Veritas Storage Foundation 5.1 SP1 for Oracle RAC Administrator"s Guide (5900-1512, April 2011)

How preferred fencing works
The I/O fencing driver uses coordination points to prevent split-brain in a VCS
cluster. At the time of a network partition, the fencing driver in each subcluster
races for the coordination points. The subcluster that grabs the majority of
coordination points survives whereas the fencing driver causes a system panic
on nodes from all other subclusters. By default, the fencing driver favors the
subcluster with maximum number of nodes during the race for coordination
points.
This default racing preference does not take into account the application groups
that are online on any nodes or the system capacity in any subcluster. For example,
consider a two-node cluster where you configured an application on one node and
the other node is a standby-node. If there is a network partition and the
standby-node wins the race, the node where the application runs panics and VCS
has to bring the application online on the standby-node. This behavior causes
disruption and takes time for the application to fail over to the surviving node
and then to start up again.
The preferred fencing feature lets you specify how the fencing driver must
determine the surviving subcluster. The preferred fencing solution makes use of
a fencing parameter called node weight. VCS calculates the node weight based on
online applications and system capacity details that you provide using specific
VCS attributes, and passes to the fencing driver to influence the result of race for
coordination points. At the time of a race, the racer node adds up the weights for
all nodes in the local subcluster and in the leaving subcluster. If the leaving
subcluster has a higher sum (of node weights) then the racer for this subcluster
delays the race for the coordination points. Thus, the subcluster that has critical
systems or critical applications wins the race.
The preferred fencing feature uses the cluster-level attribute
PreferredFencingPolicy that takes the following race policy values:
■ Disabled (default): Preferred fencing is disabled.
When the PreferredFencingPolicy attribute value is set as Disabled, VCS sets
the count based race policy and resets the value of node weight as 1.
■ System: Based on the capacity of the systems in a subcluster.
If one system is more powerful than others in terms of architecture, number
of CPUs, or memory, this system is given preference in the fencing race.
When the PreferredFencingPolicy attribute value is set as System, VCS
calculates node weight based on the system-level attribute FencingWeight.
■ Group: Based on the higher priority applications in a subcluster.
The fencing driver takes into account the service groups that are online on
the nodes in any subcluster. In the event of a network partition, the node with
higher priority service groups is given preference in the fencing race.
Overview of Veritas Storage Foundation for Oracle RAC
About preventing data corruption with I/O fencing
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