Tie breaker option for split and merge policies

You can use the tie breaker option to specify a SCSI disk or a Network File System (NFS) file that is used by the split and merge policies.

A tie breaker disk or an NFS file is used when the sites in the cluster can no longer communicate with each other. This communication failure results in the cluster splitting the sites into two, independent partitions. If failure occurs because the cluster communication links are not responding, both partitions attempt to lock the tie breaker disk or the NFS file. The partition that acquires the tie breaker disk continues to function, while the other partition reboots, or has cluster services restarted, depending on the selected action plan.

The disk or NFS-mounted file that is identified as the tie breaker must be accessible to all nodes in the cluster.

When partitions that were part of the cluster are brought back online after the communication failure, they must be able to communicate with the partition that owns the tie breaker disk or NFS file. If a partition that is brought back online cannot communicate with the tie breaker disk or the NFS file, it does not join the cluster. The tie breaker disk or NFS file is released when all nodes in the configuration rejoin the cluster.

When you configure a tie breaker disk for split and merge recovery handling, the disk must also be supported by the devrsrv command, which is part of the AIX® operating system. The SMIT interface that you use for selecting the tie breaker disk filters out any disks that do not meet this requirement. To use EMC disk as a tiebreaker disk, configure the EMC disk by using the IBM® AIX Multi Path IO (MPIO).

EMC PowerPath disks are not supported for use as a tiebreaker disk.

start of change

Site Priority configuration for split and merge policies

Starting with PowerHA® SystemMirror® Version 7.2.8, or later, and Reliable Scalable Cluster Technology (RSCT) Version 3.3.2.0, or later, configuration of priority is enabled for any PowerHA cluster site. Site Priority configuration enables RSCT to make quorum decisions. The Site Priority configuration also provides an option to add time delay for the low-priority sites so that the higher priority sites have more chance to win the tie breaker.
Note: The Site Priority feature is functional only for tie breaker configuration of SCSI PR disks.
end of change

Limitation on the tie breaker option with more than two nodes in a cluster

In a cluster with more than two nodes, after a cluster split occurs, the tie breaker disk or NFS file is reserved by the winning partition. From the winning partition, a single node makes a reservation on the tie breaker disk or NFS file. If this node fails without releasing the reservation on the tie breaker disk or NFS file, the remaining nodes cannot obtain the reservation on the tie breaker disk or NFS file and lose. Therefore, all nodes are rebooted.

After the cluster split occurs, you must quickly resolve the problem with the failed node so that the tie breaker disk reservation is released.

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.