Installation guide

Chapter 4. Fencing
Fencing is the disconnection of a node from the cluster's shared storage. Fencing cuts off I/O from
shared storage, thus ensuring data integrity. The cluster infrastructure performs fencing through the
fence daemon, fenced .
When CMAN determines that a node has failed, it communicates to other cluster-infrastructure
components that the node has failed. fenced, when notified of the failure, fences the failed node.
Other cluster-infrastructure components determine what actions to take — that is, they perform any
recovery that needs to be done. For example, DLM and GFS2, when notified of a node failure,
suspend activity until they detect that fenced has completed fencing the failed node. Upon
confirmation that the failed node is fenced, DLM and GFS2 perform recovery. DLM releases locks of
the failed node; GFS2 recovers the journal of the failed node.
The fencing program determines from the cluster configuration file which fencing method to use. Two
key elements in the cluster configuration file define a fencing method: fencing agent and fencing
device. The fencing program makes a call to a fencing agent specified in the cluster configuration
file. The fencing agent, in turn, fences the node via a fencing device. When fencing is complete, the
fencing program notifies the cluster manager.
The High Availability Add-On provides a variety of fencing methods:
Power fencing — A fencing method that uses a power controller to power off an inoperable node.
storage fencing — A fencing method that disables the Fibre Channel port that connects storage to
an inoperable node.
Other fencing — Several other fencing methods that disable I/O or power of an inoperable node,
including IBM Bladecenters, PAP, DRAC/MC, HP ILO, IPMI, IBM RSA II, and others.
Figure 4.1, “ Power Fencing Example” shows an example of power fencing. In the example, the
fencing program in node A causes the power controller to power off node D. Figure 4.2, “ Storage
Fencing Example” shows an example of storage fencing. In the example, the fencing program in
node A causes the Fibre Channel switch to disable the port for node D, disconnecting node D from
storage.
⁠Chapt er 4 . Fencing
21