Managing Serviceguard A.11.20, March 2013

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.
NOTE: Configuring a lock LUN and a lock disk on an iscsi storage device is not supported.
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 in two separate data centers, a single lock disk would
be a single point of failure should the data center it resides in suffer a catastrophic failure.
In these two cases only, a dual cluster lock, with two separately powered cluster disks, should be
used to eliminate the lock disk as a single point of failure.
NOTE: You must use Fibre Channel connections for a dual cluster lock; you can no longer
implement it in a parallel SCSI configuration.
For a dual cluster lock, the disks must not share either a power circuit or a node chassis with one
another. In this case, if there is a power failure affecting one node and disk, the other node and
disk remain available, so cluster re-formation can take place on the remaining node. For a campus
cluster, there should be one lock disk in each of the data centers, and all nodes must have access
48 Understanding Serviceguard Software Components