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Storage Foundation Cluster File System High Availability 7.2 Administrator's Guide - Solaris

Last Published: 2018-03-06

Product(s): InfoScale & Storage Foundation (7.2)

Platform: Solaris

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, system capacity, and site preference 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 0.
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, I/O fencing determines whether the VCS engine is running on all the nodes that participate in the race for coordination points. If VCS engine is running on all the nodes, the node with higher priority service groups is given preference during the fencing race.
However, if the VCS engine instance on a node with higher priority service groups is killed for some reason, I/O fencing resets the preferred fencing node weight for that node to zero. I/O fencing does not prefer that node for membership arbitration. Instead, I/O fencing prefers a node that has an instance of VCS engine running on it even if the node has lesser priority service groups.
Without synchronization between VCS engine and I/O fencing, a node with high priority service groups but without VCS engine running on it may win the race. Such a situation means that the service groups on the loser node cannot failover to the surviving node.
When the PreferredFencingPolicy attribute value is set as Group, VCS calculates node weight based on the group-level attribute Priority for those service groups that are active.
Site: Based on the priority assigned to sites in a subcluster.
The Site policy is available only if you set the cluster attribute SiteAware to 1. VCS sets higher weights to the nodes in a higher priority site and lesser weights to the nodes in a lower priority site. The site with highest cumulative node weight becomes the preferred site. In a network partition between sites, VCS prefers the subcluster with nodes from the preferred site in the race for coordination points.

See the Cluster Server Administrator's Guide for more information on the VCS attributes.

See Enabling or disabling the preferred fencing policy.