Mainframes 360
The one stop destination for System Z professionals

Wednesday, July 15, 2009

Introduction to VSAM Files

  This post is partially complete and several sections may require editing and proof-reading. The contents on the history of the organization of a VSAM File, CI and CA splits is under updation.

Access Methods
Each peripheral device such as a disk-drive or a tape cartridge has a different geometry, organization of data. Programs often process the data on a disk/tape. An application program must then contain logic to communicate with various I-O devices.

An access method is a software (bunch of tiny programs) to write device independent code. What device drivers are to Windows operating system, access methods are to mainframe zOS. Access methods assume the burden of interacting directly with the hardware. Developers can include high-level business logic, and avoid the gory details of the specific hardware in their program source-code. Access-methods are an interface between the program and the hardware device.

VSAM is one of the several access methods in the zOS. VSAM makes it easier for application programs to execute an I/O operation. There are several other access methods - QSAM, BSAM, ISAM etc.

QSAM is an access-method used to read/write data from Sequential Files. QSAM stands for Queued Sequential Access Method. QSAM queues records from the file, into an area of the main storage called System buffers. This speeds up the execution of the program.

The "V" in VSAM stands for Virtual. VSAM is an access method used to read/write data from direct-access devices, such as disk-drives.

To many people who are thrown to work at a mainframe computer on their first job, they feel lost. Mainframe people seem to speak a completely different language and that doesn't make life easy. What's more, the books and manuals are incredibly hard to comprehend.

"What on earth is a Mainframe?" is an absolute beginner's guide to mainframe computers. We'll introduce you to the hardware and peripherals. We'll talk about the operating system, the software installed on a mainframe. We'll also talk about the different people who work on a mainframe. In a nutshell, we'll de-mystify the mainframe.

Readers based in India, can buy the e-book for Rs. 50 only or the print book. International readers based in the US and other countries can click here to purchase the e-book.