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ABCs of z/OS System Programming Volume 3 - IBM Redbooks

ABCs of z/OS System Programming Volume 3 - IBM Redbooks

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The sequence for locating an existing data set<br />

The MVS system provides a service called LOCATE to read entries in a catalog. When z/<strong>OS</strong><br />

tries to locate an existing data set, the following sequence takes place.<br />

► When the master catalog is examined:<br />

– If it has the searched data set name, the volume information is picked up and the<br />

volume VTOC is used to locate the data set in the specified volume.<br />

– If the HLQ is a defined alias in the master catalog, the user catalog is searched. If the<br />

data set name is found, processing proceeds as in a master catalog find.<br />

► Finally, the requesting program can access the data set. As you can imagine, it is<br />

impossible to keep track <strong>of</strong> the location <strong>of</strong> millions <strong>of</strong> data sets without the catalog concept.<br />

For detailed information about catalogs refer to Chapter 6, “Catalogs” on page 325.<br />

Chapter 2. Data set basics 43

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