Database Mirroring Reduces Unplanned Downtime

Database mirroring supports failover between a production database and the copy of that database on a standby server. The main production database is referred to as the principal and the standby is referred to as the mirror. To maintain synchronization between the principal and mirror databases, mirroring copies the production database’s transaction logs (which record database changes, such as data inserts or deletes) to the mirror server and applies the changes recorded in those logs to the mirror database (indicated by the arrow connecting the principal and mirror database servers in the illustration).

Database mirroring offers administrators two modes of operation: high-safety mode and high-performance mode. Operating in high-safety mode ensures that all SQL Server transactions (e.g., inserts, updates, and deletes) on the principal are also recorded on the mirror, which guarantees that no data is lost in the event of a principal database (or database server) failure. However, keeping the principal and mirror synchronized consumes database server and network resources and could cause noticeable database performance impacts (for example, slowing the response of the database to application or user queries).