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VSAM - Introduction


VSAM is abbreviated as Virtual Storage Access Method. VSAM is high performance access method and data set organization, which organizes and maintains data in a catalog structure.

Virtual Storage Access Method is an IBM DASD file Storage method. VSAM file can be used in COBOL programs like other physical sequential files. VSAM are the logical datasets for storing records. The records in a VSAM file can be of fixed length or a variable Length. VSAM files can be read sequentially and randomly in COBOL program.

VSAM datasets are frequently referred to as clusters. A KSDS cluster consists of two physical parts, an index component, and a data component. ESDS and RRDS clusters consist of only a single component, the data component.


Advantage of VSAM datasets:

  • VSAM provide the facility to optimize the performance.

  • Data stored in VSAM files are structured and organized.

  • VSAM files can be used in both batch and online environment.

  • Free space in VSAM will be reused automatically.

  • It provides fast access to records in data set.

  • Data protected against unauthorized access by using passwords.


Disadvantage of VSAM datasets:

  • VSAM should be stored in only DASD (direct-access storage device).

  • VSAM files can’t be accessed directly using IPSF options 3.4. Using INSYNC or File Manager tool used to browse, edit, and copy the VSAM files.


When to use VSAM datasets?

When simple operations are involved like reading the file, writing into output file or output device, then you can use sequential files but suppose you need a file for multiple operations in the file based on a particular key value, and then you should choose a VSAM file.

Because, you can read or update a particular record in the file or you can delete a particular record in the file. This can be easily achieved by using VSAM (Virtual Storage Access Method) file over sequential file.

VSAM files are much more powerful and advanced as compared to normal sequential files.

There are multiple types of VSAM files available and you can select the VSAM file based on your business requirement.


Types of VSAM datasets:

  1. Entry Sequenced Data Set (ESDS) - Each record in ESDS is identified for access by specifying its physical location - the byte address of the first data byte of each record in relationship to the beginning of the dataset. ESDS datasets are similar to Basic Sequential Access Methid (BSAM) or Queued Sequential Access Method (QSAM) datasets.

  2. Key Sequenced Data Set (KSDS) - Each record in KSDS is identified for access by specifying its key value - a sequence of characters embedded in each data record which uniquely identify that record from all other records in the dataset. KSDS datasets are similar to Indexed Sequential Access Method (ISAM) datasets, with many of the same characteristics, but also having distinct advantages over ISAM.

  3. Relative Record Data Set (RRDS) - Each record in RRDS is identified for access by specifying its record number - the sequence number relative to the first record in the dataset. RRDS datasets are similar to Basic Direct Access Method (BDAM) datasets.


Video Tutorial: VSAM Introduction


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