CATALOG is physically another VSAM dataset. It contains information about other datasets. It provides users the ability to locate a dataset without knowing where the dataset resides. Over the course of MVS history, catalogs have gone through a number of different structures:

a.     OS CVOL(Control Volumes) – The OS CVOLS contains indexes that relate the dataset names to the serial number and device type of the volumes containing the dataset. A VTOC containing the dataset attributes resided on each volume.

b.    VSAM Catalogs – The VSAM catalogs includes for each dataset, not only the volume serial number of the volume containing the dataset but the dataset extent and other attributes. Thus keeping the information in a central VSAM catalog access was made directly to datasets on different volumes without the necessity of searching the VTOC of these different volumes. Can be used to catalog both VSAM and Non-VSAM datasets.

c.     ICF(Integrated Catalog Facility)
All of the 3 types of catalogs can co-exist on the same operating system. All datasets managed by SMS must be cataloged in ICF catalog. Functions provided for VSAM catalogs and OS CVOLS are for compatibility only.

Catalog Entries
a.     Catalog Entries for VSAM dataset
1.     Dataset Name
2.     Device Type and Volume
3.     Dataset Physical attributes (LRECL, BLKSIZE, RECFM)
4.     Dataset processing options
5.     Extent info in VTOC
b.    Catalog entries for Non-VSAM datatset
1.     Dataset Name
2.     Device Type and Volume
3.     Extent Info and Dataset physical attributes in VTOC
VTOC lists the datasets that reside on its volume, along with information about the location and size of each dataset and other dataset attributes.
For VSAM datasets, VTOC contains the extent info and for Non-VSAM datasets it contains the datatset attributes and extent info.


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