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!
- 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.
- 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.
- 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.
- 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.