Merge policy

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Depending on the cluster split policy, the cluster might have two partitions that run independently of each other. You can use PowerHA® SystemMirror® to configure a merge policy that allows the partitions to operate together again after communications are restored between the partitions.

The following options are available for configuring a merge policy:
Majority
The partition with the highest number of nodes remains online. If each partition has the same number of nodes, then the partition that has the lowest node ID is selected for standard and stretched clusters. The lowest site ID is selected for linked cluster. The partition that does not remain online is rebooted, as specified by the selected action plan. For stretched clusters to use the majority option, your environment must be running one of the following version of the AIX® operating system:
  • IBM® AIX 7.1 with Technology Level 4, or later
  • AIX Version 7.2, or later
Tie breaker
You can use a disk or an NFS file for a tie-breaker. If you use a disk for the tie-breaker, each partition attempts to acquire the tie breaker by placing a lock on the tie breaker disk. The tie breaker is a SCSI disk that is accessible to all nodes in the cluster. The partition that cannot lock the disk is rebooted, or cluster services are restarted, as specified by the chosen action plan.

If you use an NFS file for the tie-breaker, the NFS mount must exist on each of the nodes in the cluster from the selected NFS server. The partition that first reserves the NFS file continues to function. The partition that cannot lock the NFS file is rebooted, or cluster services are restarted, as specified by the chosen action plan.

If you use this option, your split policy configuration must also use the tie breaker option.
Note: The default NFS mount options are vers=4,fg,soft,retry=1,timeo=10. Modifying the default values might lead to failure in acquiring the NFS lock.
Cloud is another tie breaker option and you must have cloud communication on all the nodes of the cluster for this option. During cluster merge event, each partition attempts to acquire a lock by uploading a file to the configured Cloud service. The partition that successfully uploads the file to the configured Cloud service continues to function. The partition that cannot upload the file to the configured Cloud service is rebooted or the cluster services are restarted as specified by the chosen action plan in the policy setting.

If you use the Cloud option for the merge policy, the split policy must also be configured to use the Cloud option.

Manual
This policy option requires that you select the winning site during a merge event.

Each node in the partition presents a message to choose to continue running cluster services or recover cluster services (which restarts the node). With this option, you can specify the number of attempts and the frequency of attempts that require your input. You can also specify a default action to occur after the number of attempts that require your input is reached and you have not provided any input.

The following message is displayed for a linked cluster that specifies the manual option when a cluster split event occurs: 

Broadcast message from root@e08m138.ausprv.stglabs.ibm.com (tty) at 04:09:48 ...
A cluster split has been detected.
You must decide if this side of the partitioned cluster is to continue.
To have it continue, enter
        /usr/es/sbin/cluster/utilities/cl_sm_continue
To have the recovery action - Reboot - taken on all nodes on this partition, enter
        /usr/es/sbin/cluster/utilities/cl_sm_recover
LOCAL_PARTITION 1 e08m138 OTHER_PARTITION 2 e08m140
In this example, you can use the manual option to check whether a split event or a merger event is waiting for a manual response from the Problem Determination Tools > Manual Response to Split or Merge > Display any needed Manual Response SMIT menu.
If you want to use the manual option for stretched clusters and standard clusters, your environment must be running the following versions of software:
  • IBM AIX 7.2 with Technology Level 1, or later
  • PowerHA SystemMirror Version 7.2.1, or later
Note: For any type of cluster that uses the manual option after the number of attempts specified is reached and you have not provided any input, the partition that has the lowest node ID is chosen as the winning partition.
None
This option indicates that no action occurs when a cluster merge event occurs. To avoid any data corruption after a merge occurs, you must reboot the losing partition node manually. This option is only available from the clmgr command. If you specify none for the merge policy, you must select none for the split policy.
If you want to use the none option for stretched clusters and standard clusters, your environment must be running the following versions of software:
  • IBM AIX 7.2 with Technology Level 1, or later
  • PowerHA SystemMirror Version 7.2.1, or later