The significance of the block size keyword (BLKSIZE) is slightly different from a PDS. All PDSEs are stored on DASD as fixed 4 KB blocks. These 4 KB physical blocks are also known as pages. The PDSE is logically reblocked to the block size you specify when you open the data set. The block size does not affect how efficiently a PDSE is stored on DASD, but it can influence how efficiently the system reads from it and writes to it. The block size also affects the size of the storage buffer allocated for a PDSE. You should let the system-determined block size function calculate the best block size for your data set.