History -

VSAM was introduced in the early 1970s by IBM with three dataset organizations - sequential, indexed, and direct access.

The word virtual means it was introduced when the initial IBM virtual storage operating systems OS/VS1 and OS/VS2 introduced. So it was named virtual at the beginning.

VSAM has been continuously improved and enhanced from that point. DFSMS became a z/OS integral component from /OS V1.3.

What is VSAM?

VSAM is the short form of Virtual Storage Access Method. VSAM is an IBM invented access method that allows us to access and organize records in the disk data set. VSAM is available under the z/OS and one of the several access methods using in z/OS. VSAM works with the data that stores only on direct access storage devices (DASDs).

VSAM files can't be displayed and edited by ISPF, and it requires a tool to get them displayed and edited in ISPF.

VSAM mainly used for applications and not used for source programs, JCL, or executable modules.

The term VASM applies to data set type and access methods that are used to manage various dataset types. VSAM access method provides much more complex functions than any other disk access method. VSAM record storage format is unique and not understandable by other access methods.

An access method is a re-entrant code loaded and contained in DFSMSdfp, a component of the z/OS DFSMS product.

The access method makes an I/O operation(moving data between an I/O device and memory) easier for an application. VSAM is a type of record-oriented system.

VSAM Dataset Types -

VSAM record storage format is unique and not understandable by other access methods.

Based on the VSAM storage, access, and organizing the records, the datasets are categorized into 4 different types, and those are -

  • Key-Sequenced Data Sets (KSDS)
  • Entry-Sequenced Data Sets (ESDS)
  • Linear Data Sets (LDS)
  • Relative Record Data Sets (RRDS)

The essential difference between these data set types is the way the records are stored and accessed.

VSAM Access and retrieval options -

VSAM has three types of access to records in VSAM data sets -

  • sequential
  • direct
  • skip sequential or dynamic

VSAM provides the below access and retrieval options on the different dataset types -

  • two direct access modes (addressed or keyed)
  • two access entities (logical records and control intervals)
  • two access directions (forward and backward)
  • retrieval options (as generic key and as key greater-than-or-equal)

Access Methods and File Organization -

Data set types are classified based on the several ways of organizing the data stored on IBM disks. IBM software supports the below data set types as of today -

  • Physical Sequential (PS)
  • Partitioned Organization (PO)
  • Indexed Sequential (IS)
  • Direct Access (DA)
  • Virtual Storage Access Method (VSAM)

Each data set except VSAM has its own records organizing way depending on their purpose. VSAM is a multifunction and all-purpose access method. VSAM provides a functional equivalent for most of the dataset organizations as follows -

  • ESDS organization is the functional equivalent of Physical Sequential organization (PS).
  • KSDS organization is the functional equivalent of Indexed Sequential organization (IS).
  • RRDS organization is the functional equivalent of Direct Access organization (DA).

How VSAM is different from other datasets?

VSAM datasets are differing from other dataset organization for the below two reasons -

  • VSAM datasets are device-independent.
  • VSAM datasets can be both sequentially and directly accessed.

A record can access by RBA(addressing the record by its displacement (in bytes) from the beginning of the data set), by its key, or by its RRN(record number).

Advantages -

  • High-speed data retrieval when compared to other access methods because of its organized index.
  • Supports all types of data retrieval (sequential, random, dynamic, linear).
  • Supports fixed and variable-length records processing.
  • Supports in batch and online systems.
  • Inserting records is easy when compared to the other access methods.
  • Records can be deleted physically, and the space used by deleted records can be reused for storing newly inserting records without reorganization. This helps for faster retrieving.
  • Supports alternate index.
  • Improves efficiency.
  • Reduce the data redundancy.
  • Datasets can be distributed physically over various volumes based on key ranges.
  • Independent of storage device types.
  • VSAM datasets can be shared across systems and regions.
  • A small portion of VSAM dataset information is required to specify in JCL and doesn't require detailed information because the VSAM dataset information is stored centrally (VSAM Catalog).
  • The index is small and uses less memory to store because it uses a key compression algorithm.
  • Provides data security with password protection of dataset at different levels(read, write, update, delete, etc.).

Disadvantages -

  • VSAM files can't be displayed and edited by ISPF, and it requires a tool to get them displayed and edited in ISPF.
  • Requires more DASD space.
  • Only works with data stored on disk and can't handle data stored on other devices like tapes, etc.
  • VSAM has minimal support for data sharing across regions.
  • VSAM only provides security at the dataset level, and if any changes are required to the security level, it should be manually handled by the user.