File Structure

IOA Access Method index component records are fixed length. IOA Access Method file data component records can be fixed or variable length. Each record is represented by the relative byte address of the record (RBA), which consists of four bytes. As shown in the table, the first byte represents the extent number of the record; the next two bytes represent the block number within the extent; the last byte represents the record number within the block.

Table 31 Relative Byte Address Components

Both index file and variable data file information are stored in compressed format, resulting in reduced I/O processing overhead and optimum disk space utilization.

The first block in each IOA Access Method file extent consists of a control record. For data file components with fixed length records, the control record contains an RBA of the first free record in the database; each free record contains an RBA of the next free record. For data file components with variable length records, the control record contains a bit table indicating the blocks within the extent that contain free space for additional records. The IOADIG utility can be used to check the integrity of the list of free records. For details of the IOADIG utility, see the INCONTROL for z/OS Utilities Guide.

IOA Access Method index files function as balanced tree index structures.

In addition to index (key) and data record pointer fields, IOA Access Method file index component records can contain non‑key data, enabling non‑key selection criteria to be used without accessing the associated IOA Access Method file data component. Non‑key data fields in index records can be stored in compressed format, providing maximum space utilization benefits.