Managing Serviceguard Fifteenth Edition, reprinted May 2008

Understanding Serviceguard Software Components
How the Cluster Manager Works
Chapter 3 69
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.
Lock Requirements
A one-node cluster does not require a cluster lock. A two-node cluster
requires a cluster lock. In clusters larger than three nodes, a cluster lock
is strongly recommended. If you have a cluster with more than four
nodes, use a quorum server; a cluster lock disk is not allowed for clusters
of that size.
Use of a Lock LUN or LVM Lock Disk as the Cluster
Lock
A lock disk or lock LUN can be used for clusters up to and including four
nodes in size.
A 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. Similarly, a cluster
lock LUN is a small dedicated LUN, connected to all nodes in the cluster,
that contains the lock information.
In an LVM configuration, a disk used as a 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. A lock LUN, on the other hand, is
dedicated to the cluster lock; you cannot store any other data on it.
You specify the cluster lock volume group and physical volume, or the
cluster lock LUN, in the cluster configuration file.