Managing Serviceguard Nineteenth Edition, Reprinted June 2011

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.
Figure 11 Lock Disk or Lock LUN Operation
Serviceguard periodically checks the health of the lock disk or LUN and writes messages to the
syslog file if the device fails the health check. This file should be monitored for early detection
of lock disk problems.
If you are using a lock disk, you can choose between two lock disk options—a single or dual lock
disk—based on the kind of high availability configuration you are building. A single lock disk is
recommended where possible. With both single and dual locks, however, it is important that the
cluster lock be available even if the power circuit to one node fails; thus, the choice of a lock
configuration depends partly on the number of power circuits available. Regardless of your choice,
all nodes in the cluster must have access to the cluster lock to maintain high availability.
IMPORTANT: A dual lock cannot be implemented on LUNs. This means that the lock LUN
mechanism cannot be used in an Extended Distance cluster.
Single Lock Disk or LUN
A single lock disk or lock LUN should be configured on a power circuit separate from that of any
node in the cluster. For example, using three power circuits for a two-node cluster is highly
recommended, with a separately powered disk or LUN for the cluster lock. In two-node clusters,
this single lock device must not share a power circuit with either node, and a lock disk must be an
external disk. For three or four node clusters, the disk should not share a power circuit with 50%
or more of the nodes.
How the Cluster Manager Works 45