Tuesday, May 24, 2011

Database Mirroring

Overview of Mirroring:

Mirroring is mainly implemented for increasing the database availability. Similar to log shipping mirroring is also implemented on per database basis. Database mirroring maintains two copies of a single database that must reside on different instances of SQL Server Database Engine (server instances). Typically, these server instances reside on computers in different locations. One server instance serves the database to clients (the principal server), while the other server instance acts as a hot or warm standby server (the mirror server).
Mirroring provides a hybrid solution i.e
1. Provides a copy of the database like Log Shipping and
2. Rapid failover capabilities like Clustering

Advantages of Mirroring :

* Increases data protection ---> Depending on the mode of operation Mirroring provides minimal data loss.

* Increases availability of a database ---> In the event of a disaster, in high-safety mode with automatic failover, failover quickly brings the standby copy of the database online (with no data loss). In the other operating modes, the database administrator has the alternative of forcing service (with possible data loss) to the standby copy of the database.

* Improves the availability of the production database during upgrades ---> During service packs installation or any patch applied on Principal server which requires downtime, the standby comes into effect.

Components in Mirroring:

Database mirroring consist of the following components

1. Principal ---> The Principal is the originating server i.e it is the source server which contains the database which is configured for mirroring.There can be only one principal database and it has to be in a separate SQL Server instance than the mirror database.

2. Mirror ---> The Mirror is the receiving database in a mirror pair i.e it is the destination server which contains the mirrored database.There can be only one mirror for each principal database.The mirror needs to be on its own separate SQL Server instance preferably on separate physical server.

3. Mirrored Pair ---> A Principal and Mirror operating together are called a Mirrored Pair.The changes on the principal are reflected in the mirrored database

4. Witness ---> A Witness is optional and it monitors the Mirrored Pair. It ensures that both principal and mirror are functioning properly. The Witness is also a separate SQL Server instance preferably on a separate physical server than principal and mirror. One Witness server can monitor multiple Mirrored Pairs.

5. Quorum ---> A Quorum is the relationship between the Witness, Principal and the Mirror.

6. Endpoint ---> Endpoint is the method by which SQL Server Database engine communicates with applications. In the context of Database mirroring endpoint is the method by which the Principal communicates with the Mirror. The mirror listens on a port defined in the endpoint. The default is 5022. Each database mirror pair listens on its own unique port.


To list all the database mirror endpoints --> Run,
---> Select * from sys.database_mirroring_endpoints

To list all the endpoints --> Run,
---> Select * from sys.tcp_endpoints

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