Managing Serviceguard A.11.20, March 2013

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 by means
of a lock disk, lock LUN, or a Quorum Server.
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.
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 11.
How the Cluster Manager Works 47