ABCs of z/OS System Programming Volume 3 - IBM Redbooks

4.30 Virtual storage access method (VSAM)
Catalog
Management
CATALOG
Figure 4-40 Virtual storage access method
Virtual storage access method (VSAM)
VSAM is a DFSMSdfp component used to organize data and maintain information about the
data in a catalog. 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.
There are two major parts of VSAM:
► Catalog management - The catalog contains information about the data sets.
► Record management - VSAM can be used to organize records into five types of data sets,
that is, the VSAM access method function:
– Key-sequenced data sets (KSDS) contain records in ascending collating sequence.
Records can be accessed by a field, called a key, or by a relative byte address.
– Entry-sequenced data sets (ESDS) contain records in the order in which they were
entered. Records are added to the end of the data set.
– Linear data sets (LDS) contain 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 sets (RRDS) contains records in relative record number order, and
the records can be accessed only by this number. There are two types of relative
158 ABCs of z/OS System Programming Volume 3
Record
Management
VSAM DATA SETS
VSAM Data set types:
KSDS
ESDS
LDS
RRDS
Fixed Length
Variable Length