Arbitration For Data Integrity in Serviceguard Clusters, July 2007

Arbitration for Data Integrity in Serviceguard Clusters
Use of a Lock Disk as the Cluster Lock
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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 Cluster Lock
It is recommended 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, it is highly recommended to use three power
circuits for a two-node cluster, with a single, separately powered disk for
the cluster lock. For two-node clusters, this single lock disk may 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.
Dual Cluster Lock
In an extended distance 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 this case only, a dual cluster lock, with two separately powered
disks, should be used to eliminate the lock disk as a single point of
failure. The use of the dual cluster lock is further shown in “Use of Dual
Lock Disks in Extended Distance Clusters” on page 26.