Record format (RECFM) specifies the format and characteristics of the records in the data set. This section will be limited to a discussion of the performance considerations associated with fixed and variable length records.

There seems to be a natural affinity toward allocating MVS data sets with variable length records from the perspective of a byte oriented NFS client operating system. Such allocations enhance the feeling of transparent access and eliminate requirements to determine a fixed record length. However, the placement of incoming packets into records of varying lengths is more complex than placing packets into records with fixed lengths. Consequently, write performance for z/OS NFS is generally more efficient when MVS data sets are allocated with fixed length records. Record formats are specified with the RECFM data set creation attribute.