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SQL server can be set up as a cluster resource if necessary. This would add an extra layer of redundancy if set up correctly, as a cluster resource can be failed over to a secondary server if the primary should fail. Clustering also allows for easier upgrades of the operating system on database servers as a cluster node can be upgraded and rebooted if necessary without affecting SQL Server performance. Once the update is complete, database resources can be failed over to the updated server and the process can be repeated for the other cluster node(s) in the cluster.
In order for a SQL Server cluster to have any use, the server must also be set up with a clustered IP address for connections, and with a cluster resource for database storage such as a DAS or SAN device.