Managing Serviceguard Eighteenth Edition, September 2010

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.
When a node obtains the cluster lock, this area is marked so that other nodes will
recognize the lock as “taken.”
The operation of the lock disk or lock LUN is shown in Figure 3-2.
How the Cluster Manager Works 63