Failover Cluster Instances (FCIs) in SQL Server

High availability for SQL Server is a critical aspect of modern database management. SQL Server Failover Cluster Instances (FCIs) provide a robust solution for ensuring that your SQL Server instances remain available even in the event of hardware or operating system failures on a server node.

What is a Failover Cluster Instance?

A Failover Cluster Instance is a single installation of SQL Server that is installed across two or more nodes in a Windows Server failover cluster. The SQL Server binaries are installed on each node, but only one node owns and runs the SQL Server instance at any given time. This shared storage and clustered resource model allows for seamless failover to another node if the active node becomes unavailable.

Key Components of an FCI:

  • Shared Storage: All nodes in the cluster access the same disks where the SQL Server database files, logs, and system databases reside. This can be implemented using technologies like Storage Area Networks (SANs), iSCSI, or Shared VHDs.
  • Cluster Network: A dedicated network for cluster communication and client connections, ensuring high availability and reliable connectivity.
  • SQL Server Resource: This is a clustered resource that manages the SQL Server service, ensuring it can be brought online and taken offline on the appropriate node.
  • Network Name Resource: A virtual network name that clients connect to, abstracting the underlying physical node. This name remains the same during a failover.
  • IP Address Resource: A virtual IP address associated with the network name resource, also remaining consistent across failovers.

How Failover Works

When a failure is detected on the active node (e.g., hardware failure, OS crash, planned maintenance), the Windows Server Failover Clustering (WSFC) service initiates a failover process. This involves:

  1. Detecting the failure of the active node.
  2. Gracefully shutting down SQL Server on the failed node (if possible).
  3. Bringing the SQL Server resource group online on a different, available node.
  4. Attaching the shared disks to the new active node.
  5. Starting the SQL Server service on the new active node.
  6. Clients can then reconnect to the same virtual network name and IP address, with minimal downtime.

Benefits of SQL Server FCIs

  • High Availability: Minimizes downtime and ensures continuous operation of critical applications.
  • Disaster Recovery: Can be a component of a broader disaster recovery strategy.
  • Simplified Management: A single instance installation across multiple nodes simplifies management compared to managing separate instances.
  • Resource Efficiency: The same hardware can host multiple FCIs, improving resource utilization.

Considerations for Deployment

  • Storage Configuration: Proper configuration of shared storage is paramount for performance and reliability.
  • Networking: Robust network design with redundant paths is essential.
  • Testing: Regular testing of failover scenarios is crucial to ensure the solution works as expected.
  • Patching and Updates: Careful planning is required for patching and updating cluster nodes to minimize downtime.

Example Configuration Snippet (Conceptual):

This is a conceptual example and actual configuration will vary:


<ClusterResource Name="SQL Server" Type="SQL Server">
    <Parameter Name="InstanceName" Value="MSSQLSERVER" />
    <Parameter Name="Dependencies" Value="SQL IP Address,SQL Network Name,Shared Disk" />
</ClusterResource>

Related Topics:

FCI Architecture Diagram Storage Options for FCI SQL Server Failover Flow