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What is the difference between Database Mirroring and AlwaysOn Availability Groups?

  • 15 February 2013
  • Author: Kathi Kellenberger
  • Number of views: 36030

Last year SQL Server 2012 launched with a new choice for high availability and disaster recovery (HADR) called AlwaysOn Availability Groups. The AlwaysOn Availability Groups feature is similar to Database Mirroring which will be removed from SQL Server in a future release, but there are quite a few differences.  In both cases, a replica of the data is maintained on another server for HA or DR purposes. Transactions are automatically sent to and replayed on the replica, or secondary copy of the data.

Here is a summary of the differences between Database Mirroring and AlwaysOn Availability Groups:


Database Mirroring

AlwaysOn Availability Groups

Maximum number of secondary databases

1 Mirror

4 Secondaries

Requires Windows Clustering


Yes, however the SQL Servers can be stand-alone

Automatic failover

Yes, requires witness server and high-safety mode

Yes, one

HA or DR

Choose one

Each secondary independently configurable

Groups of databases

No, each database is configured separately

Multiple databases can be grouped to failover together

Can be used for reporting

Only against a database snapshot of the  mirror


Offload backups



Active database is called



Connection to active database

Requires special connection string for failover

Redirection handled by Windows Clustering with a virtual name. Special connection string not required

Connection string to mirror or secondary database


Can be configured with an ApplicationIntent connection string to automatically redirect or can use traditional connection string with server name.


Categories: SQL Server
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