Control Interval (CI)
- The unit of data transmission between virtual and auxiliary storage (DASD) for each physical i/o operation is defined as Control Interval. It contains data, control information and free space ( which later may be used to insert records).
- Control information in each CI contains 1) RDF (Record Definition Field) which is 3 bytes long contains the length of the records and how many records are of same length 2) CIDF (Control Information Definition Field) which is of 4 bytes long contains information about CI.
- When a VSAM gets loaded, control intervals are created and records are written into them.
- In KSDS, each control interval will have some space to insert the records later.
- In ESDS, each control interval is completely filled with records and once it is full then we start writing records in another CI
- In RRDS, control intervals are filled with fixed length slots containing either active record or dummy record. Slots containing dummy records are used to fill the new records.
- The default CI size is 4K and can be extended up to 32k.
Control Area (CA)
- A group of control intervals is called a control area.
- For ESDS and RRDS, control areas are filled with control intervals that contains records.
- For KSDS, control areas may contain CIs with complete free space which may later be used to fill records.
- A CA contains two or more CIs and a VSAM dataset consists of one or more CAs
No comments:
Post a Comment