Defining, Generating, and Punching a DMCL › Data Sharing Attributes

Data Sharing Attributes

What Attributes Can Be Specified?

The following data sharing-related attributes can be specified in a DMCL:

• The maximum number of members that can belong to the data sharing group
• The number of entries in the group's lock structure
• The default shared cache structure for the member using this DMCL
• The action that should be taken in response to a coupling facility failure

Group Membership

A DC/UCF system is specified to be a member of a data sharing group through parameters in the SYSIDMS file in the system's startup JCL. The system belongs to the specified group from the time it begins execution until it is shutdown. If the system abends, it remains a group member until it is restarted and terminated normally.

Specifying the Maximum Number of Members

The DMCL of each group member specifies the maximum number of members that can belong to the group at one time. The maximum number of members should be large enough to accommodate all anticipated systems, but since the value affects the size of the lock structure, it should not be larger than necessary.

What is a Lock Structure?

A lock structure is an object that resides in a coupling facility. It contains global locks that are used to control inter-member access to shared resources such as database areas and record occurrences.

Part of a lock structure is a table whose entries represent hash values. You specify the number of entries in this table as one of the data sharing attributes in the DMCL. The more entries in this table, the less likelihood there is that multiple resources will hash to the same table entry, a situation that increases locking overhead. However, the more entries in this table, the larger the lock structure needed to contain it.

Specifying the Number of Lock Table Entries

The value that you specify for the number of lock table entries should be at least as large as the highest SYSLOCKs value specified in the system definition of any member in the group. Performance may be improved by specifying an even larger value.

Note: For more information about sizing a lock structure, see the CA IDMS System Operations Guide.

Conflicting Group Attributes

Since the DMCL used by each member of a data sharing group specifies the maximum number of group members and the number of lock table entries, it is possible that the values specified by different members conflict. The first member to start determines the effective values and those values remain unchanged until all members of the group terminate normally. You can determine which values are in effect by issuing a DCMT DISPLAY DATA SHARING command.

What Is a Shared Cache?

A shared cache is a structure that resides in a coupling facility. It is used to contain database pages and acts as a global buffer pool shared across central versions. The use of a shared cache reduces the number of I/Os to the database.

In order to share update access to data, all files associated with a shared area must be assigned to a cache structure. One means of doing this is to specify a default shared cache for the DMCL and override the default as necessary for individual segments and files.

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

Coupling Facility Failures

In order to share update access to data, the coupling facility must be available to control access to shared resources. You may specify what action a member is to take in the event that a coupling facility structure becomes unavailable while a DC/UCF system is executing. You may direct the system to:

• Abend as soon as it detects a failure in a critical coupling facility structure
• Remain active but abend tasks that request access to shared resources

By directing the system to remain active, it can service transactions that do not access shared data. However, you will not be able to shut down the system normally since it will be unable to successfully disconnect from one or more coupling facility structures.

Note: For more information about dealing with coupling facility failures, see the CA IDMS System Operations Guide.