Editing Virtual Storage Access Method (section)

==History==
VSAM was introduced as a replacement for older access methods<ref>{{cite manual
 |     title = OS/Virtual Storage 1 Features Supplement 
 |        id = GC20-1752-0
 |      date = August 1972
 |   edition = First
 |       url = http://bitsavers.org/pdf/ibm/370/OS_VS1/GC20-1752-0_OS_VS1_Features_Supplement_Aug72.pdf
 | publisher = IBM
 }}
</ref> and was intended to add function, to be easier to use and to overcome problems of performance and device-dependence. VSAM was introduced in the 1970s when IBM announced [[virtual memory|virtual storage]] operating systems (DOS/VS, OS/VS1 and OS/VS2) for its new System/370 series, as successors of the [[DOS/360 and successors|DOS/360]] and [[OS/360 and successors|OS/360]] operating systems running on its System/360 computer series. While backwards compatibility was maintained, the older access methods suffered from performance problems due to the address translation required for virtual storage.

The KSDS organization was designed to replace [[ISAM]], the Indexed Sequential Access Method. Changes in disk technology had meant that searching for data in ISAM data sets had become very inefficient. It was also difficult to move ISAM data sets as there were embedded pointers to physical disk locations which became invalid if the data set was moved. IBM also provided a compatibility interface to allow programs coded to use ISAM to use a KSDS instead.

The RRDS organization was designed to replace [[BDAM]], the Basic Direct Access Method. In some cases, BDAM data sets contained embedded pointers which prevented them from being moved. However, most BDAM data sets did not and the incentive to move from BDAM to VSAM RRDS was much less compelling than that to move from ISAM to VSAM KSDS.

Linear data sets were added later, followed by VSAM RLS and then Transactional VSAM.