Always on feature has been introduced at SQL Server 2012 which combines the power of Database Mirroring and Clustering. Like clustering, Always on also works on top of Windows clustering but without the need of Shared storage(SAN). It enables high availability and disaster recovery for multiple SQL Server databases.
- In SQL 2012, we can have maximum of four secondary replicas for read, backup, HA and DR. Up to three of those secondary replicas can be synchronous. Any two of the sync replicas can perform automatic fail over. With SQL Server 2014, Always on supports up to eight secondary replicas. Also these secondary replicas has been enhanced to allow read workloads to continue to run even in case of primary is not available due to network failures or the loss of quorum for the windows server fail over cluster.
- SQL Server 2014 Always on feature is closely integrated with Windows Azure in which we can create one or more asynchronous secondary replicas on Windows Azure Infrastructure as a Service(Iaas) services. Also we can configure Always on availability groups with synchronous secondary replica for SQL Server databases that are hosted in Windows Azure Iaas Services to provide automatic fail over in case of server or VM failure.
- With SQL Server 2014 Always on availability groups, diagnostics and troubleshooting method has been enhanced to provide more specific information. Also included new columns as part of the Always on dashboard.
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