Managing HP Serviceguard for Linux, Tenth Edition, September 2012

IMPORTANT: For more information about the quorum server, see the latest version of
the HP Serviceguard Quorum Server release notes at http://docs.hp.com -> High
Availability -> Quorum Server.
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.
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 103).
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.
How the Cluster Manager Works 43