What is VSAM - VSAM

What is VSAM?
VSAM is one of several access methods in z/OS. It only applies to data stored in DASD devices.An access method is re-entrant code contained in DFSMSdfp™, a component of the DFSMS z/OS product. This access method makes it easier for an application to execute an I/O operation (moving data between an I/O device and memory).

VSAM access types
There are three types of access in VSAM:

  • Random: This is also referred to as direct access. The logical record needs to be located by the use of a search argument coming from the application. There is no search argument connection between two consecutive logical record accesses.
  • Sequential: The entire file is processed (for Read or Write), one logical record after the other. The application does not need to provide any search argument. The access method can implement a Read look ahead technique to load logical records in the buffers not yet required by the application program.
  • Skip sequential: A combination of the two previous types of access. The application randomly provides one search argument and from the located logical record on, all records are processed sequentially. An example is sequentially processing all the customers of a bank branch office in a file that has all the customers of the bank.

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