Virtual Storage Access Method

VSAM arranges records by an index key, relative record number, or relative byte addressing. VSAM is used for direct or sequential processing of fixed-length and variable-length records on DASD. Data that is organized by VSAM is cataloged for easy retrieval and is stored in one of five types of data sets.

Entry-sequenced data set (ESDS). Contains records in the order in which they were entered. Records are added to the end of the data set and can be accessed.
Key-sequenced data set (KSDS). Contains records in ascending collating sequence. Records can be accessed by a field, called a key, or by a relative byte address.
Linear data set (LDS). Contains data that has no record boundaries. Linear data sets contain none of the control information that other VSAM data sets do. Linear data sets must be cataloged in a catalog.
Relative record data set (RRDS). Contains records in relative record number order, and the records can be accessed only by this number. There are two types of relative record data sets.
- Fixed-length RRDS: The records must be of fixed length.
- Variable-length RRDS: The records can vary in length.
Throughout this document, the term RRDS refers to both types of relative record data sets, unless they need to be differentiated.
z/OS UNIX files. A UNIX file can be accessed as if it were a VSAM entry-sequenced data set (ESDS). Although UNIX files are not actually stored as entry-sequenced data sets, the system attempts to simulate the characteristics of such a data set. To identify or access a UNIX file, specify the path that leads to it.

Any type of VSAM data set can be in extended format. Extended-format data sets have a different internal storage format than data sets that are not extended. This storage format gives extended-format data sets additional usability characteristics and possibly better performance due to striping. You can choose for an extended-format key-sequenced data set to be in the compressed format. Extended-format data sets must be SMS managed. You cannot use an extended-format data set for certain system data sets.

Requirement: Do not use BISAM or QISAM. Use VSAM instead.