Managing Serviceguard Fifteenth Edition, reprinted May 2008

Understanding Serviceguard Software Components
How the Cluster Manager Works
Chapter 3 73
Figure 3-3 Quorum Server Operation
The quorum server runs on a separate system, and can provide quorum
services for multiple clusters.
No Cluster Lock
Normally, you should not configure a cluster of three or fewer nodes
without a cluster lock. In two-node clusters, a cluster lock is required.
You may consider using no cluster lock with configurations of three or
more nodes, although the decision should be affected by the fact that any
cluster may require tie-breaking. For example, if one node in a
three-node cluster is removed for maintenance, the cluster reforms as a
two-node cluster. If a tie-breaking scenario later occurs due to a node or
communication failure, the entire cluster will become unavailable.
In a cluster with four or more nodes, you may not need a cluster lock
since the chance of the cluster being split into two halves of equal size is
very small. However, be sure to configure your cluster to prevent the
failure of exactly half the nodes at one time. For example, make sure
there is no potential single point of failure such as a single LAN between
equal numbers of nodes, or that you don’t have exactly half of the nodes
on a single power circuit.