Windows Server Failover Clustering (WSFC) is the features for providing high availability for SQL Server workloads - for both failover clustered instances and also Availablity Groups
Depending on the quorum configuration option that you choose and your specific settings, the cluster will be configured in one of the following quorum modes:
Node majority (no witness) - Only nodes have votes. No quorum witness is configured.
Node majority with witness (disk or file share) - Nodes have votes. In addition, a quorum witness has a vote. The cluster quorum is the majority of voting nodes in the active cluster membership plus a witness vote. A quorum witness can be a designated disk witness or a designated file share witness.
No majority (disk witness only) - No nodes have votes. Only a disk witness has a vote.
The cluster quorum is determined by the state of the disk witness.
Note: A Disk Witness isn't supported with Storage Spaces Direct.
- Disk witness
Dedicated LUN that stores a copy of the cluster database
Most useful for clusters with shared (not replicated) storage
- File share witness
SMB file share that is configured on a file server running Windows Server
Does not store a copy of the cluster database
- Cloud witness
A witness file stored in Azure Blob Storage
Recommended when all servers in the cluster have a reliable Internet connection.