I just want to amplify my second bullet. When the BLKSIZE is greater than 4K, each BLKSIZE unit is recorded into multiple 4K blocks, the data blocks are not streamed. If the BLKSIZE is intermediate between a 4K multiple the result is pretty bad. As an experiment I copied SYS1.MACLIB to a PDSE without changing its DCB attributes, RECFM=FB, LRECL=80, BLKSIZE=6160. Each 6160 byte block was recorded into 2 4K data blocks in the PDSE. Obviously the final data set was much larger than the original SYS1.MACLIB. If you insist on using PDSE for source type data, use BLKSIZE=4080.