The operation of the Quorum Server is shown in Figure 12. When there is a loss of communication between node 1 and node 2, the Quorum Server chooses one node (in this example, node 2) to continue running in the cluster. The other node halts.

Figure 12 Quorum Server Operation

The Quorum Server runs on a separate system, and can provide quorum services for multiple clusters.

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 under HP Serviceguard Quorum Server Software

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 109).

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 49

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HP Serviceguard manual No Cluster Lock, Quorum Server Operation