First published on MSDN on May 13, 2010

Failover clusters ensure that workloads such as File Server, Exchange, SQL, and Virtual Machines are highly available.  These resources are considered highly available if the nodes that host resources are up, however the cluster generally requires more than half the nodes to be running, which is known as having “quorum”.

Quorum is designed to prevent “split-brain” scenarios which can happen when there is a partition in the network and subsets of nodes cannot communication with each other.  This can cause both subsets of nodes to try to own the workload and write to the same disk which can lead to numerous problems.  However this is prevented with the Windows Server Failover Clustering quorum model which forces one of these groups to have a majority of nodes running, so only one of these groups will stay online.

If the cluster loses quorum, all the nodes of a cluster will offline and any hosted resources will offline.  Choosing the right quorum mode while deploying your cluster will help ensure that the cluster stays up for as long as possible while sustaining node, disk or network failures.

Clustering supports four quorum modes. They are: Node Majority , Node and Disk Majority , Node and File Share Majority , and No Majority: Disk Only (Legacy).

Terminology

Here is some relevant terminology:

· Disk Witness Resource – A clustered disk can contribute towards the cluster’s quorum.  This disk resides in the cluster group.  Besides providing a vote for the quorum, this resource serves two other critical functions.

o   Stores a constantly-updated version of the cluster database.  This allows the cluster to maintain its state and configuration independently of individual node failures, which ensures that nodes will always have the most up-to-date copy of the database.

o   The quorum resource enforces cluster unity, preventing the “split-brain” scenario described earlier.

· File Share Witness (FSW) Resource – A file share accessible by all nodes of the cluster can contribute to the cluster’s quorum.  Besides provide a vote for the quorum, it also helps with the “split brain” scenario.  However, file share witness doesn’t contain the cluster database.

· Vote – The quorum calculation is based on votes.  Cluster nodes, disk witness resources and file share witness resources may have a vote base on the quorum configuration.  The table in the next section shows the relationship between quorum mode and votes.

Quorum Model Description

The following table describes the different quorum modes available since Windows Server 2008.

It is recommended to have an odd number of total votes in the cluster since quorum requires more than half of the votes to be online.  If I have a 4-node cluster, and only give each node a vote for 4 total votes, I need 3 nodes to stay running to maintain quorum with more than half of the votes.  This means I can only sustain a single node failure.  However, by assigning a disk or FSW a 5 ^th vote, I still need 3 votes to maintain quorum, however I can now sustain two node failures, instead of one.  So by adding these extra votes by using a disk or file share, instead of requiring the purchase of an additional node, Failover Clustering can offer higher availability at a much lower cost.

Default Configuration

When the cluster is first created, the most appropriate quorum mode is automatically assigned, which is based on the number of nodes and available cluster storage.  This can always be changed, as described in the next section.  The cluster will attempt to configure quorum so that there is always an odd number of votes.  If there is an odd number of nodes, the cluster will select Node Majority as the quorum type to keep the odd number of votes.  If there are an even number of nodes, and disks in Available Storage, the cluster will select Node and Disk Majority, giving a disk a single vote, so that there is an odd number of total votes.  If there is an even number of nodes, but no Available Storage, the cluster will select Node Majority and issue a warning message.  The cluster will never select Node and File Share Witness since it requires additional configuration, and it will never select No Majority: Disk Only as this is not recommended because it is a single point of failure.

Configuring Quorum

In Failover Cluster Manager, the quorum configuration can be changed through the Configure Cluster Quorum Wizard.  This page can be reached by right clicking on the cluster name, selecting “More Actions…” and then “Configure Cluster Quorum Settings…”

Once “Configure Cluster Quorum Setting…” is selected, the Configure Cluster Quorum Wizard appears.  This will recommend the best configuration for you based on the number of nodes and Available Storage and inform you about the number of failures you can sustain.  In the snapshot below, Node Majority quorum is recommended because it’s a 3 node cluster, which will be explained later.

In PowerShell, the Set-ClusterQuorum CMDlet is used to configure the quorum mode. The specific command to set each individual quorum configuration will be explain in the next blog post in the quorum series.  To view the TechNet topic on quorum, visit: http://technet.microsoft.com/en-us/library/cc770620(WS.10).aspx .

Thanks,
Peter Huang
Software Development Engineer in Test II
Clustering and High-Availability
Microsoft