Virtual storage access method (VSAM) an IBM disk file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and now in z/OS. Originally a record-oriented filesystem., VSAM comprises four data set organizations: Key Sequenced Data Set (KSDS), Relative Record Data Set (RRDS), Entry Sequenced Data Set (ESDS) and Linear Data Set (LDS). The KSDS, RRDS and ESDS organizations contain records, while the LDS organization (added later to VSAM) 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 documentation as a synonym of file, and Direct access storage device (DASD) because it supported other devices similar to disk drives.
VSAM records can be of fixed or variable length. They are organised in fixed-size blocks called Control Intervals (CIs), and then into larger divisions called Control Areas (CAs). Control Interval sizes are measured in bytes — for example 4 kilobytes — while Control Area sizes are measured in disk tracks or cylinders. Control Intervals are 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 a VSAM data set is defined, an integral number of Control Areas will be allocated.
The Access Method Services utility program IDCAMS is commonly used to manipulate ("delete and define") VSAM data sets.
Custom programs can access VSAM datasets through Data Definition (DD) statements in Job Control Language (JCL), via dynamic allocation or in online regions such as in Customer Information Control System (CICS).
Both IMS/DB and DB2 are implemented on top of VSAM and use its underlying data structures.
Read more about Virtual Storage Access Method: VSAM Files, VSAM Data Access Techniques, Sharing VSAM Data, History
Famous quotes containing the words virtual, storage, access and/or method:
“Tragedy dramatizes human life as potentiality and fulfillment. Its virtual future, or Destiny, is therefore quite different from that created in comedy. Comic Destiny is Fortune—what the world will bring, and the man will take or miss, encounter or escape; tragic Destiny is what the man brings, and the world will demand of him. That is his Fate.”
—Susanne K. Langer (1895–1985)
“Many of our houses, both public and private, with their almost innumerable apartments, their huge halls and their cellars for the storage of wines and other munitions of peace, appear to me extravagantly large for their inhabitants. They are so vast and magnificent that the latter seem to be only vermin which infest them.”
—Henry David Thoreau (1817–1862)
“The professional celebrity, male and female, is the crowning result of the star system of a society that makes a fetish of competition. In America, this system is carried to the point where a man who can knock a small white ball into a series of holes in the ground with more efficiency than anyone else thereby gains social access to the President of the United States.”
—C. Wright Mills (1916–1962)
“The country needs and, unless I mistake its temper, the country demands bold, persistent experimentation. It is common sense to take a method and try it. If it fails, admit it frankly and try another. But above all, try something. The millions who are in want will not stand idly by silently forever while the things to satisfy their needs are within easy reach.”
—Franklin D. Roosevelt (1882–1945)