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Database Mirroring In SQL Server to Increase Its Availability

Stacy Brown | Modified: 2016-06-28T11:17:56+05:30|High Availability | 4 Minutes Reading


The Database Mirroring in SQL Server was firstly implemented on Microsoft SQL Server 2005, which is basically designed for high-performance and high-availability resolution of database redundancy.There are so many benefits of using database mirroring which helps you to easily safeguard the database for business continuity. This article will help you to understand the use of database mirroring in SQL Server.

What is Database Mirroring in SQL Server?

Database mirroring is basically intended to increase the high availability and high performance of a SQL Server database. It can only be implemented in the databases that use the full recovery model. In SQL Server Database mirroring, a single database has its duplicate which should exist on another server of the SQL Server Database Engine. Classically, these server instances exist on computers in other locations. Creating database mirroring in SQL Server database starts up a relationship between the server instances which is also called a database mirroring session. In database mirroring a user can redo every update, insert, delete operations that happens on the primary database onto the copied database immediately.

Role of Database Mirroring in SQL Server

The SQL Server database mirroring plays an important role in SQL Server, which primarily provides a disaster recovery plan and a high accessibility technique in the SQL Server database,and it includes two similar SQL Server instances on the common or different machines.

First, SQL Server instance works as a preliminary instance called the principal and the second instance works as a mirrored instance called the mirror. A third SQL Server instance available for some special cases which acts as a witness.

Implementation of Database Mirroring Modes In SQL Server

There are two different database mirroring modes in SQL Server. The first one is high-safety mode and the second is high-performance mode.

High-Safety Mode:

The first operating mode is high-safety mode which maintains synchronous operation. When a session starts in high-safety mode, both the mirror Server and mirror database synchronize simultaneously with the primary database as fast as possible.A transaction is committed after the synchronization of both the databases at the rate of increased transaction delay.

High-Performance Mode:

The second operating mode in SQL Server is high-performance mode, which runs asynchronously. The log records which are sent by the principal Server is maintained by the mirror server. Mirror database may slow somewhat from the principal database. However, the slowness between both the primary and mirror databases is small. But the gap can turn out to be important, if the principal server is in a heavy process or the system of the mirror server is congested.

SQL Server Database Mirroring Advantages

  • Minimizes data loss
  • Provides a high availability of the database
  • Maintains full-text catalogs
  • Do not need any singular hardware
  • Provides automatic server failover in high safety mode
  • The log-transfer method provides a solution for database redundancy

Disadvantages of Database Mirroring in SQL Server

  • Data loss is possible,if it is in asynchronous operation mode.
  • User operation is not possible in the mirror server/database.
  • Late delay possibilities can occur during data transfer.
  • Delay possibilities can only be prevented with a good hardware.
  • SQL Server database mirroring can only be possible in full recovery mode.

Background of Database mirroring

The mirrored transaction log backups and a full database should be created and restored on the SQL Server instance. The WITH NORECOVERY option executes the restore process. The principal server starts the setup process of the database mirroring with the help of SQL Server T-SQL code or Management Studio wizard.

At the starting of the setup process, there’s an alternative for choosing a third SQL Server instance which is only necessary if the high safety with automatic fail over mode is required.


It is clear that the database mirroring in SQL Server plays an important role. if it fails it displays SQL Error 1418. It minimizes the data lost chances in SQL Server. Moreover, it provides a high availability of the database. It also helps in case of corruption of principal database and the mirror database takes the place of the principal database and is used as a principal database.