VSAM Control Area
The term Virtual Storage Access Method (VSAM) applies to both a data set type and the access method used to manage various user data types.As an access method, VSAM provides much more complex functions than other disk access methods. VSAM keeps disk records in a unique format that is not understandable by other access methods.
VSAM is used primarily for applications. It is not used for source programs, JCL, or executable modules. VSAM files cannot be routinely displayed or edited with ISPF.
You can use VSAM to organize records into four types of data sets: key-sequenced, entry-sequenced, linear, or relative record. The primary difference among these types of data sets is the way their records are stored and accessed.
A D V E R T I S E M E N T
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
Multiple CIs are placed in a control area (CA). A VSAM data set consists of control areas and index records. One form of index record is the sequence set, which is the lowest-level index pointing to a 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.
Control intervals are grouped together into control areas. The rules used for filling and writing control areas are similar to those which apply for control intervals. For ESDS and RRDS clusters, control areas are filled with control intervals that contain records. For KSDS clusters, some of the control intervals in each control area may consist entirely of free space that can be used for dataset expansion.
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