OPTBLKSZ—standard form

Generally, the larger block size fits more records on a track, which achieves better utilization of disk space and reduces the number of I/Os that are required to read the data set. Very small data sets might not benefit significantly from the denser record storage. However, because the minimum allocation by z/OS is a single track, allocating larger block sizes is generally more effective. The only disadvantage is that larger block sizes require larger buffers in memory. The following table shows the effect of different block sizes on DASD capacity.