Your WSFC Quorum is like a Republic, or more accurately, a Democracy. There are many articles out there regarding Quorum voting logic but most are somewhat lengthy. I decided to set out to see how few words I could effectively explain Quorum rules in, so here we go. Don't count this part. Or this. Wait... er... start counting.... NOW.
Windows 2008 and higher:
- A Quorum is the act of n nodes agreeing on a majority.
- A node is a cluster member, shared disk, or fileshare witness.
- A cluster can have a shared disk or fileshare, but not both.
- A majority is defined by greater than 50% consensus. (a tie is not majority)
- Fractions (only) are rounded up to the nearest integer. (2.5=3)
- There is a legacy quorum method called "disk only" wherein one (defined quorum) disk is the only vote. This is considered obsolete because it creates a single point of failure.
Windows 2008/r2 with Hotfix 2494036 or Higher:
"Nodeweight" was added to revoke a node of its voting privileges (NodeWeight=0). You can use this for nodes in a different site or to ensure that shared disk/fileshare casts the deciding vote. This is generally used in cross-site clusters.
Windows 2012 or Higher:
"Dynamic Clustering" changes the nodeweight of downed cluster member and effectively reduces the number of participating nodes by one. This works under the following circumstances:
- Prior to the outage, the cluster has achieved quorum (normal under most circumstances)
- Nodes must go down one at a time so the remaining nodes can agree to removed the downed member. If multiple nodes go down simultaneously the dynamic removal will not take place.
Windows Server Failover Clustering is an excellent option for SQL Server, Hyper-V, and other services. Hopefully this understanding of cluster failover behavior enables you to design solutions that better meet the needs of your clients.
Note: It is important to consider how a Quorum is formed when considering patching strategy.
TechNet: Understanding Quorum Configurations in a Failover Cluster
Aeval Shah's Blog: Windows Server 2012 Failover Clustering Dynamic Quorum
Configure Cluster Quorum NodeWeight Settings
Microsoft Support: Cluster NodeWeight hotfix for 2008/r2
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