Managing Serviceguard 11th Edition, Version A.11.16, Second Printing June 2004

Understanding Serviceguard Software Components
How the Cluster Manager Works
Chapter 366
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 or perform a TOC. 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.
Lock Requirements
A one-node cluster does not require a cluster lock. A two-node cluster
requires a cluster lock. In clusters larger than 3 nodes, a cluster lock is
strongly recommended. If you have a cluster with more than four nodes,
a cluster lock disk is not allowed, but a quorum server may be used.
Use of an LVM Lock Disk as the Cluster Lock
A lock disk may be used for clusters up to and including four nodes in
size. The cluster lock disk is a special area on an LVM disk located in a
volume group that is shareable by all nodes in the cluster. When a node
obtains the cluster lock, this area is marked so that other nodes will
recognize the lock as “taken.”
The lock disk is not dedicated for use as the cluster lock; the disk can be
employed as part of a normal volume group with user data on it. The
cluster lock volume group and physical volume names are identified in
the cluster configuration file.
The operation of the lock disk is shown in Figure 3-2.