Merge policy A PowerHA merge policy defines the behavior of a cluster when a cluster merge occurs. 

Quarantine policy A PowerHA quarantine policy defines how a standby node isolates or quarantines  an active node or partition from the shared data to prevent data corruption when a cluster split occurs. 

Critical resource group When multiple resource groups (RGs) are configured in a cluster, the RG that is considered as most important or critical to the user is defined as the critical RG for a quarantine policy. For more information, see 9.3.1, “Active node halt quarantine policy” on page 337. 

Standard cluster A standard cluster is a traditional PowerHA cluster. 

Stretched cluster A stretched cluster is a PowerHA V7 cluster with nodes that are in sites within the same geographic location. All cluster nodes are connected to the same active and backup repository disks in a common storage area network (SAN). 

Linked cluster A linked cluster is a PowerHA V7 cluster with nodes that are in sites in different geographic locations. Nodes in each site have their own active and backup repository disks. The active repository disks in the two sites are kept in sync by Cluster Aware AIX (CAA). 

## 9.2 PowerHA cluster split and merge policies (before PowerHA V7.2.1) 

This section provides an introduction to PowerHA split and merge policies before PowerHA for AIX V7.2.1. 

## 9.2.1 Split policy 

For more information, see the following IBM Redbooks: 

GLYPH<SM590000> IBM PowerHA SystemMirror for AIX Cookbook , SG24-7739 

GLYPH<SM590000> IBM PowerHA SystemMirror V7.2 for IBM AIX Updates , SG24-8278 

Before PowerHA V7.1, when a cluster split occurs, the backup node tries to take over the resources of the primary node, which results in a split-brain situation. 

The PowerHA split policy was first introduced in PowerHA V7.1.3 with two options: 

GLYPH<SM590000> None 

This is the default option where the primary and backup nodes operate independently of each other after a split occurs, resulting in the same behavior as earlier versions during a split-brain situation. 

GLYPH<SM590000> Tiebreaker 

This option is applicable to only clusters with sites configured. When a split occurs, the partition that fails to acquire the Small Computer System Interface (SCSI) reservation on the tiebreaker disk has its nodes restarted. For a two-node cluster, one node is restarted, as shown in Figure 9-3 on page 327. 

