The logical block size can affect the performance of a PDSE. In general a large block size improves performance. IBM recommends that you use the system-determined block size function for allocating data sets. It chooses the optimal block size for the data set. A PDSE is given the maximum block size of 32 760 bytes for variable records. For fixed-length records, the block size is the largest multiple of record size that is less than or equal to 32 760.

Applications can use different logical block sizes to access the same PDSE. The block size in the DCB is logical and has no effect on the physical block (page) size being used.