Managing Serviceguard 13th Edition, February 2007

Understanding Serviceguard Software Components
How the Cluster Manager Works
Chapter 370
Figure 3-2 Lock Disk Operation
Serviceguard periodically checks the health of the lock disk and writes
messages to the syslog file when a lock disk fails the health check. This
file should be monitored for early detection of lock disk problems.
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.
Single Lock Disk
HP recommends that you use a single lock disk. A single lock disk 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 single, separately powered disk for the
cluster lock. For two-node clusters, this single lock disk must not share a
power circuit with either node, and it 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.