VSAM has been available for VSE users since 1973, when DOS/VS Release 28 became available. Today, it is estimated that over 90 percent of the data in VSE environments is managed by VSE/VSAM. The care and feeding of VSE/VSAM (the proper establishment of the VSAM environment and definition of the data sets and other objects within it) can pay immense rewards.

In the mid-1980s, IBM established a performance benchmarking center in Dallas, and the Swami was the first technical staff member of that organization. In his performance and benchmarking experience, most of the performance benefit possible from optimum tuning of batch and on-line workloads comes from simple tuning of VSAM data sets and the program access specifications (JCL, CICS FCT options, etc.) for them.

If you wanted to know how to do this, you've found the right place!

Terminology and Some Useful Jargon

VSAM has been available for VSE users since 1973, when DOS/VS Release 28 became available. Today, it is estimated that over 90 percent of the data in VSE environments is managed by VSE/VSAM. The care and feeding of VSE/VSAM (the proper establishment of the VSAM environment and definition of the data sets and other objects within it) can pay immense rewards.

In the mid-1980s, IBM established a performance benchmarking center in Dallas, and the Swami was the first technical staff member of that organization. In his performance and benchmarking experience, most of the performance benefit possible from optimum tuning of batch and on-line workloads comes from simple tuning of VSAM data sets and the program access specifications (JCL, CICS FCT options, etc.) for them.

If you wanted to know how to do this, you've found the right place!

VSAM

The Virtual Storage Access Method, a collection of interfaces, commands, and structures along with the programming subsystems that implement them, for storing and retrieving data on disk (sometimes referred to as DASD, for Direct Access Storage Devices) in IBM e-server mainframe systems, such as VSE, zVM and zOS.

Record (Logical Record)

A collection of data items (fields) that describe an entity. All the fields of a record are stored or retrieved together. A collection of records is called a file or a data set.

Field

An elementary data component of a record -- one or more bytes with a particular use. For example, LASTNAME, FIRSTNAME, ADDRESS, ACCOUNT BALANCE and BIRTHDATE could all be fields in a record. Some fields are numeric, others are simply text strings.

File

A collection of records serving a common purpose. The PAYROLL FILE would contain records indicating hours worked, pay rate, name, social security and tax information.

Data Set

A file by any other name is still a file. Lost in the background of MVS, back in the OS/360 days, they began calling their files data sets.

Block

A physical record containing potentially multiple logical records. A block of data is read or written from/to external devices (disks, tapes) as a single transfer. VSAM reads and writes Control Intervals, which can be comprised of one or more blocks (or physical records).

Control Interval

The smallest unit of data VSAM will read or write as a single unit. A control interval (CI) can be composed of multiple physical records or blocks. A control interval will contain VSAM control information, free space and some number of (logical) records.

Control Area

A group of control intervals containing data (if a keyed file) managed by a single index control interval. In sequential processing, a control area represents the largest amount of data VSAM would potentially read or write at one time.

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.

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