Managing Serviceguard Eighteenth Edition, September 2010

Figure 3-2 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.
Dual Lock Disk
If you are using disks that are internally mounted in the same cabinet as the cluster
nodes, then a single lock disk would be a single point of failure, since the loss of power
to the node that has the lock disk in its cabinet would also render the cluster lock
unavailable. Similarly, in a campus cluster, where the cluster contains nodes running
64 Understanding Serviceguard Software Components