Upgrading Windows Server Failover Cluster Functional Level: Impact on SQL Server Always On Availability Groups
Introduction
Windows Server Failover Clustering (WSFC) is the backbone of high-availability solutions like SQL Server Always On Availability Groups (AGs). Periodically, Microsoft introduces new cluster functional levels to enable advanced features and improvements.
But what happens when you upgrade the cluster functional level? Does it affect SQL Server Always On? Are there risks? This blog explains:
✅ What a cluster functional level is
✅ How upgrading impacts SQL Server Always On
✅ Best practices for a smooth update
1. What is a Cluster Functional Level?
The cluster functional level determines which features are available in a WSFC, similar to an Active Directory functional level.
Supported Levels (as of Windows Server 2022)
| Version | Functional Level | Notes |
|---|---|---|
| Windows Server 2016 | 8 | Default for 2016 |
| Windows Server 2019 | 9 | Introduces VM resilience improvements |
| Windows Server 2022 | 10 | Supports Storage Replica enhancements |
Why Upgrade?
- Enables new clustering features (e.g., better VM resiliency, storage replication).
- Required when adding newer Windows Server nodes.
- Improves security and performance.
2. Does Upgrading Affect SQL Server Always On?
✅ No Direct Impact on Existing Availability Groups
- SQL Server continues running without interruption.
- No automatic failover or restart of AG resources.
- No changes to AG configurations.
🚨 Key Considerations
| Factor | Impact | Recommendation |
|---|---|---|
| Mixed OS Versions | Upgrade blocked if nodes run different Windows versions | Ensure all nodes are on the same or higher version |
| SQL Server Version | SQL 2016+ fully supported; older versions (2014-) may need testing | Verify compatibility before upgrading |
| Rollback | Not possible without rebuilding the cluster | Test in non-production first |
| Quorum & Networking | Newer levels optimize quorum voting | Validate cluster health before upgrading |
3. Best Practices for Upgrading
🔹 Before Upgrading
- Check the current functional level:
Get-Cluster | Select-Object ClusterFunctionalLevel- Ensure all nodes are on the same Windows version.
- Run a cluster health check:
Test-Cluster -Node "Node1,Node2,Node3" -Include "Storage","Network"- Back up SQL Server and cluster configurations.
🔹 During Upgrade (Maintenance Window Recommended)
Update-ClusterFunctionalLevel -Cluster "YourClusterName" -Force- No downtime, but a maintenance window is safest.
- Monitor AG status post-upgrade:
SELECT ag.name, ar.replica_server_name, ars.synchronization_health_desc
FROM sys.availability_groups ag
JOIN sys.availability_replicas ar ON ag.group_id = ar.group_id
JOIN sys.dm_hadr_availability_replica_states ars ON ar.replica_id = ars.replica_id;🔹 After Upgrading
- Add new nodes (if planning to expand).
- Validate failover testing to ensure AGs work as expected.
4. Real-World Example: Upgrading from 2019 to 2022
Scenario
- Cluster: 3-node WSFC (Windows Server 2019 → 2022)
- SQL Server: 2019 Always On AGs
Steps Taken
- Upgraded all nodes to Windows Server 2022.
- Ran
Update-ClusterFunctionalLevel. - Verified AG synchronization:
Get-ClusterResource | Where-Object {$_.State -ne "Online"}- Confirmed no impact on database availability.
Result
- Successfully upgraded to functional level 10.
- No SQL Server downtime or AG reconfiguration needed.
5. Troubleshooting Potential Issues
| Issue | Solution |
|---|---|
| “ClusterFunctionalLevel upgrade failed” | Ensure all nodes are on the same Windows version |
| SQL AG shows “Not Synchronizing” | Restart the SQL Server AG resource (Move-ClusterGroup) |
| Node eviction after upgrade | Rejoin the node and verify network/storage connectivity |
Conclusion
Upgrading the cluster functional level is safe for SQL Server Always On, provided:
✔ All nodes run compatible Windows versions.
✔ SQL Server is 2016 or later.
✔ The cluster is healthy before upgrading.
By following best practices, you can enable new clustering features without disrupting SQL Server high availability.
📢 Have you upgraded your cluster functional level? Share your experience in the comments!
Further Reading
Would you like a step-by-step video guide on this process? Let me know! 🚀
