VSAM Control Interval

VSAM is a kind of record-oriented file system. In this kind of dataset, information is stored as a collection of records. VSAM records can be of any length; they need not be of one set length. They are, however, organized into blocks called Control Intervals, which are measured in bytes. These Control Intervals are further organized into Control Areas, which are measured in much larger units.

Successful processing with this program reduces these tasks from more than hour of work to only a few minutes for a disk. This program is used after the preliminary formatting of the disk is completed using the DOS disk formatting facility which makes the disk ready for VSAM catalog and dataset definitions.

VSAM works with a logical data area known as a control interval (CI) that is diagrammed in Figure 1. The default CI size is 4K bytes, but it can be up to 32K bytes. The CI contains data records, unused space, record descriptor fields (RDFs), and a CI descriptor field.

Simple VSAM control interval

In non-VSAM data management methods, the unit of data that is moved between memory and the storage device is defined by the block. In VSAM, the unit of data that is transferred in each physical I/O operation is defined as a control interval. A control interval contains records, control information, and (in the case of KSDS clusters) possibly free space which may later be used to contain inserted records.

When a VSAM dataset is loaded, control intervals are created and records are written into them. 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.