AlwaysOn is a new integrated high availability (HA) and disaster recovery (DR) solution that provides redundancy within a datacenter and across datacenters to help enable fast application failover during planned and unplanned downtime. Specifically, AlwaysOn delivers failover cluster instances, multiple secondaries, active secondaries, and connection director capabilities (multi-subnet and read-only intent).
AlwaysOn is the next evolution of database mirroring. Database mirroring is relatively easy to set up, and it provides an almost instant failover when it’s used in High Safety mode. However, database mirroring has several important limitations. First, database mirroring is limited to a single failover partner. In addition, database mirroring is capable of failing over only a single database at a time. Database mirroring must be implemented either synchronously or asynchronously—you have to choose one or the other. Finally, the databases on the mirroring server are in a state of recovery, so they can’t be directly accessed. To use the data, you must take point-in-time snapshots of the databases. AlwaysOn addresses all of these issues.
With AlwaysOn you can have up to four replicas (the new name for mirrored servers). These replicas can be connected both synchronously (High Safety, for HA) and asynchronously (High Performance, for DR). A new feature called Availability Groups lets you fail over multiple databases simultaneously. The result is increased availability and uptime.
These replicas are not just passive secondaries, they are active and provide read-only access, so you can do things like take your reporting infrastructure off the primary and put it on one of the secondaries. You can do a lot of sophisticated load balancing by distributing your overall workload across the various secondaries that you have.
Asynchronous secondaries will allow a company to set up either a regional or global high-availability environment, where the secondaries are spread across data centers. This solution can provide a buffer for catastrophic site failures. So, if one of your data centers were to experience an outage or an earthquake or something along those lines, it would immediately fail over to another data center, which could be in another state or even in another country.
AlwaysOn brings the technologies (mirroring, clustering, log shipping, and replication) together into a single solution. And it’s easy: there’s a new management interface, which is wizard based, that basically walks you through the process of setting it up. There’s also a dashboard that allows you to monitor the health and hygiene of the high-availability environment over time
From Brent Ozar: SQL Server Denali AlwaysOn Rocks
TechEd Video: Microsoft SQL Server Code-Named “Denali” AlwaysOn Series,Part 1: Introducing the Next Generation High Availability Solution and Microsoft SQL Server Code-Named “Denali” AlwaysOn Series,Part 2: Building a Mission-Critical High Availability Solution Using AlwaysOn