The space required by system software data sets, except for PDSE data sets, is affected by the block sizes you choose for those data sets. Generally, data sets with larger block sizes use less space to store the same data than those with smaller block sizes. Data sets that store more data in less space usually offer better DASD performance than those that use more space to store the same data.

There are some exceptions to the general rule that larger block sizes result in better space utilization. For example, fixed block (FB) record format data sets should not be allocated with block sizes larger than half the track length of the DASD they are allocated on. Doing so will cause considerable DASD space to be wasted, because current DASD track lengths are less than twice the maximum block size of 32,760 bytes. However, some data sets are best allocated using specific block sizes. When this is true of system software data sets, IBM® recommends specific block sizes for them.

Generally, system-determined block sizes (SDB) makes the best choice for block size for fixed block (FB), variable block (VB), and variable-block spanned (VBS) record format data sets. You should use SDB for all system software data sets with these record formats except those for which IBM specifically recommends other block sizes. One way to do this is specifying BLKSIZE=0 in the DCB parameter of a DD statement when allocating new data sets using JCL. For details about how to specify system-determined block sizes, see z/OS MVS JCL Reference.

Note that system determination of block sizes affects the block size and number of blocks used. It does not affect the amount of space allocated in a data set. The amount of space is defined by IBM (in sample jobs and program directories).