Before get into Dynamic Quorum, we should understand what is Quorum model and types of Quorum used in Exchange high availability feature.
What is Quorum
The quorum configuration in a failover cluster determines the number of failures that the cluster can sustain while still remaining online. If an additional failure occurs beyond this threshold, the cluster will stop running. Generally quorum is used to avoid split brain on the cluster environment.
Quorum Types
Majority Node Set [MNS] is a Windows Clustering model used since early versions of Exchange. This model requires 50% of the voters (servers and/or one file share witness) to be up and running.
We should understand two major types of quorum which we used frequently in exchange servers
1. Node and File share majority
DAGs with an even number of members use the failover cluster's Node and File Share Majority quorum mode, which uses an external witness server that acts as a tie-breaker. In this quorum mode, each DAG member gets a vote. In addition, the witness server is used to provide one DAG member with a weighted vote.
Any member of the DAG that can communicate with the witness server can place a Server Message Block [SMB] lock on the witness server's witness.log file. The DAG member that locks the witness server (the locking node) retains an additional vote for quorum purposes.
2. Node Majority
DAGs with an odd number of members use the failover cluster's Node Majority quorum mode. In this mode, each member gets a vote and each member's local system disk is used to store the cluster quorum data. If the configuration of the DAG changes, that change is reflected across the different disks.
What is Dynamic Quorum
Windows Server 2012 introduced a new model called Failover Clustering Dynamic Quorum, which we can use with Exchange. When using Dynamic Quorum, the cluster dynamically manages the vote assignment to nodes based on the state of each node. When a node shuts down or crashes, it loses its quorum vote. When a node successfully re-joins the cluster, it regains its quorum vote. By dynamically adjusting the assignment of quorum votes, the cluster can increase or decrease the number of quorum votes that are required to keep it running. This enables the cluster to maintain availability during sequential node failures or shutdowns.
The advantage this brings, is that it is now possible for a cluster to run even if the number of nodes remaining in the cluster is less than 50%! By dynamically adjusting the quorum majority requirement, the cluster can sustain sequential node shutdowns down to a single node and still keep running.
We can enable Dynamic Quorum by editing DAG properties from failover cluster management console.
How it works
To explain this consider three node DAG. Normally we should keep two nodes up to keep cluster online. But by using Dynamic Quorum feature we can run cluster even with one node up.
When the first node is down, dynamic quorum removes the vote from the node with the lowest ID. This is because with only two nodes remaining in a cluster we cannot have a majority as the majority of two is two. So, in order to avoid the cluster from shutting down, one of the votes is removed, thus only requiring one vote to maintain the cluster.
Example
DAG and Database setup
Cluster setup
Cluster after node1 is down
Databases after node1 is down
Cluster after node2 is down
Database after node2 is down
Even after two nodes are down from a three node DAG, cluster is still up and databases are mounted on the server which is remaining up.
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