KEYLEN specifies the length (0 to 255) in bytes of an optional key that precedes each block on direct access storage devices. The value of KEYLEN is not included in BLKSIZE or LRECL, but must be included in BUFL if buffer length is specified. Thus, BUFL=KEYLEN+BLKSIZE. See Using KEYLEN with PDSEs for information about using the KEYLEN parameter with PDSEs.

Rule: Do not specify nonzero key length when opening a PDSE or extended format data set for output.

IBM recommends not coding KEYLEN or coding KEYLEN=0. A nonzero value generally will make your program run slower.