Managing HP Serviceguard A.11.20.10 for Linux, December 2012

Figure 9 Quorum Server to Cluster Distribution
IMPORTANT: For more information about the quorum server, see the latest version of the HP
Serviceguard Quorum Server release notes at http://www.hp.com/go/hpux-serviceguard-docs
(Select HP Serviceguard Quorum Server Software).
3.2.10 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 re-forms 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, and that
you don’t have exactly half of the nodes on a single power circuit.
3.2.11 What Happens when You Change the Quorum Configuration Online
You can change the quorum configuration while the cluster is up and running. This includes changes
to the quorum method (for example, from a lock disk to a quorum server), the quorum device (for
example, from one quorum server to another), and the parameters that govern them (for example,
the quorum server polling interval). For more information about the quorum server and lock
parameters, see “Cluster Configuration Parameters ” (page 86).
When you make quorum configuration changes, Serviceguard goes through a two-step process:
1. All nodes switch to a strict majority quorum (turning off any existing quorum devices).
2. All nodes switch to the newly configured quorum method, device and parameters.
40 Understanding Serviceguard Software Components