DASD, the block size is the track length of the output volume for devices whose track length is less than 32KB. It is one half the track length for devices whose track length is greater than 32KB. You can select a different block size for tape or DASD by coding DCB=BLKSIZE=block size in the corresponding data set definition (DD) statement. The minimum block size is 7 892 bytes; the maximum is 32 760 bytes.

Note: To include the block size specification in the tape label, specify the BLKSIZE parameter in the tape DD statement.

You can also use the following options to save dump space:

Dump only the used space (the default if you do not use keywords ALLDATA or ALLEXCP), instead of all allocated space, in sequential and partitioned data sets or in data sets with a null DSORG field. For VSAM key sequenced data sets, the VALIDATE keyword (the default) dumps only the used data instead of all of the allocated space.
Use the COMPRESS keyword.
Note:
1. DFSMSdss ignores the COMPRESS keyword if you specify it during a logical data set dump for physical sequential extended-format data sets.
2. If your tape drive has the improved data recording capability (IDRC) and you want to use hardware data compaction, you do not need to use the COMPRESS keyword with the DUMP command. If you want software compression, specify the COMPRESS keyword, but you do not need to specify DCB=TRTCH=COMP in the JCL. In most cases, hardware data compaction without software data compression gives the best performance. However, you can use software compression and hardware compaction at the same time.
Perform incremental data set backup instead of volume backup. This reduces the amount of dumped data and decreases processing time.
When dumping to DASD, specify an output dump data set that is extended format in the compressed format. Avoid specifying the COMPRESS or HWCOMPRESS keywords when the output dump data set is extended format in the compressed format. Performance could be degraded since the data may be compressed twice.