Managing Serviceguard A.11.20, March 2013

to both lock disks. In the event of a failure of one of the data centers, the nodes in the remaining
data center will be able to acquire their local lock disk, allowing them to successfully reform a new
cluster.
NOTE: A dual lock disk does not provide a redundant cluster lock. In fact, the dual lock is a
compound lock. This means that two disks must be available at cluster formation time rather than
the one that is needed for a single lock disk. Thus, the only recommended usage of the dual cluster
lock is when the single cluster lock cannot be isolated at the time of a failure from exactly one half
of the cluster nodes.
If one of the dual lock disks fails, Serviceguard will detect this when it carries out periodic checking,
and it will write a message to the syslog file. After the loss of one of the lock disks, the failure
of a cluster node could cause the cluster to go down if the remaining node(s) cannot access the
surviving cluster lock disk.
Use of the Quorum Server as the Cluster Lock
A Quorum Server can be used in clusters of any size. The quorum server process runs on a machine
outside of the cluster for which it is providing quorum services. The quorum server listens to
connection requests from the Serviceguard nodes on a known port. The server maintains a special
area in memory for each cluster, and when a node obtains the cluster lock, this area is marked so
that other nodes will recognize the lock as “taken.”
If communications are lost between two equal-sized groups of nodes, the group that obtains the
lock from the Quorum Server will take over the cluster and the other nodes will perform a system
reset. Without a cluster lock, a failure of either group of nodes will cause the other group, and
therefore the cluster, to halt. Note also that if the Quorum Server is not available when its arbitration
services are needed, the cluster will halt.
The operation of the Quorum Server is shown in Figure 12. When there is a loss of communication
between node 1 and node 2, the Quorum Server chooses one node (in this example, node 2) to
continue running in the cluster. The other node halts.
Figure 12 Quorum Server Operation
The Quorum Server runs on a separate system, and can provide quorum services for multiple
clusters.
IMPORTANT: For more information about the Quorum Server, see the latest version of the HP
Serviceguard Quorum Server release notes at http://www.hp.com/go/hpux-serviceguard-docs
under HP Serviceguard Quorum Server Software
How the Cluster Manager Works 49