The secondary database copy maintained in a SQL Server Database Mirroring configuration, kept synchronized with the principal database via transaction log streaming.
- The mirror database is kept in a continuous RESTORING state: It applies transactions streamed from the principal database and cannot be accessed directly for read operations.
- Database Mirroring is deprecated and was removed in SQL Server 2022: Organizations should migrate to Always On Availability Groups, which provide similar functionality along with significant enhancements.
- In synchronous (High Safety) mode, a transaction is not committed on the principal until it is hardened on the mirror: This ensures zero data loss during failover.
- Mirror database status can be monitored using: The Database Mirroring Monitor in SQL Server Management Studio (SSMS) or the
sys.database_mirroringdynamic management view (DMV). - Related terms: Database Mirroring, Always On Availability Groups, High Availability, Failover.
