BLKSIZE Parameter

Purpose

The BLKSIZE (Block Size) parameter specifies the size, in bytes, of physical blocks used to store records in a dataset. This parameter is crucial for I/O efficiency, as it determines how many logical records are stored in each physical block and affects both storage utilization and performance.

Where BLKSIZE is Used

• In DD statements as part of the DCB parameter
• In TSO ALLOCATE commands
• In the DCB macro for assembler programs
• In dynamic allocation services (SVC 99)
• Within SMS data class definitions

Basic Syntax

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//ddname DD DSN=dataset.name,
//        DCB=(BLKSIZE=nnnnn,...),
//        ...other parameters...

Where nnnnn is the block size in bytes.

System-Determined Block Size

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//ddname DD DSN=dataset.name,
//        DCB=(RECFM=FB,LRECL=80),
//        ...other parameters...

Note that BLKSIZE is omitted here, allowing z/OS to determine the optimal block size automatically.

BLKSIZE Value Ranges

BLKSIZE and RECFM Interaction

Block size requirements and calculations depend on the record format (RECFM):

System-Determined Block Size (SDB)

Modern z/OS systems can automatically determine optimal block sizes. This is recommended in most cases as it ensures efficient use of device capabilities.

• To use system-determined block size, simply omit the BLKSIZE parameter
• The system will calculate an optimal BLKSIZE based on:
  • Device type characteristics
  • RECFM and LRECL values
  • Track capacity considerations
  • System performance settings
• SDB is available for both SMS-managed and non-SMS-managed datasets
• SDB can be controlled using the SYSTEM BLKSIZE parameter in SMS data classes

Common BLKSIZE Values

Usage Examples

Fixed-Length Blocked Dataset

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//SRCLIB   DD DSN=USER.SOURCE.LIB,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(10,5,50)),
//            DCB=(RECFM=FB,LRECL=80,BLKSIZE=27920)

This example creates a partitioned dataset with fixed-length 80-byte records and a block size of 27920 bytes, which fits 349 records per block.

System-Determined Block Size

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//DATAFILE DD DSN=USER.DATA.FILE,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(5,2)),
//            DCB=(RECFM=FB,LRECL=400)

This example creates a dataset with fixed-length 400-byte records but allows the system to determine the optimal block size automatically.

Variable-Length Blocked Dataset

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//LOGFILE  DD DSN=USER.APPLICATION.LOG,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(5,2)),
//            DCB=(RECFM=VB,LRECL=1028,BLKSIZE=32760)

This example creates a variable-length dataset with a maximum record length of 1028 bytes and the maximum block size of 32760 bytes.

Tape Dataset

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//BACKUP   DD DSN=USER.BACKUP.FILE,
//            DISP=(NEW,CATLG,DELETE),
//            UNIT=TAPE,
//            VOL=SER=TAPE01,
//            DCB=(RECFM=FB,LRECL=8000,BLKSIZE=32000)

This example creates a tape dataset with a block size of 32000 bytes, chosen to be a multiple of the record length and close to the maximum allowed.

BLKSIZE Optimization Techniques

• Track Capacity Utilization: Choose a BLKSIZE that maximizes track usage
  • For 3390 DASD: Aim for block sizes that are efficient divisors of 56,664 bytes (track capacity)
  • Half-track blocking: Approximately 27,998 bytes
  • Full-track blocking: Approximately 56,664 bytes, minus overhead
• Fixed-Length Record Optimization:
  • Make BLKSIZE a multiple of LRECL to avoid wasted space
  • Calculate records per block: floor(BLKSIZE ÷ LRECL)
  • Recalculate optimal BLKSIZE: records_per_block × LRECL
• Large Block Interface (LBI):
  • Allows block sizes larger than 32,760 bytes for certain devices (primarily tape)
  • Can significantly improve tape utilization and backup/restore performance
  • Requires hardware and software support

Performance Implications

Common Issues and Troubleshooting

Best Practices

1. Use System-Determined Block Sizes: In most cases, let the system determine optimal block sizes by omitting the BLKSIZE parameter.
2. For Fixed-Length Records: If manually specifying BLKSIZE, make it a multiple of LRECL for efficient storage.
3. For Sequential Processing: Use large block sizes to minimize I/O operations and maximize throughput.
4. For Tape Datasets: Use the maximum supported block size for most efficient tape utilization.
5. For PDS Libraries: Use half-track or optimal track blocking to balance directory access and data access.
6. Documentation: Document the rationale for non-standard block sizes to aid future maintenance.
7. Consistency: Use consistent block sizes for similar datasets to simplify management.
8. Consider Memory Constraints: Remember that larger block sizes require more buffer space in programs.

JES2 vs JES3 Considerations

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

Related Concepts

• RECFM Parameter - Record format specification that affects BLKSIZE requirements
• LRECL Parameter - Logical record length, crucial for calculating appropriate BLKSIZE
• DSORG Parameter - Dataset organization that affects BLKSIZE considerations
• DCB Parameter - Data Control Block parameters that include BLKSIZE
• SPACE Parameter - Space allocation, which interacts with BLKSIZE for storage efficiency