Mastering SQL Server Always On AG Quorum Types

 In SQL Server Always On Availability Groups (AG), quorum is essential to maintain high availability and ensure the correct functioning of the cluster. Quorum determines how many voting nodes must be online and in agreement for the cluster to operate. Understanding quorum types helps in configuring and managing Always On AG effectively.

Types of Quorum in SQL Server Always On AG:

1. Node Majority:
• Definition: Quorum is maintained by a majority of the nodes in the cluster. Suitable for clusters with an odd number of nodes.
• Usage: Common in multi-node clusters where having a majority is easier to achieve.
• Example: A 3-node cluster requires at least 2 nodes to be online for quorum.
2. Node and Disk Majority:
• Definition: A majority is formed using both the nodes and a disk witness. The disk witness acts as an additional vote.
• Usage: Useful for clusters with an even number of nodes to avoid split-brain scenarios.
• Example: A 4-node cluster with a disk witness requires 2 nodes and the disk witness to maintain quorum.
3. Node and File Share Majority:
• Definition: Similar to Node and Disk Majority, but uses a file share witness instead of a disk. The file share witness provides an additional vote.
• Usage: Preferred in virtualized or cloud environments where a shared disk is not feasible.
• Example: A 4-node cluster with a file share witness requires 2 nodes and the file share witness to maintain quorum.
4. No Majority (Disk Only):
• Definition: Only the disk witness has a vote, and quorum is maintained as long as the disk witness is online.
• Usage: Rarely used in Always On AG, as it creates a single point of failure. Typically seen in traditional failover clusters.
• Example: A 2-node cluster using only a disk witness, where the disk witness decides quorum.

5. Node and Cloud Witness Majority:
• Definition: Combines the voting power of the cluster nodes with a cloud-based witness stored in Microsoft Azure Blob Storage. The cloud witness acts as an additional vote in the cluster.
• Usage: Ideal for geographically distributed clusters, hybrid environments, or clusters without reliable access to a shared disk or file share.
• Example: In a 4-node cluster, 2 nodes and the cloud witness must be online to maintain quorum.

Benefits of Cloud Witness:

• No additional hardware requirements: Unlike a disk witness, which needs shared storage, or a file share witness, which needs a reliable file server, Cloud Witness only requires an internet connection and an Azure Blob Storage account.
• Resilience in multi-site clusters: Cloud Witness is perfect for multi-site or distributed clusters, as it remains independent of local infrastructure, adding a layer of high availability.
• Simplified management: Azure's integration with the Windows Failover Cluster Manager simplifies the configuration and management of the witness.

When to Use Cloud Witness:

• Cloud or hybrid environments: When running SQL Server Always On AG in cloud or hybrid environments, Cloud Witness provides a simple, scalable solution.
• No shared storage: In scenarios where traditional shared storage for disk witness or file share witness is impractical.
• Geo-distributed clusters: Cloud Witness is ideal for clusters spanning multiple geographical regions where local file share or disk-based solutions aren't feasible.

Choosing the Right Quorum:

• Odd number of nodes: Use Node Majority for simplicity and effective fault tolerance.
• Even number of nodes: Use Node and Disk Majority or Node and File Share Majority to add a tie-breaking vote and avoid split-brain scenarios.
• Virtual/Cloud Environments: Node and File Share Majority is often preferred as it avoids the complexity of disk dependencies.

 

By adding Cloud Witness to the list of quorum types, SQL Server Always On AG gains more flexibility and fault tolerance, especially in hybrid or cloud-native architectures.

By mastering these quorum types, administrators can configure SQL Server Always On AG for better high availability, ensuring continuous operation and minimizing downtime.