Microsoft Windows Failover Clustering is a high-availability option designed to increase the uptime of SQL Server instances. A cluster includes two or more physical servers, called nodes; identical configuration is recommended. One is identified as the active node, on which a SQL Server instance is running the production workload, and the other is a passive node, on which SQL Server is installed but not running. If the SQL Server instance on the active node fails, the passive node becomes the active node and begins to run the SQL Server production workload with some minimal failover downtime. Additionally, you can deploy a Windows Failover Cluster to have both nodes active, which means running different SQL Server instances where any SQL Server instances can failover to the other node.
Clustering is designed to improve the availability of the physical server hardware, operating system, and SQL Server instances but excluding the shared storage. Should any of these aspects fail, the SQL Server instance fails over. The other node in a cluster automatically takes over the failed SQL Server instance to reduce downtime to a minimum.
Additionally, the use of a Windows Failover Cluster can help reduce downtime when you perform maintenance on cluster nodes. For example, if you need to update hardware on a physical server or install a new service pack on the operating system, you can do so one node at a time.
Key Concept: A Windows Failover Cluster uses shared storage– typically, this shared storage is on a SAN. When a SQL Server instance is installed on the cluster, system and user databases are required to be on the shared storage. That allows the cluster to move the SQL instance to any server (or “node”) in the cluster whenever you request, or if one of the nodes is having a problem. There is only one copy of the data, but the network name and SQL Server service for the instance can be made active from any cluster node.
Translation: A failover cluster basically gives you the ability to have all the data for a SQL Server instance installed in something like a share that can be accessed from different servers. It will always have the same instance name, SQL Agent jobs, Linked Servers and Logins wherever you bring it up. You can even make it always use the same IPAddress and port– so no users of the SQL Server have to know where it is at any given time.
Clustering using Availability Groups in SQL Server 2012
In order to use the Availability Group feature in SQL Server 2012, the Failover Clustering feature must be enabled in Windows. If you’re using Windows Server 2008 or prior, this feature is only available in Datacenter and Enterprise edition of Windows Server, so that feature isn’t free. This feature is now included in Windows Server 2012 for all editions.
Even though you’re enabling the Failover Cluster feature, you are NOT required to have shared storage to use Availability Groups. You have the option to use a Failover Cluster in an Availability Group, but you can also run your Availability Groups with entirely independent storage subsystems if you desire.