Managing HP Serviceguard for Linux, Sixth Edition, August 2006

Understanding Serviceguard Software Components
How the Cluster Manager Works
Chapter 342
The operation of the lock LUN is shown in Figure 3-2.
Figure 3-2 Lock LUN Operation
Serviceguard periodically checks the health of the lock LUN 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
also configure the Disk Monitor to check the lock LUN.
Use of the Quorum Server as a Cluster Lock
The cluster lock in Linux can also be implemented by means of a
quorum server. 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.” A quorum server can
be used in clusters of any size. In a two-node cluster, you are required to
configure a quorum server or lock LUN. If communications are lost
between these two nodes, the node that obtains the cluster lock will take
over the cluster and the other node will perform a TOC (transfer of
control). Without a cluster lock, a failure of either node in the cluster will
cause the other node, and therefore the cluster, to halt. If the quorum
server is not available during an attempt to access it, the cluster will
halt.