Microsoft SQL Server 2012 Bible

High Availability/AlwaysOn

AlwaysOn Availability Groups are the next evolution of mirroring technologies in SQL Server. As reviewed in the first half of this chapter, mirroring allows you to mirror only on a per-database (1:1 ratio). With SQL Server 2012, you can define multiple databases together in a logical container called an availability group that enables the databases to fail over together as a single unit.

In addition to grouping databases together, you now get up to four readable secondary copies of your database. This gives you the flexibility to offload reporting from your primary transactional databases or even take backups from your secondaries! AlwaysOn Availability Groups offers the following benefits:

• Multiple secondary replicas (one primary and up to four secondary replicas).
• Readable secondary replicas.
• Flexible failover policies, for more granular control over conditions that cause automatic failovers for an availability group.
• No requirement for a witness server to achieve high-availability/automatic failover for your databases.
• Multiple availability modes, including Asynchronous-commit mode and Synchronous-commit mode. Asynchronous is a disaster-recovery solution recommended when secondary replicas are distributed over large distances (for example across a WAN). In Asynchronous mode, the primary does not wait for any of the secondary replicas to harden their logs; instead, after writing the log record locally to its log file, it then sends the transaction ...