Advanced Topics › Communication Methods › Cross-system Communication Methods › Virtual Control File Communication Methods › How You Select Master Systems

How You Select Master Systems

When running CA MII, you can gain the most benefit by selecting the system with the most managed enqueue activity to be the master system. Because the master system reads the VCF directly from its own storage, it incurs no device I/O overhead, providing the best enqueue response time to the master system. You determine the number of managed enqueue requests by issuing the CA MII DISPLAY SERVICE command.

The master CA MIM address space manages the VCF, therefore, it uses more storage and performs more I/O operations than client CA MIM address spaces.

To define the master candidate list, issue the following statement:

GLOBALVALUE VCFMASTER=(sysa,sysb,sysc)

The master candidate list is examined from left to right during initialization to determine which CA MIM address space of the system is to be the preferred master. The master candidate list is also examined in recovery situations when the current master becomes unavailable due to hardware or system-related errors. In this case, management responsibility of the VCF is passed to the next master candidate.

You can change the master candidate list after synchronization by executing another GLOBALVALUE statement to any single CA MIM address space. The new values are passed to all systems participating in the MIMplex.

The GLOBALVALUE statement has the following parameters:

• ANYELIGIBLE = YES | NO
• The VCFMASTER parameter defines your preferred master candidate list. However, internally, CA MIM builds an eligible master list. The master candidate list and the eligible master list may or may not include the same systems. For example, in a four-system MIMplex (SYSA,SYSB,SYSC,SYSD) where the CA MIM address space on each system has CTC connectivity to each of the other systems, all systems are eligible to become a master. Therefore, all are included in the internally built eligible master list. However, you may have defined GLOBALVALUE VCFMASTER=(SYSA,SYSB). This means the eligible master list consists of (SYSA,SYSB,SYSC,SYSD), while the defined master candidate list consists of only (SYSA,SYSB).
• The GLOBALVALUE ANYELIGIBLE parameter is used in VCF recovery situations when none of the defined master candidate systems (SYSA,SYSB) are available to handle the needed recovery. In this case, specifying ANYELIGIBLE=YES would allow CA MIM to automatically switch the VCF recovery to an available system found in the eligible master list (SYSC or SYSD). The default value for the ANYELIGIBLE parameter is YES.
• MOSTPREFERRED = YES| NO

  The most preferred master is always the leftmost system defined in the VCFMASTER parameter. The most preferred master may or may not be the currently active master. This parameter determines whether master status should be transferred from the currently active master to a more preferred master. The MOSTPREFERRED parameter value is checked when a system rejoins an existing MIMplex, and when all systems participating in the MIMplex are restarted.

  For example, consider this scenario:

  • A four-system MIMplex is defined (SYSA,SYSB,SYSC,SYSD)
  • VCFMASTER=(SYSA,SYSB)
  • MOSTPREFERRED=NO
  • SYSA is down and freed
  • The currently active master is SYSB
  • Systems SYSB, SYSC, and SYSD are synchronized
  • CA MIM is restarted on SYSA

  When SYSA rejoins the MIMplex, the MOSTPREFERRED parameter value is checked and found to be NO. Therefore, SYSB retains master status. If the MOSTPREFERED parameter value were YES, then master status would be transferred from SYSB to SYSA, because SYSA is a more preferred master in the VCFMASTER list. Transferring of the master status from SYSB to SYSA occurs automatically without any operator intervention.

  Another helpful example involves a global shutdown. Use the same basic scenario as before:

  • SYSA is down and freed.
  • Systems SYSB, SYSC, and SYSD are synchronized.
  • A global shutdown is performed, whereby the CA MIM address spaces on SYSB, SYSC, and SYSD are terminated.
  • CA MIM is restarted on all four systems-SYSA, SYSB, SYSC, and SYSD.

  Synchronization completes with SYSB as the currently active master because it was the previous active master. At this point, the MOSTPREFERRED parameter value is checked and found to be NO. SYSB retains master status. If the MOSTPREFERRED parameter value was set to YES, then master status would be transferred from SYSB to SYSA. The default value for the MOSTPREFERRED parameter is NO.

• NOMASTER = WAIT/TERMINATE

  The NOMASTER parameter defines how client systems react when the currently active master CA MIM address space terminates or becomes unresponsive, and there are no other eligible masters available in the MIMplex. This parameter determines whether the customer CA MIM address spaces should terminate or wait for an eligible master to rejoin the MIMplex. This parameter is of consequence only when using the CTCONLY or XCF communication methods. The same “no eligible master” situation with CTCDASD results in all systems migrating down to, and remaining on, the backup DASD control file. The default value for the NOMASTER parameter is WAIT.

• VCFMASTER = (sysa, sysb, ...sysn)

  The VCFMASTER parameter defines your master candidate list. The leftmost system in the list is your most preferred master. An eligible master has connectivity to each system in the MIMplex. If a system has connectivity to each system in the MIMplex, then that system is added to the eligible master list. If there are no systems in the eligible master list, then initialization terminates. If a VCFMASTER parameter is defined having systems that are not eligible to become master systems, CA MIM attempts to select a master based on the eligible master list built internally.