This provides server-level redundancy on a certified Microsoft Windows Cluster Services Configuration. A failover cluster is setup with a minimum of two servers. In the cluster setup, each server is called a node. All the nodes are connected to shared disk resources. Only one node is active at a time for a SQL Server instance and serves all the requests for that instance of SQL Server. When the active node fails, failover takes place and one of the other available nodes will become the active node. More information can be read from this MSDN library article.
- This is an appropriate option for mission critical applications where automatic failover is needed for the entire instance of SQL Server.
- Recovery Time Objective (RTO) - Almost immediately, because processing is handed over to another node in the cluster.
- Recovery Point Objective (RPO) - If there is no data corruption or data loss (due to data deletion), there is minimal to no data loss during the failover.
- It provides automatic failover.
- It protects an entire SQL Server instance.
- Can apply service packs to one node at a time to minimize downtime.
- If secondary server is used in a passive mode (only for failover), additional SQL Server licensing is not needed.
- It is more expensive since it requires special hardware for the setup as well as redundant hardware.
- It is not supported with all SQL Server editions. (see chart below)
- It is a single shared data resource.