(Failover Cluster) Witness in Clustering

🔹 What is a Witness in Clustering?

• A witness acts as a “tie-breaker” to help the cluster reach quorum (i.e., majority agreement among nodes) in the event of a node or site failure.
• Without quorum, the cluster goes offline to prevent data corruption or “split-brain.”

🔸 1. Cloud Witness

FeatureDetails

Location	Microsoft Azure (uses Azure Blob Storage)
Use Case	Ideal for multi-site clusters or when you want a low-maintenance, off-site witness
Cost	Very low — just basic Azure Blob storage charges
Pros	No need to maintain a 3rd location server, works over the Internet
Cons	Requires Azure and Internet access

 

✅ Best for: Stretch clusters, disaster recovery, or when you don't have a 3rd site for a file share or disk.

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🔸 2. File Share Witness

FeatureDetails

Location	Any accessible SMB file share (can be on a domain controller, file server, etc.)
Use Case	Useful in 2-node clusters, no shared storage, or stretched clusters
Pros	Easy to configure, doesn't need Azure
Cons	The file share must be highly available; if it's down, quorum can be lost

 

✅ Best for: Simple clusters or when shared storage or Azure is not an option.

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🔸 3. Disk Witness

FeatureDetails

Location	A shared disk (LUN) accessible by all cluster nodes
Use Case	Traditional on-prem clusters with SAN or S2D setups
Pros	No need for external services or Internet
Cons	Can be a single point of failure if the disk fails, not suitable for stretched clusters (due to split-brain risk)

 

✅ Best for: On-premise clusters with reliable shared storage (e.g., SAN).