MAINFRAMES....

VSAM stands for Virtual Storage Access Method.

 What is Virtual storage?

There are two types of storage supported by Z/OS (IBM Mainframes OS)

       1)    Physical storage.

  2)    Virtual storage.

As you know IBM mainframe is used by multiple user at a time from their work location so every time accessing data, updating data  etc . take place. So every time instead of using the physical storage where actual data resides the data called in virtual storage as per the job requirement.

Storage used by Mainframe operating system is Direct Access Storage Device (DASD) volume. In mainframe computer there is two types of storage one is Actual physical storagei.e memory and another is Virtual storage to which user has an access.z/OS keeps only the active portions of each program in central storage. 

Virtual storage  is a memory management technique by which programs are made to work as if they have more memory available to them than is actually attached to the computer. After running program on mainframe the output is reflect on physical memory or can be placed on disk.

So it will speed up the process of data manipulation.So for updation also data manipulated is updated in physical storage.

Why IBM has introduced VSAM

VSAM is also a file system like a normal PS file but it overcomes the drawbacks of PS file.AS no any system is perfect VSAM file also have some drawbacks.

So to speed up the data manipulation operation  IBM introduced VSAM in 1973.

VSAM advantages over PS dataset

1)AS the KSDS data set has an INDEX component the searching operation will be faster than normal PS files.

2)Records can be physically deleted.

3) All VSAM files are stored in VSAM catlog so we can refer any VSAM dataset no need to detailed in  JCL.

4) Records can be accessed sequentially and randomly.

5) The JCL required to access the VSAM dataset is much simpler.

Disadvantages

Space requirement is higher as there is free space reuired after every record.

 I will write the VSAM tutorial in two parts

      1) VSAM with JCL used to create,delete,alter,load the VSAM files from PS,GDG files

      2) VSAM with COBOL used to read,write,Rewrite the VSAM file.

  Types of VSAM

)      

          1)   KSDS (Key Sequential Data set)

          2)  ESDS (Entry sequential Data set)

           3)RRDS(Relative Record Data Set)