Understanding Operating Modes

There are essentially three operating modes for database mirroring, as follows:

• High-safety mode (synchronous)— After a database mirroring session is initiated under high-safety mode, the mirror server database synchronizes with the principal server database and then writes the logs to disk. After the logs have been written to disk, all transactions are committed on both servers. Although this mode guarantees no data loss between the two servers, it does come with the expense of increased transaction latency.

• High-availability mode (synchronous)— Also referred to as high-safety mode with automatic failover, this operating mode is made available only with the presence of a witness server. This mode is the recommended operating mode for database mirroring within a SharePoint farm because of its ability to provide increased high availability through automatic failover. Similar to high-safety mode, the mirror server synchronizes the mirror database with the principal database and proceeds by committing transaction logs on both servers after the mirror server has written the logs to disk.

• High-performance mode (asynchronous)— Running under asynchronous operation, this operating mode provides an increased level of performance by sending logs from the principal server to the mirror server and then immediately committing the transactions on the principal server without waiting on the mirror server to write the logs to disk. By minimizing the latency of committed transactions, this mode increases the overall performance of the principal server. However, it also increases the risk of data loss between the two partners. Asynchronous mirroring is available only with the Enterprise edition of SQL Server, whereas the other two modes are available in either the Standard or Enterprise editions.

Although it is possible to switch between operating modes once a database mirroring session is configured, it is essential that administrators understand each operating mode and their respective advantages and disadvantages to successfully meet the availability requirements of their organization.