High Quality Content by WIKIPEDIA articles! VSAM maintains extensive information about data sets and the DASD space they occupy. It is kept along with a wide range of other information in a central location called the VSAM catalog.The VSAM catalog is very important because a VSAM data set cannot exist until it is first defined in the catalog. This means that information about the data set is recorded in the catalog in the form of entries. Also, a VSAM data set cannot be accessed unless information about it can be obtained from the catalog. Therefore, the catalog must be available for use.