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

• CLUSTER
  • 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