Alternate Index (AIX) is itself a KSDS. The data structure used by a KSDS is nowadays known as a B+ tree. Relative VSAM organization An RRDS may have an index defined to it to enable access via keys, by defining an Alternate Index. Linear VSAM organization An LDS is an unstructured VSAM dataset with a control interval size of a multiple of 4K. It is used by certain system services. VSAM Data Access Techniques There are four types of access techniques for VSAM data: Local Shared Resources (LSR), is optimised for "random" or direct access. LSR access is easy to achieve from CICS. Global Shared Resources (GSR) Non-Shared Resources (NSR), which is optimised for sequential access. NSR access has historically been easier to use than LSR for batch programs. Distributed File Management (DFM), an implementation of a Distributed Data Management Architecture server, enables programs on remote computers to create, manage, and access VSAM files. Sharing VSAM data Sharing of VSAM data between CICS regions can be done by VSAM Record-Level Sharing (RLS). This adds record caching and, more importantly, record locking. Logging and commit processing remain the responsibility of CICS which means that sharing of VSAM data outside a CICS environment is severely restricted. Sharing between CICS regions and batch jobs requires Transactional VSAM, DFSMStvs. This is an optional program that builds on VSAM RLS by adding logging and two-phase commit, using underlying z/OS system services. This permits generalised sharing of VSAM data. History VSAM was introduced as a replacement for older access methods 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 storage operating systems (DOS/VS, OS/VS1 and OS/VS2) for its new System/370 series, as successors of the DOS/360 and 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