Managing Serviceguard 12th Edition, March 2006

Understanding Serviceguard Software Components
How the Cluster Manager Works
Chapter 370
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 in this type of cluster, 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. 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 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 surviving 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