Quorum Basics
Quorum is the mechanism used to ensure that in the event of a break in
communication between parts of the cluster or the loss of parts of the cluster, you
always have a majority of cluster resources for the cluster to function. Quorum is the
reason it is common to have a shared disk or file share that can be used in arbitration
when an even number of nodes exists in different parts of the cluster.
Imagine that you have a cluster of four nodes without any shared disk or file share
used for quorum and arbitration. A split occurs, and for some reason each node can
contact only the node next to it. Then, each half of the cluster has two nodes, which is
a disaster because both halves may think that they should own all of the services and
applications. That is why the quorum model is based on a majority—that is, more than
half is needed for the cluster to function. In our example of two nodes on each side,
neither side has a majority (half is not a majority), so no cluster resources would be
serviced. This is far better than multiple nodes trying to service the same resources.
The behavior in this scenario, with exactly half the nodes in each partition of the
cluster, changed in Windows Server 2012 R2, so that services would be offered by one
of the partitions. Each node can be seen as having a vote. By adding an extra vote with
a file share, disk, or cloud storage account in Windows Server 2016, you can ensure
that one part of the cluster can always get more than 50 percent by claiming the file
share, disk, or cloud storage account vote.
Let’s look in detail at quorum. Prior to Windows Server 2012, various quorum models
were available, and even with Windows Server 2012, there was specific guidance about
when to use a file share witness or disk witness. In Windows Server 2012 R2, this all
changed.
Prior to Windows Server 2012, various cluster models existed, but Windows Server
2012 simplified this to a single model. Within a cluster, by default, each node has a
vote. These votes are used in times of arbitration to decide which partition of a cluster
can make quorum—that is, has more than half the number of votes. When creating a
cluster, you also define either a disk witness or a file share witness, which also has a
vote.
Prior to Windows Server 2012 R2, a file share witness or disk witness was configured
only if you had an even number of nodes. An even number of nodes would equal an
even number of votes, and therefore, in the event of partitioning of the cluster, neither
partition would have more votes than the other side. When you configure the file
share witness or disk witness, the extra vote ensures that one partition of the cluster
could claim that vote and therefore have more than 50 percent of the votes and make
quorum. Only when the witness is required to make quorum is it locked by one
partition of the cluster. For a file share witness, the lock is performed by locking the
witness.log file on the share by one of the cluster partitions. To lock a disk witness,
the disk has a SCSI persistent reservation made by one of the partitions. Both types of