Catalog maintains the unit and volume where dataset resides. Catalog is used for retrieval of datasets. Non-VSAM datasets create a catalog entry by means of Disposition Parameter in JCL. VSAM datasets maintains its own catalog in form of KSDS cluster. In following image you can see the type of VSAM catalogs −
Master catalog is itself a file which monitors and manages operations of VSAM. Their is only one master catalog in any system which contains entries about system datasets and VSAM datasets. VSAM and Non-VSAM datasets may have entry in master catalog but this is not a good practice. The master catalog is created during system generation process and resides on system volume. Master catalog owns all VSAM resources in operating system. All files used in VSAM are controlled by master catalog. Master catalog is responsible for following operations −
When any of the above file attributes changes they are automatically updated in master catalog. Master catalog is defined using IDCAMS programs.
User catalog has same structure and concepts as the master catalog. It is present at next hierarchy level after master catalog. User catalog is not mandatory in the system but it is used to enhance security of VSAM system. Master catalog points to VSAM files but if User catalog is present then master catalog points to user catalog. User catalogs can be many in number as per the system requirement. In VSAM structure if master catalog is removed then it will not affect the user catalog. User catalog contain entries about application specific datasets. The information of user catalog is stored in master catalog.
Data space is an area of the direct access storage device that is exclusively allocated for VSAM use. Data space must be created before creating VSAM clusters. The area occupied by the data space is recorded in the Volume Table of Contents (VTOC), so that the space will not be available for allocation to any other use, either VSAM or non-VSAM. VTOC has entry of area occupied by space. VSAM creates a data space to hold the user catalog entries. VSAM takes control of this space and monitors and maintains this space as needed by VSAM files.
Unique Clusters consists of a separate data space which is utilized completely by the cluster created within it. Unique clusters are created out of unallocated space on direct access storage.
A sub-allocated VSAM file shares the VSAM space with other sub-allocated files. It specifies that file should be sub-allocated within existing VSAM space. Sub-allocation is used for easier management and control of VSAM spaces.
Non-VSAM datasets resides on both tape and direct access storage. Non-VSAM datasets may have entries in both master catalog and user catalogs. The main function of cataloging non-VSAM datasets is to retain unit and volume serial information.