Database mirroring is a software solution introduced in SQL Server 2005, service pack 1, to increase the availability of a SQL Server database. It uses two database instances (normally on different hosts), which contain copies of the same SQL Server database. These databases are identical in both name and content. The copies are the principal and the mirror. The mirror serves as a hot standby to the principal, where transactions take place. The mirror is very closely synchronized with the principal through transaction log porting. It is immediately available in case the principal fails.

The primary consideration when you establish your backup and restore procedures for database mirroring is that these operations are only available on the principal database.

For a complete description of database mirroring refer to the SQL Server 2005 Books online.