The minimum number of cluster nodes or votes required to keep a Windows Server Failover Cluster (WSFC) online, preventing split-brain scenarios where multiple nodes simultaneously believe they are the active cluster owner.
- Role in SQL Server high availability: SQL Server Always On Availability Groups and Failover Cluster Instances rely on WSFC quorum to determine which nodes are authoritative and can host SQL Server resources.
- Quorum models: WSFC supports several quorum configurations, including Node Majority, Node and Disk Majority, Node and File Share Majority, and No Majority (Disk Only), each offering different levels of resilience.
- Impact of quorum loss: Loss of quorum causes the entire WSFC to go offline, including all SQL Server instances hosted on it, making quorum configuration a critical high availability design decision.
- Cloud witness: An Azure storage account can be used as a cloud witness and is the recommended quorum witness for SQL Server clusters in hybrid and cloud environments, replacing traditional physical disk witnesses.
- Cluster planning: DBAs and infrastructure teams must collaborate on quorum design to ensure the cluster can withstand anticipated failure scenarios such as single-node failures, network partitions, or complete site outages.
- Related terms: Always On Availability Groups, Failover Cluster Instance, High Availability, WSFC.
