- Accidental DBA
- Posts
- SQL Server Failover Cluster Instances
If you’ve been around SQL Server for a while, you’ve heard of Failover Cluster Instances (FCIs). They’ve been part of SQL’s high availability toolbox since long before Availability Groups showed up, and they’re still relevant today, especially if you want protection at the instance level, not just the database level.
Let’s break down what they are, how they differ from AGs, and where they make sense in your environment.
What Is an FCI?
A Failover Cluster Instance is a single SQL Server instance installed across multiple Windows Server nodes. At any given time, one node is active, while the others are standing by. If the active node fails, Windows Server Failover Clustering (WSFC) brings the SQL instance online on another node.
The big requirement? Shared storage. All nodes point to the same database files, whether that’s on a SAN, SMB file share, or Storage Spaces Direct.
How FCIs Differ from Availability Groups
Storage:
FCI: one copy of the data on shared storage.
AG: multiple synchronized copies on separate storage.
Scope:
FCI: protects the whole instance (system databases, SQL Agent jobs, logins, SSIS packages, everything).
AG: protects only selected user databases.
Client Connections:
FCI: uses a Virtual Network Name (VNN). Apps connect to a single name, and after failover, that name points to the new active node.
AG: uses an AG Listener, which looks and behaves similarly, but is tied to a group of databases instead of the entire instance.
Use Cases:
FCI: protects against server or instance failure.
AG: adds database-level resilience, readable secondaries, and cross-datacenter flexibility (Distributed AG)
FCI Version & Edition Requirements
Windows Server: Requires WSFC.
SQL Server Editions:
Standard: limited to 2 nodes.
Enterprise: supports multiple nodes, multi-subnet clusters, and advanced options.
Storage: Must be shared, unless you’re on newer Windows with Storage Spaces Direct.
Benefits of FCIs
Instance-level protection: Everything tied to SQL Server moves during failover.
Automatic failover: Fast recovery from node crashes, no manual intervention required.
No data divergence: One data copy = no sync overhead.
Connection simplicity: The Virtual Network Name hides node-level details from applications.
Downsides & Risks
Shared storage dependency: If the SAN or file share fails, the cluster fails.
No readable secondaries: Unlike AGs, you can’t offload reporting or backups.
Longer failover times: The whole instance must restart on the new node.
Complex setup: Requires WSFC expertise, quorum planning, and strong storage design.
When to Choose FCI vs AG
Pick FCI if:
You want full-instance protection, including jobs, logins, and system databases.
You already have a reliable SAN or shared storage.
You need a single connection name (VNN) that hides failovers from applications.
Pick AG if:
You want multiple data copies for extra resilience.
You need readable secondaries for reporting or backups.
You want DR across data centers without shared storage.
You have personnel to maintain the underlying WSFC components
Clusterless / Read-Scale AGs are an option
The Bottom Line:
Failover Cluster Instances aren’t flashy, but they’re still solid. They protect more than just databases, they’re available in Standard Edition, and for many shops they’re the right balance of reliability and simplicity.
Just remember: your cluster is only as good as your storage.
New Pocket DBA® clients get the first month FREE!
Book a call, and mention “Newsletter”
SQL tidBITs:
When connecting to an FCI, always use the Virtual Network Name (VNN) — never the node name. This way, your applications don’t care which node is currently active. Failover happens, your apps keep talking, and you don’t get that 3 a.m. call.
Reply