Windows Server Failover Clustering provides high availability for workloads. These resources are considered highly available if the nodes that host resources are up; however, the cluster generally requires more than half the nodes to be running, which is known as having quorum.
Quorum is designed to prevent split-brain scenarios which can happen when there is a partition in the network and subsets of nodes cannot communicate with each other. This can cause both subsets of nodes to try to own the workload and write to the same disk which can lead to numerous problems. However, this is prevented with Failover Clustering’s concept of quorum which forces only one of these groups of nodes to continue running, so only one of these groups will stay online.
In Windows Server 2016, there are two components of the system that have their own quorum mechanisms on an Storage Spaces Direct cluster:
- Cluster Quorum: This operates at the cluster level (i.e. you can lose nodes and have the cluster stay up)
- Pool Quorum: This operates on the pool level (i.e. you can lose nodes and drives and have the pool stay up). Storage pools were designed to be used in both clustered and non-clustered scenarios, which is why they have a different quorum mechanism.