Managing HP Serviceguard for Linux Ninth Edition, April 2009

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 LUN or a
quorum server. A cluster lock is required on two-node clusters.
The cluster lock is used as a tie-breaker only for situations in which a running cluster
fails and, as Serviceguard attempts to form a new cluster, the cluster is split into two
sub-clusters of equal size. Each sub-cluster will attempt to acquire the cluster lock. The
sub-cluster which gets the cluster lock will form the new cluster, preventing the
possibility of two sub-clusters running at the same time. If the two sub-clusters are of
unequal size, the sub-cluster with greater than 50% of the nodes will form the new
cluster, and the cluster lock is not used.
If you have a two-node cluster, you are required to configure a cluster lock. If
communications are lost between these two nodes, the node that obtains the cluster
lock will take over the cluster and the other node will halt (system reset). Without a
cluster lock, a failure of either node in the cluster will cause the other node, and therefore
the cluster, to halt. Note also that if the cluster lock fails during an attempt to acquire
it, the cluster will halt.
Use of a Lock LUN as the Cluster Lock
A lock LUN can be used for clusters up to and including four nodes in size. The cluster
lock LUN is a special piece of storage (known as a partition) that is shareable by all
nodes in the cluster. When a node obtains the cluster lock, this partition is marked so
that other nodes will recognize the lock as “taken.”
NOTE: The lock LUN is dedicated for use as the cluster lock, and, in addition, HP
recommends that this LUN comprise the entire disk; that is, the partition should take
up the entire disk.
The complete path name of the lock LUN is identified in the cluster configuration file.
The operation of the lock LUN is shown in Figure 3-2.
How the Cluster Manager Works 45