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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 it 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

    • Data component
      • Control Interval (CI)
      • Control Area (CA)
    • Index component
      • Sequence set
      • Index set
  • VSAM Splits CI Split and CA Split
  • Spanned records

Each of them are explained in detail in upcoming aricles

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