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What is Database Mirroring?
Database mirroring is a solution for increasing the availability of a SQL Server database. Mirroring is implemented on a per-database basis and works only with databases that use the full recovery model.
Database mirroring is the creation and maintenance of redundant copies of a database. The purpose is to ensure continuous data availability and minimize or avoid downtime that might otherwise result from data corruption or loss, or from a situation when the operation of a network is partially compromised.
What are the benefits of Database Mirroring?
- It increases the 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.
- It Increases the data protection.Database mirroring provides complete or almost complete redundancy of the data, depending on whether the operating mode is high-safety or high-performance.
- It Improves the availability of the production database during upgrades.To minimize downtime for a mirrored database, you can sequentially upgrade the instances of SQL Server that are hosting the failover partners.
How Database Mirroring works in SQL Server?
Database mirroring maintains two copies of a single database that must reside on different server instances of SQL Server Database Engine. Typically, these server instances reside on computers in different locations. Starting database mirroring on a database, initiates a relationship, known as a database mirroring session, between these server instances.
One server instance serves the database to clients (the principal server). The other instance acts as a hot or warm standby server (the mirror server), depending on the configuration and state of the mirroring session.
The principal and mirror servers communicate and cooperate as partners in a database mirroring session. The two partners perform complementary roles in the session: the principal role and the mirror role. At any given time, one partner performs the principal role, and the other partner performs the mirror role.
Database mirroring involves redoing every insert, update, and delete operation that occurs on the principal database onto the mirror database as quickly as possible. Redoing is accomplished by sending a stream of active transaction log records to the mirror server, which applies log records to the mirror database, in sequence, as quickly as possible.
What is a automatic failover?
The process by which, when the principal server becomes unavailable, the mirror server to take over the role of principal server and brings its copy of the database online as the principal database.
What is a principal database principal server?
Principal database is a read-write database whose transaction log records are applied to mirror database and Principal server is an instance on which the principal database resides.
What is a mirror database and mirror server?
The copy of the database that is typically fully synchronized with the principal database and the mirror server is an instance on which the mirror database resides.
What is a failover partners?
The two server instances (the principal server or the mirror server) that act as role-switching partners for a mirrored database.
What is a forced service?
A failover initiated by the database owner upon the failure of the principal server that transfers service to the mirror database while it is in an unknown state.
What is a High-performance mode of the Mirroring?
The database mirroring session operates asynchronously and uses only the principal server and mirror server. The only form of role switching is forced service.
What is a High-safety mode of the Mirroring?
The database mirroring session operates synchronously and, optionally, uses a witness, as well as the principal server and mirror server.
What is a redo queue?
It receives transaction log records that are waiting on the disk of a mirror server.
What is a send queue?
It holds the unsent transaction log records that have accumulated on the log disk of the principal server.
What is role switching?
Within the context of a database mirroring session, the principal and mirror roles are typically interchangeable in a process known as role switching. Role switching involves transferring the principal role to the mirror server. In role switching, the mirror server acts as the failover partner for the principal server. When a role switch occurs, the mirror server takes over the principal role and brings its copy of the database online as the new principal database. The former principal server, if available, assumes the mirror role, and its database becomes the new mirror database. Potentially, the roles can switch back and forth repeatedly.
What is Transaction safety?
A mirroring-specific database property that determines whether a database mirroring session operates synchronously or asynchronously. There are two safety levels: FULL and OFF.
What is Witness server?
To support automatic failover, a database mirroring session must be configured in high-safety mode and also possess a third server instance, known as the witness.Unlike the two partners, the witness does not serve the database. Supporting automatic failover is the only role of the witness.
What happened, when a DBA paused the session?
Pausing preserves the session state while removing mirroring. When a session is paused, the principal server does not send any new log records to the mirror server. All of these records remain active and accumulate in the transaction log of the principal database. As long as a database mirroring session remains paused, the transaction log cannot be truncated.
What are the Prerequisites to set up a Database Mirroring?
- The partners and the witness, if any, must be running on the same version of SQL Server.
- The two partners, that is the principal server and mirror server, must be running the same edition of SQL Server. The witness, if any, can run on any edition of SQL Server that supports database mirroring.
- The database must use the full recovery model. The simple and bulk-logged recovery models do not support database mirroring.
- Verify that the mirror server has sufficient disk space for the mirror database.
- When you are creating the mirror database on the mirror server, make sure that you restore the backup of the principal database specifying the same database name WITH NORECOVERY.
