VSAM -- an Access Method

VSAM, the Virtual Storage Access Method, is a general purpose access method intended to assist users in storing and retrieving data on disk devices of various types. In IBM jargon, these disk devices are often referred to as DASD, or Direct Access Storage Devices.

Because of the complexity of performing I/O operations on disk devices -- especially CKD and ECKD devices, access methods such as SAM (the Sequential Access Method) and VSAM were developed.

VSAM could be considered to have several principal components:

Access Method Services

AMS is a comprehensive utility package used to

• Define space on disk volumes
• Define catalogs to manage disk space and files
• Define clusters (or datasets or files) and their components
• Define and create alternate indexes
• Copy data into or out of VSAM files
• Print contents of VSAM files
• List catalog information
• Export and import information to and from other systems
• Backup and restore files and collections of files

Catalog Services

Access method components which handle allocating of disk space, opening and closing of files, and protecting of data from concurrent use in ways that might be destructive to the data.

Record Management

Access method components which handle the actual writing of user's logical data records from program work areas to VSAM buffer areas and then to disk, and the actual reading of this data from disk into VSAM's buffers and moving it to the user's work area for processing by the application program.

Record management components are responsible for the actual creation of channel programs used to write data from VSAM's buffers to disk and to read data from disk into VSAM's buffers, as well as the transfer between user work areas and VSAM's buffers.

Record management will handle retrieval sequentially or directly, by key or by address, as needed. Thus, a simple retrieve request may involve index searches to locate the position of the data record as well as the ultimate operation of reading the data from disk.

VSAM I/O Device Use

VSAM uses disk devices for storage of catalogs and files, and optionally to save backup copies of files:

CKD Devices

These represent the oldest types of disk subsystems used by VSAM. Included in this group were IBM 2314, 3330, 3340, and 3380 devices.

ECKD Devices

These represent current types of disk subsystems used by VSAM. This group includes IBM 3390, and the RAMAC, RAMAC Virtual Array (RVA), and the SHARK subsystems which simulate 3390 devices. Internal Disk subsystems on IBM Multiprise 2000 and Multiprise 3000 processors can simulate these types of disk subsystems as well.

FBA Devices

These represent older types of disk subsystems, but which are still currently in use or which are simulated by current technology, such as Virtual Disks. The IBM 3310, 3370, 9332, 9335, and 9336 devices fall into this group.

A distinction is made between these ("real FBA") devices and the Virtual FBA (VFBA) virtual disk implementations of VSE and VM because of the volitile nature of the VFBA storage.

VSAM also uses other I/O devices:

Tape Devices

Tape devices are used by the AMS utility program for

• Export and Import
• Backup and Restore
• Copy data between VSAM files and sequential file format on tape

Printer Devices

The AMS utility program uses printer output to log command processing actions, list catalog contents, and to print VSAM file contents (this is particularly useful for debugging, etc.).

Card Devices

The AMS utility reads its input command stream as card images. In some cases, card image data can be copied into a VSAM file as well.