If the database needs to be trustworthy after a failover, additional setup steps are necessary. For information about enabling automatic decryption of the database master key of a mirror database, see Set Up an Encrypted Mirror Database. Take at least one log backup on the principal database. Create either a full database backup or a differential database backup of the principal database. Typically, you need to take at least one log backup on the principal database.
However, a log backup might be unnecessary, if the database has just been created and no log backup has been taken yet, or if the recovery model has just been changed from SIMPLE to FULL.
Unless the backups are on a network drive that is accessible from both systems, copy the database and log backups to the system that will host the mirror server instance. Before you can start a database mirroring session, you must create the mirror database. You should do this just before starting the mirroring session. This example uses the AdventureWorks sample database, which uses the simple recovery model by default.
To use database mirroring with the AdventureWorks database, modify it to use the full recovery model:. This is useful to separate the backups under the full recovery model from any previous backups made under the simple recovery model. The restore command depends on whether the paths of principal and mirror databases are identical. 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. 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. There are two different database mirroring modes in SQL Server. The first one is high-safety mode and the second is high-performance mode. In the above two modes, synchronous operations are occurring while in this mode we are having an asynchronous operation. As we know, one will be primary and another one will be the mirror server.
Figure 4: Mirror Database Server Now we have to set up some of the basic things. So, now we will be going to configure Mirroring in the next steps.
Once you click this option, you will see a popup as shown in the image. You can also open this popup by Properties and then cab mirror the page.
Note: It will show some information related to Mirroring. For not showing up on the starting page repetitively you can check on the checkbox shown on the screen to skip this page. Click on the Next button, if this page shows up again otherwise click on the continue option to go ahead with the further process.
In the next step, it will ask you to configure the witness server which is used when we have asynchronous transfer with automatic failover.
As we are implementing basic mirroring, we can skip that step by click on No and then Next as shown in the image. Figure 6: Include Witness Server if needed At the next step. Now it will ask for some additional information related to the Principal server.
In this step, we will be able to create the endpoint using SQL server objects that can and can easily communicate with each other over a single network.
The endpoint name is Mirroring and Listener Port is Information which is auto-populated so that you can cross-check and click on the Next button as shown in the image. Follow the steps shown below image. It asks for the Mirror server credentials. To minimize downtime for a mirrored database, you can sequentially upgrade the instances of SQL Server that are hosting the failover partners. This will incur the downtime of only a single failover.
This form of upgrade is known as a rolling upgrade. For more information, see Upgrading Mirrored Instances. High-performance mode The database mirroring session operates asynchronously and uses only the principal server and mirror server. The only form of role switching is forced service with possible data loss. High-safety mode The database mirroring session operates synchronously and, optionally, uses a witness, as well as the principal server and mirror server.
Optionally, the role of witness is performed by a third server instance. After 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.
Transaction safety A mirroring-specific database property that determines whether a database mirroring session operates synchronously or asynchronously. Witness For 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. Unlike the two failover partners, the witness does not serve the database.
Supporting automatic failover is the only role of the witness. 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. When a database mirroring session is synchronized, database mirroring provides a hot standby server that supports rapid failover without a loss of data from committed transactions. When the session is not synchronized, the mirror server is typically available as a warm standby server with possible data loss.
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. Each partner is described as owning its current role. The partner that owns the principal role is known as the principal server , and its copy of the database is the current principal database.
The partner that owns the mirror role is known as the mirror server , and its copy of the database is the current mirror database. When database mirroring is deployed in a production environment, the principal database is the production database. 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. Unlike replication, which works at the logical level, database mirroring works at the level of the physical log record.
Beginning in SQL Server , the principal server compresses the stream of transaction log records before sending it to the mirror server.
This log compression occurs in all mirroring sessions. A given server instance can participate in multiple concurrent database mirroring sessions with the same or different partners.
A server instance can be a partner in some sessions and a witness in other sessions. The mirror server instance must be running the same edition of SQL Server. A database mirroring session runs with either synchronous or asynchronous operation.
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