Professional Microsoft® SQL Server® 2008 Administration

17.1. Overview of Database Mirroring

Database mirroring is a high-availability solution at the database level, implemented on a per-database basis. To maximize database availability, you need to minimize planned as well as unplanned downtime. Planned downtime is very common, such as changes you have to apply to your production system, hardware upgrades, software upgrades (security patches and service packs), database configuration changes, or database storage upgrades. These all require your database or server to be unavailable for short periods of time if everything goes as planned. Unplanned downtime can be caused by hardware failures such as storage failure, by power outages, by human error, or by natural disasters, all of which can cause the production server or data center to be unavailable.

Figure 17-1 illustrates a number of mirroring concepts, which we'll discuss in detail.

Database mirroring involves two copies of a single database, residing on separate instances of SQL Server, usually on different computers. You can have separate instances of SQL Server 2008 on the same computer, but that would most likely not fit your high-availability requirements other than for testing purposes. At any given time, only one copy of the database is available to clients. This copy of the database is known as the principal database. The SQL Server that hosts this principal database is known as the principal server. Database mirroring works by transferring and applying the stream of database ...