Introduction
In a Windows Failover Cluster, quorum is essential for ensuring the cluster functions correctly and maintains data integrity. Quorum loss occurs when a node is no longer part of the active cluster membership, and this can significantly impact the cluster’s operation. This blog post explores the mechanisms behind quorum, its interaction with SQL Server Always On Availability Groups, and the effects of quorum loss.
How Nodes Communicate
Nodes in a Windows Failover Cluster communicate over designated networks, which must be set to “Allow cluster network communication on this network.” They use heartbeat packets to monitor each other’s status. A heartbeat packet is essentially a ping sent from one node to another to confirm its operational status. The receiving node responds, indicating its status.
Monitoring Heartbeats
Each node monitors these heartbeat packets to ensure network and node functionality. For instance, if NODE2 sends a heartbeat packet to NODE1 and receives a response, NODE2 knows that NODE1 is operational and the network between them is functional. Conversely, if NODE1 does not receive a response from NODE2 after sending a heartbeat packet, it marks this as a failed heartbeat.
Handling Failed Heartbeats
A single missed heartbeat is not immediately critical. Nodes are configured to tolerate a limited number of missed heartbeats before marking a node or network route as down. By default, if a node does not receive a response to five consecutive heartbeats within five seconds, it considers that route down. This means NODE1 will mark the route to NODE2 as down if NODE2 fails to respond to five consecutive heartbeat packets.
Rejoining the Cluster
When a node like NODE2 is removed from the cluster due to network issues, its Cluster Service terminates and attempts to restart. This process helps the node re-establish its connection and rejoin the cluster by sending heartbeats to other nodes, hoping to receive responses and re-establish communication.
Cluster Health Monitoring
Windows clusters monitor the health of member servers. If a health issue is detected, a server may be removed from the cluster, causing cluster resources, including the availability group role, to go offline or automatically fail over to an availability group replica partner if configured.
Quorum Voting Configuration Guidelines
To determine the recommended quorum voting configuration for the cluster, follow these guidelines:
- No Vote by Default: Assume each node should not vote without explicit justification.
- Include All Primary Nodes: Nodes hosting an Always On Availability Group primary replica or preferred owner of an Always On Failover Cluster Instance should have a vote.
- Include Possible Automatic Failover Owners: Nodes that could host a primary replica or FCI, as a result of an automatic failover, should have a vote.
- Exclude Secondary Site Nodes: Avoid giving votes to nodes at a secondary disaster recovery site to prevent them from contributing to decisions that might take the cluster offline unnecessarily.
- Odd Number of Votes: If needed, add a witness file share, a witness node (with or without a SQL Server instance), or a witness disk to prevent ties in the quorum vote.
Quorum Configuration Options
Several options can be used to configure quorum, each ensuring enough votes to maintain cluster health and avoid split-brain scenarios:
- Node Majority: Suitable for clusters with an odd number of nodes, where each node gets a vote. The cluster can sustain failures of up to (N/2) – 1 nodes, where N is the number of nodes.
- Node and Disk Majority: Includes the votes of both nodes and a witness disk, suitable for even-numbered node clusters to avoid ties.
- Node and File Share Majority: Uses a file share instead of a witness disk, useful when a shared disk is unavailable, for even-numbered node clusters.
- Disk Only (Disk Witness): Uses only a witness disk to determine quorum, typically for two-node clusters.
- File Share Only (File Share Witness): Uses a file share to maintain quorum, also typical for two-node clusters without a shared disk.
Examples of Quorum Configuration
- Node Majority: In a three-node cluster, each node has one vote, tolerating the failure of one node.
- Node and Disk Majority: A four-node cluster with a witness disk has five votes in total, tolerating the failure of up to two votes.
- Node and File Share Majority: A two-node cluster with a file share witness can tolerate the failure of one vote.
- Disk Only: A two-node cluster with a witness disk relies on the disk’s availability for maintaining quorum.
- File Share Only: A two-node cluster with a file share witness relies on the file share for quorum.
Choosing the Right Quorum Configuration
Consider these factors when deciding on the quorum configuration for your cluster:
- Number of Nodes: Odd-numbered node clusters typically use node majority, while even-numbered benefit from additional witnesses.
- Location of Nodes: For geographically dispersed clusters, avoid giving votes to secondary site nodes to prevent inappropriate decisions.
- Network Reliability: Ensure reliable network infrastructure, especially when using file share or disk witnesses.
- High Availability Requirements: Choose a configuration that supports your high availability and disaster recovery needs.
Always On Availability Groups and Quorum
SQL Server Always On Availability Groups use Windows Failover Clustering for high availability and disaster recovery. The health of the Windows Failover Cluster directly affects the availability of the Always On Availability Groups.
Role of Quorum in Always On
Quorum ensures enough votes to maintain cluster health and prevent split-brain scenarios. Maintaining quorum is crucial for consensus on primary and secondary replicas in Always On Availability Groups.
Impact of Quorum Loss on Always On
Quorum loss can lead to:
- Automatic Failover: If the primary replica is among nodes that lost quorum, automatic failover might occur.
- Availability Impact: The cluster might not perform as expected, causing potential downtime for Always On Availability Groups.
- Manual Intervention: Administrators may need to manually resolve quorum issues to ensure proper operation.
Conclusion
Quorum loss in a Windows Failover Cluster is a mechanism to ensure cluster stability and data integrity. By monitoring heartbeats and enforcing communication protocols, the cluster can quickly identify and isolate problematic nodes. This isolation helps prevent data corruption and ensures the remaining nodes continue functioning.
Maintaining quorum is vital for SQL Server Always On Availability Groups to ensure high availability and disaster recovery. Proper network configuration and monitoring can minimize quorum loss and maintain the health of both your Windows Failover Cluster and Always On Availability Groups. Understanding these processes is crucial for managing these environments effectively.