Managing Serviceguard Fifteenth Edition, reprinted May 2008

Understanding Serviceguard Software Components
How the Cluster Manager Works
Chapter 368
• An SPU or network failure was detected on an active node.
• An inactive node wants to join the cluster. The cluster manager
daemon has been started on that node.
• A node has been added to or deleted from the cluster configuration.
• The system administrator halted a node.
• A node halts because of a package failure.
• A node halts because of a service failure.
• Heavy network traffic prohibited the heartbeat signal from being
received by the cluster.
• The heartbeat network failed, and another network is not configured
to carry heartbeat.
Typically, re-formation results in a cluster with a different composition.
The new cluster may contain fewer or more nodes than in the previous
incarnation of the cluster.
Cluster Quorum to Prevent Split-Brain Syndrome
In general, the algorithm for cluster re-formation requires a cluster
quorum of a strict majority (that is, more than 50%) of the nodes
previously running. If both halves (exactly 50%) of a previously running
cluster were allowed to re-form, there would be a split-brain situation
in which two instances of the same cluster were running. In a split-brain
scenario, different incarnations of an application could end up
simultaneously accessing the same disks. One incarnation might well be
initiating recovery activity while the other is modifying the state of the
disks. Serviceguard’s quorum requirement is designed to prevent a
split-brain situation.
Cluster Lock
Although a cluster quorum of more than 50% is generally required,
exactly 50% of the previously running nodes may re-form as a new
cluster provided that the other 50% of the previously running nodes do
not also re-form. This is guaranteed by the use of a tie-breaker to choose
between the two equal-sized node groups, allowing one group to form the
cluster and forcing the other group to shut down. This tie-breaker is
known as a cluster lock. The cluster lock is implemented either by
means of a lock disk, lock LUN, or a quorum server.