Control Interval-VSAM

Control interval
A CI is a contiguous area of direct access storage that VSAM uses to store data records and control information that describes the records. A CI is the unit of information that VSAM transfers between the storage device and the processor during one I/O operation.

A CI consists of:
  • Logical records stored from beginning to end
  • Free space, for data records to be inserted into or lengthened
  • Control information, which is made up of two types of fields; one control interval definition field (CIDF) per CI, and several record definition fields (RDF) describing the logical records.
    • CIDF is a 4-byte field. – It contains information about the amount and location of free space.
    • RDF is a 3-byte field. – It describes the length of records. For fixed length records there are two RDFs, one with the length and other with how many with the same length.

For random access, it is recommended to use small data CIs, to avoid bringing uneeded logical records into memory. For sequential access, it is recommended that you define large CIs to decrease the number of I/O operations

An LDS does not contain CIDFs and RDFs in its CI.

With KSDS clusters, the entire control interval is usually not filled.  Some percentage of free space is left available for expansion.  With ESDS clusters, each control interval is completely filled before records are written into the next control interval in sequence.  With RRDS clusters, control intervals are filled with fixed-length slots, each containing either an active record or a dummy record.  Slots containing dummy records are available for use when new records are added to the dataset.


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