Defining Segments, Files, and Areas › Planning › Specifying Data Set Name Information

Specifying Data Set Name Information

Specifying a Data Set Name

When you access a file, you must provide information to the operating system to locate the file on disk storage. You can specify this information in one of two ways:

• In the JCL used to execute your local mode or central version system
• For z/OS and z/VM operating systems and for z/VSE with DYNAM/D, by supplying dataset information in the FILE statement

Reasons to Specify Dataset Information on the FILE Statement

The advantages of specifying the data set name or other operating system information on the FILE statement are:

• You can specify fewer statements in the system execution JCL.
• If you change the location of a file, only its definition needs changing and not every set of execution JCL.
• By not supplying an external file name (a ddname), you can ensure that only the correct file is accessed, since the dataset name cannot be overridden in the execution JCL.
• In a z/OS operating system, you can access more files if dynamic allocation is used to reference them.

Controlling the Use of Dynamic Allocation in Local Mode

By default, data set information included on the FILE statement will be used in both central version and local mode environments to dynamically allocate a data set unless the identifying information is overridden through a JCL statement.

A site may control whether dynamic allocation is used by default for local mode operations and the default behavior can be overridden for an individual job step. Both of these actions are effected through the use of the LOCAL_DYNAMIC_ALLOCATION SYSIDMS parameter. The default behavior can be established by compiling a SYSIDMS options module and it can be overridden by specifying a LOCAL_DYNAMIC_ALLOCATION parameter in the SYSIDMS file associated with the job step.

Note: For more information about the SYSIDMS parameter file, see the CA IDMS Common Facilities Guide.