HP 3PAR StoreServ Cluster Extension Software Administrator Guide

Node Majority quorum in Windows Clustering
The Node Majority 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.
Node Majority with file share witness
The file share witness feature is an improvement to the Node Majority 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 Node Majority quorum cluster deployment.
Consider a two-node Node Majority quorum cluster. Because an Node Majority quorum cluster
can only run when the majority of the cluster nodes are available, a two-node Node Majority
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 Node Majority 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 Node Majority quorum cluster. Therefore, with this feature enabled,
a two-node Node Majority 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 Node Majority 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.
NOTE: See Microsoft documentation for more details on Microsoft Failover quorum configuration.
14 HP 3PAR StoreServ Cluster Extension features