VSAM (Virtual STORAGE ACCESS Method) describes an access method to files, which were put on on IBM large computer systems under the operating system z/OS. The naming is based on the idea to be able to address file contents such as cells in (virtual) main storage which is supported with the help of a RBA (relative byte address). For the first time it was made possible thereby to have to take on physical characteristics of the storage media (plate cylinders for example) no more consideration.

VSAM files are called also cluster (corresponding data heaps). There are different forms of VSAM cluster:

• KSDS (key sequential DATA set): With this file type the data access is made by an index, which is stored on operating system level in its own DATA set. Equally the sequential access one supports

• ESDS (entry sequential DATA set): With this file type sequentially the volume of data of the file one accesses. By alternative indices a direct access can later be made possible by indices (key fields).

• RRDS (relative records DATA set): That access takes place by logical record numbers.

• LDS (linear DATA set): Is a completely unstructured byte stream, which is arbitrarily interpretable by the application software however.

• zFS (zSeries files system): An advancement of the HFS is DATA sets, which are used by the Unix system services (hierarchical Unix contains file structure).

VSAM files are put on and administered under z/OS with the help of the Utilitys IDCAMS.

Data base systems such as CIM or DB2 use VSAM cluster for the storage of their data.

There the key usually of several information, as for instance "“posting date, article, quantity"” exists and the VSAM cluster is, arises an efficient and fast accessability sorted according to the key on all records, which begin with a partial key.

For example all records (data records) are to be selected, whose posting date was in January 2003. But VSAM offers the option GENE KEY, which stands for "“gene Eric key"”. With first vintages hands one to the READ instruction over a key, which consists only of 6 bytes (JJJJMM). From there out one can read on sequentially, until the first 6 bytes of the key do not agree any more. In pseudo code could look in such a way:

• Set END on "“N"”
• Open the file in an VSAM environment with generic key.
• Read the first data record, whose key begins with "“200301"”.
• So long END unequally "“J"”:
  • Write data record on the printer
  • Read the next data record
  • If file end reaches or at the beginning of the key of the evenly read data record no more does not agree:
    • Set END on "“J"”
• The file closes

The first implementations of VSAM in the MVS 3,8 (MVS: Multiple Virtual STORAGE) was disturbance susceptible. In particular the VSAM Cataloge at that time, the volume Ownership and the Suballocatable DATA spaces caused problems. At this time the proverb "“VSAM developed is cruel"”.

Only with the improvements, which brought the ICF Cataloge (MVS XA), VSAM really became generally accepted.

Articles in category "VSAM"

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Related Websites

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• File Status Keys or File Return Code
  A quick reference of the VSAM and QSAM File Status Codes for an IBM mainframe ... This does not apply to VSAM sequential files. File status 0 is returned. ...


• The VSAM-QSAM Connection
  The VSAM and QSAM Connection provides a suite of sample programs describing the techniques used for managing VSAM Data Sets and QSAM flat sequential files.


• Video Surveillance and Monitoring Homepage
  The objective of the VSAM project was to develop automated video ... If something is missing that you used to see, click here for the old VSAM web site. ...


• VSAM Demystified
  Virtual Storage Access Method (VSAM) is one of the access methods used to process data. Many of us have used VSAM and work with VSAM data sets daily, ...


• What is VSAM - A Word Definition From the Webopedia Computer ...
  This page describes the term VSAM and lists other pages on the Web where you can find additional information.