The organization of data in all VSAM data sets, except linear data sets, is arranged in records, also called logical records. A logical record is the user record requested from, or given to, the VSAM record management function.

Logical records of VSAM data sets are stored differently from logical records in non-VSAM data sets. VSAM stores records in control intervals. A control interval is a continuous area of direct access storage that VSAM uses to store data records and control information that describes the records. Whenever a record is retrieved from direct access storage, the entire control interval containing the record is read into a VSAM I/O buffer in virtual storage. The desired record is transferred from the VSAM buffer to a user-defined buffer or work area. Figure 1 shows how a logical record is retrieved from direct access storage.

Figure 1. VSAM Logical Record Retrieval