Introduction of vsam files

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Introduction of VSAM files

  • VSAM stands for Virtual Storage Access Method.
  • VSAM files are datasets which are NOT sequential Files.
  • Virtual Storage Access Method is an IBM DASD file Storage method.
  • The records in a VSAM file can be of Fixed Length or a variable Length.
Introduction of VSAM files

When You should Choose a VSAM file over normal Sequential file?

  • When you want to do multiple operations in the file based on a particular key value, then you should choose a VSAM file.
  • If you want to update a particular record in the file or you want to delete your files as a normal activity, then you choose VSAM file over sequential file because your intention is to perform multiple operations on the file.
  • VSAM files are much more powerful  and advanced as compared to normal sequential  files.
  • You can access the records of Sequential file in sequential order only but you can access the records of a VSAM file in multiple ways based on your requirement. You can access the records sequentially or using a key or dynamically in case of VSAM files.
  • There are multiple types of VSAM files and you can select the VSAM file based on your requirement.
  • When simple operations are involved like reading the file, writing the output to SPOOL(Simultaneous Peripheral Operations Online) or output device, then you can use sequential files but suppose you need a file wherein you need to frequently change the contents of the file periodically and multiple operations are required like different way of accessing etc. then you can use VSAM files.
  • If your file has a particular key value, then you can use that key value to access the records. You can access the records using primary key or even alternate keys.
  • If you need to update, insert or delete the records frequently, then VSAM is a better option compared to sequential files.
  • If you want to access the file directly using IPSF options 3.4, then VSAM is not a good choice.
  • You have too see the performance issues as well.

Important points about a VSAM file

  • You cannot directly modify the VSAM file using ISPF or using option 3.4 of TSO/ISPF.
  • VSAM files cannot access the data stored in Tape but can access the data from or to a DASD(Direct Access Storage Device).
  • The Records can be accessed in multiple ways like  sequential order or you can also place the records in logical sequence using a primary or secondary key or you can also access the records in random order or you can also access the records using RRN(Relative Record Number).

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