Blocking Factor

Enhanced Definition

The blocking factor is the number of logical records grouped together to form a single physical block of data on a storage device (like disk or tape). Its primary purpose in z/OS is to optimize I/O operations by reducing the number of physical reads and writes required to process a dataset.

Key Characteristics

- I/O Efficiency: A higher blocking factor generally leads to fewer physical I/O operations, as more logical records are transferred in a single read or write, thereby improving job performance.
- Storage Utilization: It influences how efficiently storage space is used, especially on tape, where inter-record gaps (IRGs) are minimized by grouping records.
- JCL Specification: The blocking factor is implicitly determined by the BLKSIZE (block size) and LRECL (logical record length) parameters in the DCB (Data Control Block) of a JCL DD statement. For fixed-length records, BLKSIZE must be a multiple of LRECL.
- Buffer Management: Processing blocked records requires memory buffers large enough to hold at least one physical block, which the access method uses for deblocking (separating logical records).
- Record Formats: Applicable to various record formats, including FB (Fixed Blocked), VB (Variable Blocked), and U (Undefined).
- Device Dependency: Optimal blocking factors can vary depending on the storage device type (e.g., DASD track sizes, tape drive characteristics) and the access method used (e.g., QSAM, BSAM).

Use Cases

- Sequential File Processing: Widely used for QSAM (Queued Sequential Access Method) datasets on both DASD and tape to enhance the performance of batch applications reading or writing large volumes of sequential data.
- Batch Job Optimization: Critical for COBOL or Assembler batch programs that perform extensive file I/O, where a well-chosen blocking factor can significantly reduce elapsed time.
- Tape Archiving and Backup: Maximizing the blocking factor for tape datasets improves the speed of backup and restore operations and optimizes tape cartridge capacity utilization.
- Sort Utility Input/Output: Sort programs like DFSORT often benefit from appropriately blocked input and output files to minimize I/O overhead during sorting large datasets.

Related Concepts

The blocking factor is intrinsically linked to LRECL (Logical Record Length) and BLKSIZE (Block Size). For fixed-length records, BLKSIZE is typically LRECL multiplied by the blocking factor. It directly impacts the number of physical I/O operations performed by access methods like QSAM or BSAM, which manage the grouping and ungrouping of logical records within I/O buffers. A larger blocking factor generally reduces I/O calls but increases the memory required for I/O buffers, influencing overall system performance and resource utilization.

Best Practices:

Optimize BLKSIZE: For fixed-length records, always ensure BLKSIZE is a multiple of LRECL. For DASD, strive for BLKSIZE values that fill a full track or half-track to minimize wasted space and I/O operations.
Standardize for Tape: For tape datasets, a common practice is to use a BLKSIZE of 32760 bytes (32K-8 bytes, allowing for control information) or 65520 bytes (64K-8 bytes) as it often provides good performance across various tape drives.
Balance Performance and Memory: While larger blocking factors reduce I/O, they require larger I/O buffers. Ensure that the chosen BLKSIZE does not lead to excessive virtual storage consumption or page faults, especially in memory-constrained environments.
Use BLKSIZE=0 for System Determination: For new datasets on DASD, specifying BLKSIZE=0 in the DCB allows the system (or SMS Data Class) to determine an optimal block size, often based on device characteristics and LRECL.
Consistency: Maintain consistent blocking factors for datasets that are frequently passed between jobs or systems to avoid unnecessary data conversions or performance degradation.