RECFM Parameter

Purpose

The RECFM (Record Format) parameter specifies the format and characteristics of records in a dataset. This parameter is crucial for defining how data is physically stored within datasets, affecting both storage efficiency and processing methods.

Where RECFM is Used

• In DD statements within the DCB parameter
• In IDCAMS utility DEFINE commands
• In TSO ALLOCATE commands
• In the DCB macro for assembler programs
• In dynamic allocation (SVC 99) calls

Basic Syntax

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//ddname DD DSN=dataset.name,
//        DCB=(RECFM=xx,...),
//        ...other parameters...

Where xx is a combination of one or more record format codes.

RECFM Values

RECFM values are composed of one or more of the following codes, combined to define the complete record format:

Basic Record Format Codes

Optional RECFM Modifiers

Common RECFM Combinations

Record Format Structure Details

Fixed-Length Records (F, FB)

┌────────────────────┐
│ Record 1 (80 bytes)│
├────────────────────┤
│ Record 2 (80 bytes)│
├────────────────────┤
│ Record 3 (80 bytes)│
└────────────────────┘
              

Variable-Length Records (V, VB)

┌────┬──────────────────┐
│BDW │Block length      │
├────┼──────────────────┤
│RDW │Record 1 (varying)│
├────┼──────────────────┤
│RDW │Record 2 (varying)│
├────┼──────────────────┤
│RDW │Record 3 (varying)│
└────┴──────────────────┘
              

ASA and Machine Control Characters

When 'A' (ASA) or 'M' (Machine) control characters are specified in RECFM, the first byte of each record contains a control character that indicates carriage control operations for printers:

Usage Examples

Fixed-Blocked Dataset

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//SRCLIB   DD DSN=USER.SOURCE.LIB,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(10,5)),
//            DCB=(RECFM=FB,LRECL=80,BLKSIZE=32720)

This example creates a new dataset with fixed-length blocked records, typically used for source code or data files with fixed record structures.

Variable-Blocked Dataset

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//LOGFILE  DD DSN=USER.APPLICATION.LOG,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(5,2)),
//            DCB=(RECFM=VB,LRECL=1024,BLKSIZE=32760)

This example defines a log file with variable-length records, which allows flexibility in record sizes for logging varying message lengths.

Print File with ASA Control

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//REPORT   DD DSN=USER.MONTHLY.REPORT,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(1,1)),
//            DCB=(RECFM=FBA,LRECL=133,BLKSIZE=13300)

This example defines a report file with ASA carriage control characters. The LRECL is 133 bytes: 1 byte for the ASA control character plus 132 bytes for the print line.

Load Module Library

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//LOADLIB  DD DSN=USER.LOAD.LIBRARY,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(10,5,10)),
//            DCB=(RECFM=U,BLKSIZE=32760)

This example creates a library for executable load modules using the undefined record format, which is appropriate for binary executable programs.

IDCAMS DEFINE for VSAM Dataset

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//IDCAMS   EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN    DD *
  DEFINE CLUSTER (NAME(USER.VSAM.KSDS) -
         RECORDSIZE(80 80) -         /* LRECL specification */
         INDEXED -
         KEYS(8 0) -
         CYLINDERS(5 1))
/*

For VSAM datasets, RECFM is implied by the dataset type (KSDS, ESDS, etc.) and is specified differently using RECORDSIZE.

RECFM and Performance

Record format has significant implications for I/O performance and storage efficiency:

• Blocking (B): Blocked records improve I/O efficiency by reducing the number of physical I/O operations needed to process a dataset. Always use blocked records (FB, VB) unless there's a specific reason not to.
• Fixed vs. Variable: Fixed-length records provide more predictable performance but might waste space if many records don't use their full length. Variable-length records are more space-efficient but have slightly higher processing overhead.
• BLKSIZE Considerations: For optimal performance, set BLKSIZE to a multiple of LRECL for fixed-length records, and as large as practical (up to 32760 bytes) for both fixed and variable records.
• System-Determined BLKSIZE: Modern z/OS systems can determine optimal block sizes if you omit BLKSIZE in the DCB parameter, which is often the best approach.

Common Issues and Troubleshooting

Best Practices

1. Use Blocked Records: Always use blocked record formats (FB, VB) for improved I/O performance unless there's a specific requirement for unblocked records.
2. Match RECFM to Data Characteristics: Choose fixed-length records for data with consistent size and variable-length for data with significant size variations.
3. Optimize BLKSIZE with RECFM: Set BLKSIZE to maximize efficiency based on the RECFM and storage device characteristics, or let the system determine the optimal value.
4. Be Consistent: Maintain consistent RECFM/LRECL/BLKSIZE specifications across related datasets to avoid processing errors.
5. Document Special RECFMs: If using unusual RECFM combinations (like FBS or VBS), document the reasons and implications for maintenance purposes.
6. Consider Standard Values: Use standard RECFM/LRECL combinations for common datasets:
   • Source code: RECFM=FB, LRECL=80
   • Print files: RECFM=FBA, LRECL=133
   • Load libraries: RECFM=U, BLKSIZE=32760
   • Variable data: RECFM=VB, LRECL=(appropriate size), BLKSIZE=32760

JES2 vs JES3 Considerations

The RECFM parameter is part of the data management system rather than the job entry subsystem, so it functions identically in both JES2 and JES3 environments.

However, there are a few considerations regarding spool datasets:

Related Concepts

• DSORG Parameter - Dataset organization that works with RECFM
• LRECL Parameter - Logical record length, which defines the size of each record
• BLKSIZE Parameter - Block size, which determines how many records are stored in each physical block
• DCB Parameter - Data Control Block parameters that include RECFM, LRECL, and BLKSIZE
• DSNTYPE Parameter - Dataset type specification, which can affect valid RECFM choices