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AlwaysOn is a new feature of SQL Server 2012 for high-availability and disaster recovery. The AlwaysOn feature is an extension to the principles of SQL Server Database Mirroring. However, it includes enhancements that go beyond the existing high-availability solutions that Database Mirroring and Database Replication offer.

For more information about SQL Server Database Mirroring, see SAP note 965908.

For more information about SQL Server 2012 AlwaysOn, see SAP note 1772688.

Features

Some of the main features and benefits of the AlwaysOn feature are the following:

  • Provides four secondary replicas besides the primary replica

  • Two of the mirrors work with synchroneous data replication whereas the additional two mirrors work with asynchroneous data replication

  • Reading from a synchronous or asynchronous mirror real time is possible

  • Inclusion of multiple user databases in a mirroring relationship (no system databases allowed). Increased scalability and improved failover time due to comprehensive design changes

  • Multi-subnet support included for the SQL Server failover cluster functionality

  • Support of multiple databases as failover units (with Windows Server Failover Clustering only the complete SQL Server instance can fail over, with database mirroring only one database can fail over)

  • New health detection for SQL Server, which is used for shared disk and non-shared disk deployments

  • Microsoft Failover Clustering APIS can be installed without shared storages

  • Enabling of backups from secondary replicas

This graphic is explained in the accompanying text.

AlwaysOn Running on Two Windows Server Failover Cluster Nodes