![]() Typically, these server instances reside on computers in different locations. Overview of Database Mirroringĭatabase mirroring maintains two copies of a single database that must reside on different server instances of SQL Server Database Engine. Supporting automatic failover is the only role of the witness. Unlike the two failover partners, the witness does not serve the database. There are two safety levels: FULL and OFF.įor use only with high-safety mode, an optional instance of SQL Server that enables the mirror server to recognize when to initiate an automatic failover. The relationship that occurs during database mirroring among the principal server, mirror server, and witness server (if present).Īfter a mirroring session starts or resumes, the process by which log records of the principal database that have accumulated on the principal server are sent to the mirror server, which writes these log records to disk as quickly as possible to catch up with the principal server.Ī mirroring-specific database property that determines whether a database mirroring session operates synchronously or asynchronously. Unsent transaction log records that have accumulated on the log disk of the principal server. The taking over of the principal role by the mirror. Optionally, the role of witness is performed by a third server instance. The principal server and mirror server perform complementary principal and mirror roles. Received transaction log records that are waiting on the disk of a mirror server. In database mirroring, the partner whose database is currently the principal database. In database mirroring, a read-write database whose transaction log records are applied to a read-only copy of the database (a mirror database). In a database mirroring configuration, the server instance on which the mirror database resides. The copy of the database that is typically fully synchronized with the principal database. The database mirroring session operates synchronously and, optionally, uses a witness, as well as the principal server and mirror server.Ī failover initiated by the database owner, while the principal server is still running, that transfers service from the principal database to the mirror database while they are in a synchronized state. The only form of role switching is forced service (with possible data loss). The database mirroring session operates asynchronously and uses only the principal server and mirror server. The two server instances (the principal server or the mirror server) that act as role-switching partners for a mirrored database.Ī 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. 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. For more information, see Upgrading Mirrored Instances. This form of upgrade is known as a rolling upgrade. This will incur the downtime of only a single failover. To minimize downtime for a mirrored database, you can sequentially upgrade the instances of SQL Server that are hosting the failover partners. Improves the availability of the production database during upgrades. For more information, see Automatic Page Repair (Availability Groups: Database Mirroring). If this request succeeds, the unreadable page is replaced by the copy, which usually resolves the error. The partner that is unable to read a page requests a fresh copy from the other partner. For more information, see Operating Modes, later in this topic.Ī database mirroring partner running on SQL Server 2008 Enterprise or later versions automatically tries to resolve certain types of errors that prevent reading a data page. For more information, see Role Switching, later in this topic.ĭatabase mirroring provides complete or almost complete redundancy of the data, depending on whether the operating mode is high-safety or high-performance. 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. ![]() In the event of a disaster, in high-safety mode with automatic failover, failover quickly brings the standby copy of the database online (without data loss). Benefits of Database Mirroringĭatabase mirroring is a simple strategy that offers the following benefits: ![]() For information about support for database mirroring, restrictions, prerequisites, recommendations for configuring partner servers, and recommendations for deploying database mirroring, see Prerequisites, Restrictions, and Recommendations for Database Mirroring.
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