Quorum Configuration in Windows Server Failover Clustering (WSFC)

Quorum is a critical component in Windows Server Failover Clustering (WSFC) that determines the availability of a cluster and prevents a split-brain scenario .

Types of Quorum Configurations in WSFC

1. Node Majority

How It Works:

Each node in the cluster has a vote. The cluster will remain online as long as more than half (majority) of the nodes are available. If less than half of the nodes remain, the cluster goes offline to avoid split-brain.
✅ Advantages:
Best for odd-numbered clusters (e.g., 3, 5, 7 nodes) since an absolute majority is always possible.
No dependency on external resources (e.g., disk or file share).
❌ Disadvantages:
Not ideal for even-numbered clusters because there’s a risk of a 50/50 split, leading to a potential outage. If multiple nodes fail, the cluster may go offline unless a sufficient majority remains.

Best Use Case:
Odd-numbered clusters (e.g., 3, 5, or 7 nodes). Local data centers where no shared storage is available.

2. Node and Disk Majority

How It Works:

Each node gets a vote plus a shared disk (witness disk).
The majority of votes (including the disk) determines cluster availability.
If nodes fail, the disk acts as a tiebreaker.
✅ Advantages:
Ideal for even-numbered clusters (e.g., 2, 4, 6 nodes).
Prevents split-brain scenarios by ensuring a majority decision.
Automatic failover if one site goes down in a two-node cluster.
❌ Disadvantages:
Requires shared storage, which may not be available in cloud or multi-site clusters.
Single point of failure if the disk witness is lost.

Best Use Case:
Even-numbered clusters (4, 6 nodes) with a shared disk available.
Single-site clusters where shared storage can be reliably accessed.

3. Node and File Share Majority

How It Works:

Each node gets a vote plus a file share witness (instead of a disk witness). If an equal split occurs, the file share acts as a tiebreaker.
✅ Advantages:
Best for multi-site clusters where a shared disk is not feasible.
Redundant failover option for disaster recovery setups.
Can be placed in an independent location for better resilience.
❌ Disadvantages:
Requires an accessible file share (if the file share fails, it can cause problems).
More complex configuration than disk-based quorum.

Best Use Case:
Multi-site clusters where shared storage is unavailable. Azure or hybrid cloud clusters.

4. No Majority: Disk Only (Legacy Mode, Not Recommended)

How It Works:
Only the disk quorum has a vote (nodes do NOT vote).
If the quorum disk is lost, the entire cluster goes offline.
✅ Advantages:
Works in very small clusters (e.g., 2-node cluster) where shared storage is available.
Easy to configure.
❌ Disadvantages:
Single point of failure (if the quorum disk fails, the entire cluster goes down).
Not recommended in modern SQL Server Always On environments