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Posted by Ami Levin on Sunday, 06 November 2011 

In a previous post, I’ve presented the enhancements to failover cluster instances. In the mean time, Microsoft announced some radical changes to it’s licensing and editions scheme. In SQL Server 2012, the standard and BI editions will support basic failover clustering of up to 2 nodes.

AlwaysOn Availability Groups

The AlwaysOn Availability Groups feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring.

NOTE: In CTP3, some features of AlwaysOn Availability Groups are only at preview quality and might have more bugs than other features. The preview-quality features are under the control of trace flag 9532. Enable this trace flag to test the AlwaysOn preview features.

An availability group is a container that defines a set user databases (availability databases) to fail over as a single unit, and a set of availability replicas to host copies of each availability database. Each availability group requires at least two availability replicas: the primary availability replica and one secondary availability replica.

The main exciting new features and characteristics of availability groups are:

  • Multiple secondary replicas (up to 4).
  • Alternative availability modes: Asynchronous-commit mode and Synchronous-commit mode.
  • Several failover modes: automatic failover, planned manual failover, and forced manual failover.
  • Active secondary replicas allow Read-only access to replicas and the ability to perform backup operations on secondary replicas.
  • Availability group listeners provide fast application failover after an availability group fails over.
  • flexible failover policy for each availability group to provide some control over the automatic failover process.
  • Automatic page repair for protection against page corruption.
  • Forcing WSFC quorum (forced quorum).
  • Encryption and compression, which provide a secure, high performing transport.
  • Interoperation with Change data capture, Change tracking, Contained databases, FILESTREAM, FileTable, Remote Blob Store (RBS), Service Broker, and SQL Server Agent.

AlwaysOn availability groups can be deployed and maintained by a new set of T-SQL statements and a host of new wizards in SSMS and powershell cmdlets. A new AlwaysOn availability groups dashboard monitors AlwaysOn availability groups, availability replicas, and availability databases and evaluates results for AlwaysOn policies.

For detailed info, see the AlwaysOn Availability Groups page in BOL.

Remember to watch the AlwaysOn team blog for updates and announcements.

NOTE: The features described here for SQL Server 2012 are available in CTP3 (Community Technology Preview) version and may or may not make it to the RTM (Release To Manufacture) version. It has happened before…