cluster. For any cluster, the total number of votes is a hard number that can be
changed only through one of the actions previously mentioned. Problems can occur,
though. Consider a five-node cluster with no witness configured, which means that
there are five possible votes and three votes must be available for the cluster to make
quorum. Consider the following sequence of actions:
1 . An administrator performs patching on a node, which requires reboots. The node is
unavailable for a period of time, and therefore its vote is not available. This leaves
four out of the five possible votes available, which is greater than 50 percent, so
the cluster keeps quorum.
2 . The administrator starts to perform maintenance on another node, which again
requires reboots, losing the vote of the additional node and leaving three out of the
five possible votes available. That is still greater than 50 percent, which keeps
quorum and the node stays functional.
3 . A failure in a node occurs, or the administrator is an overachiever and performs
maintenance on another node, losing its vote. Now there are only two votes out of
the five possible votes, which is less than 50 percent. The cluster loses quorum, the
cluster services stop on the remaining two nodes, and all services in the cluster are
no longer offered.In this scenario, even though planned maintenance is going on and even though there
are still two healthy nodes available, the cluster can no longer make quorum because
less than 50 percent of the votes are available. The goal of clustering is to increase
availability of services, but in this case it caused services to become unavailable.
Windows Server 2012 changed how the vote allocation works and cures the scenario
just described with a feature called dynamic quorum. With dynamic quorum, the total
number of votes available in the cluster changes as node states change; for example, if
a node is taken down as part of maintenance, the node removes its vote from the
cluster, reducing the total number of votes in the cluster. When the node comes out of
maintenance, it adds its vote back, restoring the total number of possible votes to the
original value. This means that the cluster has greater resiliency when it comes to
problems caused by a lack of votes. Consider the preceding scenario in Windows
Server 2012 with dynamic quorum:
1 . An administrator performs patching on a node, which requires reboots, so the node
is unavailable for a period of time. As the node goes into maintenance mode, it
removes its vote from the cluster, reducing the total number of votes from five to
four.
2 . The administrator starts to perform maintenance on another node, which again
requires reboots. The node removes its vote, reducing the total number of votes in
the cluster to three.
3 . A failure in a node occurs, or the administrator is an overachiever and performs
maintenance on another node, losing its vote. Now only two votes are left, out of
the three total votes, which is greater than 50 percent, so the cluster stays running!