Defining, Generating, and Punching a DMCL › Adding Segments to the DMCL › File Overrides

File Overrides

The following information can be specified or overridden at the file level:

• External file name (DDNAME)
• Dataset disposition for dynamic allocation
• Dataspace usage
• Buffer association
• Shared cache association

Overriding the External File Name

If your DMCL contains files defined with duplicate external file names, use the file override clause to resolve the conflict.

Dataspace Usage

Use file overrides to indicate that a file is to reside in a dataspace. If a dataspace is used, whenever a page is read from disk it will be cached in the dataspace. All future reads will receive a copy of the page in the dataspace, thus reducing I/O requests. The page will remain in the dataspace until the file is closed.

The DCMT VARY FILE command allows the dataspace specification to be changed dynamically while the system is running.

Shared Cache Association

You can associate a shared cache with a file either through a file override or by specifying a default shared cache for the file's segment. The latter is then used for all files within the segment, unless a file override specifies a different shared cache.

A default shared cache can also be specified for the DMCL. This is used only in a data sharing environment for file's whose associated area is designated as shared and for which no cache has otherwise been assigned.

Note: For more information about using shared cache, see the CA IDMS System Operations Guide.