Database Mirroring Operating Modes
Database Mirroring is configured for three different operating modes. These modes are high availability, high performance and high protection. Each offers a different set of functionality and must be understood in order to select the appropriate configurations.
- High Availability Operating Mode
- The High Availability Operating Mode provides durable, synchronous transfer of data between the principal and mirror instances including automatic failure detection and failover. With this functionality comes a performance hit. This is because a transaction is not considered committed until SQL Server has successfully committed it to the transaction log on both the principal and the mirror database. As the distance between the principal and the mirror increases, the performance impact also increases.
- In addition, there is an continuous ping process between all three nodes (if a witness is used) to detect failover. If witness server is not visible from the mirror, you must either reconfigure the operating mode for the database mirroring session or turn off the witness. Alternatively, you can manually fail over a database mirroring session at the mirror in High Availability Mode by issuing the ALTER DATABASE SET PARTNER FAILOVER command at the principal. The same command can also be issued if you have to take principal down for maintenance.
- High Performance Operating Mode
- With the High Performance Operating Mode the overall architecture acts as a warm standby and does not support automatic failure detection or failover. The data transfer between the principal and mirror instances is asynchronous. As such, this mode provides better performance and permits geographic separate between the principal and mirror SQL Server instances. Unfortunately, this mode increases latency and can lead to greater data loss in the event of primary database failure if it is not managed properly.
- High Protection Operating Mode
- The High Protection Operating Mode operates very similar to the High Availability Mode except the failover and promotion (mirror to principal) process is manual. With this mode the data transfer is synchronous. This mode is typically not recommended except in the event of replacing the existing witness SQL Server. After replacing or recovering the witness SQL Server, the operating mode should be changed back to High Availability Operating Mode