Block

Key Characteristics

• • Physical I/O Unit: A block represents the actual amount of data transferred between main memory and an external storage device in a single I/O operation.
  • Contains Logical Records: It typically encapsulates multiple logical records, which are the application-defined units of data (e.g., a single customer record, a line of text).
  • Defined by BLKSIZE: The size of a block is specified by the BLKSIZE parameter in the Data Control Block (DCB) for a dataset, often within JCL or dynamically in a program.
  • Performance Impact: Larger block sizes generally lead to fewer I/O operations and better throughput for sequential processing, but can increase buffer memory requirements.
  • Fixed or Variable Length: Blocks can contain fixed-length logical records (where BLKSIZE is a multiple of LRECL) or variable-length logical records (where BLKSIZE must be large enough to hold the largest record plus control information).
  • Managed by Access Methods: Access methods like QSAM (Queued Sequential Access Method) or BSAM (Basic Sequential Access Method) handle the blocking and deblocking of logical records to and from physical blocks.

Use Cases

• • Sequential File Processing: Optimizing read/write operations for large sequential datasets (PS or PO members) used in batch jobs, such as processing transaction logs, master files, or flat files.
  • Tape Storage Optimization: Maximizing data density and transfer rates on magnetic tapes by grouping many logical records into large blocks, which reduces the number of inter-block gaps and improves tape drive efficiency.
  • DASD Storage Efficiency: Efficiently utilizing disk space and reducing access arm movement on DASD (Direct Access Storage Device) by reading/writing larger chunks of data at once, especially for datasets with high sequential access.
  • Batch Report Generation: When a COBOL program reads an input file and writes an output report, proper blocking ensures efficient data flow for both input and output datasets, minimizing I/O waits.

Related Concepts