ⓘ Virtual Storage Access Method
Virtual Storage Access Method is an IBM DASD file storage access method, first used in the OS/VS1, OS/VS2 Release 1 and Release 2 operating systems, later used throughout the Multiple Virtual Storage architecture and now in z/OS. Originally a record-oriented filesystem, VSAM comprises four data set organizations: Key Sequenced Data Set, Relative Record Data Set, Entry Sequenced Data Set and Linear Data Set. The KSDS, RRDS and ESDS organizations contain records, while the LDS organization simply contains a sequence of pages with no intrinsic record structure, for use as a memory-mapped file.
IBM uses the term data set in official documents as a synonym of file, and direct access storage device hard, because it supported other devices similar to disks.
VSAM records can be fixed length or variable length. They are organized in fixed-size blocks called control intervals CIS and then into larger divisions called control areas CASS. The interval size control is measured in bytes, e.g. 4 KB and the size of the control zone are measured in disk tracks or cylinders. Intervals control the units of transfer between disk and computer so a read request will read one complete control interval. Control areas are the units of allocation so, when you define a VSAM data set a whole number of control areas will be highlighted.
Access method services IDCAMS utility is often used to manipulate the delete and define the VSAM datasets.
Custom programs can access VSAM datasets through data definition DD statements in the job Control language JCL by using dynamic allocation or in online regions such as in the information control system CICS.
Both IMS / DB and DB2 are implemented on the basis of VSAM and use its underlying data structures.