The Access Method Services utility program IDCAMS is commonly used to manipulate ("delete and define") VSAM data sets.
Custom programs can access VSAM datasets through Data Definition (DD) statements in Job Control Language (JCL), via dynamic allocation or in online regions such as in Customer Information Control System (CICS).
The size of the CI and CA is determined by the Access Method Services (AMS), and the way in which they are used is normally not visible to the user.
You can use three types of record-orientated file organization with VSAM (the contents of linear data sets have no record structure): An ESDS may have an index defined to it to enable access via keys, by defining an Alternate Index.
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.
VSAM was introduced as a replacement for older access methods[14] and was intended to add function, to be easier to use and to overcome problems of performance and device-dependence.
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.
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.