What're the advantages/disadvantages of RECFM=FB over RECFM=F?

Question

While defining a dataset to be created, one of the JCL parameters, DCB has a positional sub-parameter RECFM, has possible values of F,FB,V,VB etc.. What're the advantages/disadvantages of RECFM=FB over RECFM=F or RECFM=VB over RECFM=V? And which case prefers to use what RECFM format?

Answer 1

RECFM is short for record format.

F represents fixed length records, unblocked. FB represents fixed length records, blocked. Blocking stores multiple records in a disk block, while the unblocked format stores one record in a disk block. At one time, disk drives were so slow that the unblocked format provided relative speed, while the blocked format provided better disk usage. Today, with modern disk drives, there's no advantage to using the unblocked format.

V represents variable length records, unblocked. VB represents variable length records, blocked. You would use these formats if you have variable length records, rather than fixed length records. You need to add 4 to the maximum record length in the LRECL to account for the record length field.

There's an additional attribute character, A. Used with fixed blocked (FBA) or variable blocked (VBA), this tells the system that the first byte of your record is a printer control character.