Split Brain and Amnesia

Two types of problems can arise from cluster partitions: split brain and amnesia. Split brain occurs when the cluster interconnect between Solaris hosts is lost and the cluster becomes partitioned into subclusters, and each subcluster believes that it is the only partition. A subcluster that is not aware of the other subclusters could cause a conflict in shared resources such as duplicate network addresses and data corruption.

Amnesia occurs if all the nodes leave the cluster in staggered groups. An example is a two-node cluster with nodes A and B. If node A goes down, the configuration data in the CCR is updated on node B only, and not node A. If node B goes down at a later time, and if node A is rebooted, node A will be running with old contents of the CCR. This state is called amnesia and might lead to running a cluster with stale configuration information.

You can avoid split brain and amnesia by giving each node one vote and mandating a majority of votes for an operational cluster. A partition with the majority of votes has a quorum and is enabled to operate. This majority vote mechanism works well if more than two nodes are in the cluster. In a two-node cluster, a majority is two. If such a cluster becomes partitioned, an external vote enables a partition to gain quorum. This external vote is provided by a quorum device. A quorum device can be any disk that is shared between the two nodes.

Table 2–1 describes how Sun Cluster software uses quorum to avoid split brain and amnesia.

Table 2–1 Cluster Quorum, and Split Brain and Amnesia Problems

Partition Type	Quorum Solution
Split brain	Enables only the partition (subcluster) with a majority of votes to run as the cluster (only one partition can exist with such a majority). After a node loses the race for quorum, that node panics.
Amnesia	Guarantees that when a cluster is booted, it has at least one node that was a member of the most recent cluster membership (and thus has the latest configuration data).