VSAM Structure

VSAM LDS VSAM Cluster Components


VSAM Structure contains below four major parts.

  • The Master catalog
  • The User catalog
  • The Data space
  • The Cluster

Let’s discuss about individuals one by one.

Master Catalog

Every system that uses VSAM has one and only one master catalog.

The master catalog contains entries about system datasets and VSAM structures.

Any VSAM or non-VSAM dataset can be cataloged in the master catalog

In most of the computer systems, the Systems Programming team will create user catalogs, which are cataloged in the master catalog

All the users of the system will be allowed to catalog datasets in user catalogs created.

The master catalog is created during the System Generation process and normally resides on the System Residence volume.

The master catalog "owns" all other VSAM resources in a computer system.

User Catalogs

A user catalog is created to contain entries of application specific datasets.

The user catalog defining information is stored into catalog entry in the master catalog.

VSAM Data Space

A data space must be created, before VSAM clusters can be created on a volume.

A data space is an area of the direct access storage device that is exclusively allocated for VSAM use.

That is, the area occupied by the data space is recorded in the Volume Table of Contents (VTOC) of the volume as allocated to a dataset, so that the space will not be available for allocation to any other use, either VSAM or non-VSAM. There are three data spaces shown in the chart (#4, #5, and #6).

Actually, when either the master catalog or a user catalog is defined, VSAM creates a data space to hold the user catalog entries, allocating the amount of space specified for the user catalog from the space available on the volume as a data space which will be completely allocated to the catalog..

The name of the files that are recorded in the VTOC for space allocated to catalogs and data spaces is generated by VSAM. However, they can be easily recognized for what they contain from the high level qualifier of the generated name. For catalogs (both master and user), the high level qualifier is Z9999992. For data spaces, the high level qualifier is Z9999994.


VSAM dataset which we have seen in detail in our earlier section when defined logically takes up the form as a VSAM cluster. Before proceeding to see about VSAM cluster it is very important to know primarily what a cluster is. A cluster nothing but a association of the index, sequence set and data portions of the dataset. The operating system takes the responsibility of giving program access to the cluster by which access to given to all parts of the dataset simultaneously. The space occupied by a VSAM cluster is divided into contiguous areas called control intervals (CI) and about this concept we have seen in detail in our earlier section.

There are two main components present in a VSAM cluster and they are as given below:

• The data component

• The index component

Data Component of VSAM Cluster: Data records are present in the data component of a VSAM cluster.

Index Component of VSAM Cluster: Index records are present in the index component of a VSAM key-sequenced cluster.

VSAM LDS VSAM Cluster Components