HP 3PAR Cluster Extension Software Administrator Guide for Windows (5697-1821, April 2012)

NOTE: HP 3PAR Cluster Extension does not support shared disk quorum with Windows Clustering.
MNS or Node Majority quorum in Windows Clustering
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.
Majority Node Set with File Share Witness
The file share witness feature is an improvement to the Majority Node Set (MNS) quorum model.
This feature lets you use a file share that is external to the cluster as an additional "vote" to determine
the status of the cluster in a MNS quorum cluster deployment.
Consider a two-node MNS quorum cluster. Because an MNS quorum cluster can only run when
the majority of the cluster nodes are available, a two-node MNS quorum cluster is unable to sustain
the failure of any cluster node. This is because the majority of a two-node cluster is two. To sustain
the failure of any one node in an MNS quorum cluster, you must have at least three devices that
can be considered as available. The file share witness feature enables you to use an external file
share as a witness. This witness acts as the third available device in a two-node MNS quorum
cluster. Therefore, with this feature enabled, a two-node MNS quorum cluster can sustain the failure
of a single cluster node. Additionally, the file share witness feature provides the following two
functions:
• It helps protect the cluster against a problem that is known as a split brain. This problem occurs
if the two nodes in a MNS quorum cluster cannot communicate with each other. In this situation,
each cluster node is unable to determine whether the loss of communication occurred because
the other cluster node failed, or whether the loss of communication occurred because of a
problem with the network. The file share witness can designate one of the cluster nodes as
the surviving cluster node. That cluster node can then determine that it should continue to run
the cluster. In this scenario, the surviving cluster node can determine that the other cluster node
failed, or that the other cluster node was not sanctioned by the file share witness.
• It helps protect the cluster against a problem that is known as a partition in time. This problem
occurs if the following conditions are true:
◦ Cluster node A is running, but cluster node B is not running.
◦ Cluster node A stops running.
◦ Cluster node B tries to run the cluster.
In this situation, cluster node B may not have the cluster state information that was updated
on cluster node A. Therefore, cluster node B may run the cluster by using incorrect state
information. The file share witness feature helps prevent this problem by detecting that
the cluster state has changed. The file share witness feature prevents the cluster node that
contains outdated cluster state information from running the cluster.
Planning for HP 3PAR Cluster Extension 15