WHAT IS VSAM

What is VSAM? VSAM, for Virtual Storage Access Method, is an IBM access method intended to provide, in one coherent system, most of the fi...

What is VSAM?
VSAM, for Virtual Storage Access Method, is an IBM access method intended to provide, in one coherent system, most of the file organization needs for application programmers.

An access method such as VSAM is not a database management system. The two are easily confused. An access method stores and retrieves data. It does to provide for a relationship among the data, aside from data. It does not provide for a relationship among the data, aside from the fact that sequential datasets are in sequential order. A database system such as DB2 or IMS is often implemented using VSAM.  They differ in that the database also carries information that shows the relationship among the data. To use the book example, a book becomes a database when it references information in other books.

As such, it provides sequential, indexed, relative record and linear datasets. In VSAM terminology, these four types of data organization are named as follows:

1.ESDS.  Entry-Sequenced Datasets. These are the familiar sequential datasets, and they can be read or written only in sequential order – that is, the order of entry.
2.KSDS.  Key-Sequenced Datasets. These are datasets stored in order of a key field in the record. Individual records can be accessed randomly by the record key. The keys are stored in a separate index so that record access is a two-stage process: first locating the key in the index and then using the information in the index for the key to locate the record.
3.RSDS.  Relative record Datasets. These are datasets stored in some sequential order. The records have no internal keys, but individual records can be accessed randomly by giving the relative position of the record in the dataset, such as 1,2, and so on.
4.LDS.  Linear Datasets.  These are datasets that consist of a long stream of bytes. They aren’t considered to have records.