HP P6000 Cluster Extension Software Administrator Guide (5697-0986, June 2011)

MNS quorum in MSCS
An MNS is a single quorum resource that allows you to build clusters with nodes that are geographically
separated. In an MNS cluster, unlike in a quorum disk cluster, data is stored on multiple disks across
the cluster. The MNS resource ensures that the cluster's configuration data is stored locally on the
nodes and that it is kept consistent across the different disks. This allows cluster topologies where the
nodes do not need shared access to a quorum disk.
(Number of nodes in the cluster/2) +1
This ensures that more than 50% of the nodes will have an up-to-date copy of the configuration
information.
The cluster service does not start (and bring resources online) if there are 50% or less of the configured
nodes up and running. The cluster service waits, trying to restart, until a quorum is established when
more nodes join. This feature guarantees that the cluster has the latest and most up-to-date configuration.
This also means that, in a geographically dispersed cluster, you must distribute the nodes evenly
between two data centers and have an arbitrator node in a third site or separate protected area to
be able to survive a single data center failure.
SLE HA cluster setup considerations
Follow the guidelines in this section when you configure clusters for use with P6000 Cluster Extension.
Quorum
In an SLE HA cluster, quorum is defined as a strict majority of the defined cluster (more than 50%).
With certain failures, a cluster might be divided into two subclusters. In an SLE HA cluster, a subcluster
with more than 50% of the nodes wins the quorum. The subcluster that wins the quorum re-forms the
cluster and fences the subcluster that lost the quorum. The behavior of the subcluster that lost the
quorum depends on the defined no-quorum policy. This behavior is in effect until the cluster is fenced.
When the cluster is fenced, the resources owned by the fenced nodes fail over to active cluster nodes.
STONITH
STONITH is an SLE HA cluster fencing method. SLE HA cluster provides STONITH plug-ins for devices
such as UPS, PDU, Blade power control devices, and lights out devices. Some plug-ins can STONITH
more than one node (for example, Split Brain Detector STONITH) and some can STONITH only one
node (for example, HP iLO STONITH).
HP iLO STONITH uses the power control functions of an HP iLO device to STONITH a node that has
lost quorum and needs to be fenced.
IMPORTANT:
If all of the iLO devices in a cluster are connected using a single network, a single switch failure might
disable iLO, preventing nodes from being fenced. This failure might be difficult to detect, especially
before a node failure where iLO features would be required.
The STONITH action can be set to power off or reset, depending on the environment requirements.
• Power off: The STONITH agent powers off the nodes in the errant subcluster.
• Reset: The STONITH agent resets the nodes in the errant subcluster, and the nodes try to automat-
ically rejoin the cluster.
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