Physical Database DDL Statements › DMCL Statements › Entity Currency › Usage

Usage

Ordering Definitions

You must define one or more database buffers for the DMCL before you add segments.

Assigning Buffers for Native VSAM Files

The following restrictions apply to buffers assigned to native VSAM files:

• If the buffer is defined as NSR, all files using it must be associated with a single area.
• If the file access method is KSDS, ESDS, PATH, or RRDS, then the associated buffer must be defined as NSR or LSR. Likewise, if the buffer is defined as NSR or LSR, only KSDS, ESDS, PATH, and RRDS files can use it
• All PATH files associated with an area mapped to a KSDS or ESDS file must use the same buffer as the KSDS or ESDS file.

Assigning Buffers for Other Files

The page size of the buffer must be greater than or equal to the page size of all areas whose files are assigned to the buffer. If the file's access method is VSAM, the page size of the buffer must be greater than or equal to the file's control interval size.

External File Name

All non-blank external file names, including those for both database and journal files, must be unique within a DMCL. If necessary, use file overrides to assign unique names.

An external file name must be specified unless dynamic allocation will be used to access the file.

Note: For more information about dynamic file allocation in various operating systems, see 7.14.3, “Usage".

Archive Journal Block Size

Upon generation, the block size associated with an archive journal is checked to ensure it is not less than the block size of the disk journals. Since the block size of the disk journals is derived from the page size of the journal buffer, if the archive journal's block size is less than the page size of the journal buffer, the page size of the journal buffer is used and a warning message issued.

Caching Files in Memory

You can reduce retrieval I/O operations by caching a file in memory using the MEMORY CACHE clause of the file override specification. File caching is not supported for native VSAM files.

Note: For more information about using memory caching, see Reducing I/O.

Dataspace Versus Memory Cache

The MEMORY CACHE clause replaces the use of the DATASPACE clause. The latter is still accepted for upward compatibility, but is no longer generated on displays.

Controlling Memory Cache

Use the DMCL-wide MEMORY CACHE options to control where and how much memory cache storage can be allocated.

Insufficient Storage for Memory Cache

If MEMORY CACHE YES is specified for a file, and not enough storage is available to cache the file in memory, processing continues as if MEMORY CACHE NO was specified.

Dynamically Changing Memory Cache Specification

The MEMORY CACHE specification can be changed dynamically:

• Use DCMT VARY DMCL to change DMCL-wide MEMORY CACHE options
• Use DCMT VARY FILE to change the MEMORY CACHE specification for a file.

Note: For more information about DCMT VARY DMCL and DCMT VARY FILE, see the CA IDMS System Tasks and Operator Commands Guide.

Specifying Data Sharing Attributes

Each data sharing group has an associated coupling facility lock structure. The first CA IDMS system to become a member of the group, establishes the attributes of the lock structure. These attributes remain in effect until all members of the group have terminated normally. As long as any CA IDMS system is either active or has failed and not yet been restarted, the existing lock structure attributes remain in effect. Lock structure attributes include the number of lock entries and the maximum number of members. Both of these attributes affect the size requirements for the lock structure and should be chosen carefully.

Note: For more information about specifying data sharing attributes, see 24.4, “Reducing I/O”. Also see the CA IDMS System Operations Guide.