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Module 2: VSAM Concepts

VSAM Overview

  • VSAM stands for Virtual Storage Access Method and it was introduced by IBM in 1970’s.
  • As name suggests, it is an access method intended to function in a virtual storage environment of MVS, z/OS.
  • VSAM is high performance access method and acts as an interface between application program and physical operation of storage devices.
  • VSAM’s role is to provide an easiest way for program to store/retrieve data to/from an external device without concerning about physical details of transferring data bytes between program and I/O devices.
  • VSAM can be used in COBOL or CICS in the mainframe for data manipulation on files
  • VSAM facilitates both sequential and random processing
  • IBM introduced VSAM as a replacement for several existing access methods provided by them earlier
    • KSDS (Key Sequenced Data Set) to replace ISAM (Indexed Sequential Access Method)
    • ESDS (Entry Sequenced Data Set) to replace PS/QSAM(Queued Sequential Access Method)
    • RRDS (Relative Record Data Set) to replace BDAM (Basic Direct Access Method)
  • KSDS, ESDS and RRDS mentioned above are types of VSAM data organization and will be explained later in this module

Advantages of VSAM over other Access methods

  • VSAM logically structures and organizes data more properly and thus enables faster access to data
  • Data retrieval can be sequential, random or dynamic
  • Portable across systems i.e. VSAM dataset and catalogs cab be easily transported between operating systems
  • VSAM can be made password protected to avoid unauthorized access
  • Deletion of records are done at physical level (not logical level). Thus, free space is more easily available
  • Supports alternate index
  • One versatile utility, IDCAMS, to manage all file operations
  • AMS commands can be executed as TSO commands
  • Batch and on-line processing is supported
  • VSAM is provided with performance parameter which helps in optimizing performance

Disadvantages of VSAM

  • VSAM occupies more storage space as compared to other type of datasets; this is because VSAM data set also holds control information in them, in addition to the actual data that they contains.
  • VSAM cannot be stored on TAPE volume. It is always stored on Disk i.e. DASD storage.
  • Since it requires number of cylinders to store data which makes it less cost-effective
  • For KSDS, primary key cannot be changed
  • VSAM are device independent but Integrity of data across region and system has to be controlled by user

VSAM Terminologies and Concepts

Each of them are explained in detail in upcoming aricles


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