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Windows Server has several High Availability (HA) features. These features are designed to provide fault tolerance to applications running on Windows Server. Ideally a single instance of Windows Server is designed to be as resilient as possible in order to minimize or eliminate disruptions to critical applications. The next line of defense is to provide an additional host or hosts and allow applications to fail over to another instance as quickly as possible. This typically results in a small disruption to applications as they are restarted on another host.
The following sections describe how Windows Server features such as Multipath (MPIO) and Windows Server Failover Cluster (WSFC) are used to provide high availability.
Multipath I/O (MPIO) is a key feature used by Windows Server for storage resiliency. MPIO allows multiple storage paths from Windows Server to the same storage volume. It also provides different load balancing and optimization strategies for using these paths. The ability to use multiple paths is key to providing redundancy to a single storage appliance and enables products such as metro node to provide redundancy across multiple storage appliances.
Windows Server Failover Clustering (WSFC) is the basis of many enterprise Windows deployments. A basic WSFC runs on two or more Windows Servers configured as a single cluster and requires shared storage between the servers. Dell Storage appliances such as PowerVault are used as shared storage. Shared storage volumes are owned by one of the nodes in the cluster. Applications that support WSFC such as SQL Server have a virtual address but run on one of the nodes in the cluster. WSFC coordinates failover activities between nodes in the cluster. Prior to restarting the application on another node, any dependencies such as storage volumes and other Windows services are also moved and restarted by the WSFC service.
For example, if an issue is detected on the server running the SQL Server instance, that SQL Server instance (leveraging WSFC functionality) will be restarted on another server in the WSFC and the SQL Server instance will bring all related databases online. For complete information, see the Microsoft Always On Failover Cluster Instances page.