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If the witness is not visible from the mirror
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If the witness server is not visible from the mirror, you must either reconfigure the operating mode for the database mirroring session or turn off the witness.
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Alternatively, you can manually fail over a database mirroring session in High Availability operating mode by issuing the following command at the principal:
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ALTER DATABASE SET PARTNER FAILOVER
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Manual failover
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In High Availability operating mode, you would normally issue this statement to manually fail over from the principal to the mirror before taking the principal offline for maintenance.
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Lesson 5: Failing Over a Database Mirror
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A failure of the principal in either High Performance or High Protection operating mode leaves the mirror in a restoring state and inaccessible to transactions. When this occurs, you must connect to the mirror and initiate a manual failover from there.
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How to Initiate a Failover
The most common scenario for initiating a failover in a production environment is to do so from the mirror. Initiating a failover is a straightforward process. However, you cannot manually initiate a failover at the mirror from the Database Properties page within SSMS because the mirror database is in a recovering state. In this state, the database is inaccessible. Because the only time you can manually initiate a failover from the graphical user interface (GUI) is when the principal is online, which is not the typical situation, you should always use the ALTER DATABASE Transact-SQL command to initiate a failover. The complete ALTER DATABASE command you issue at the mirror database is as follows:
ALTER DATABASE <mirror_database> SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS
IMPORTANT
Data loss possible
Manually forcing the failover can cause data loss. With the original principal unavailable, the partners cannot communicate with each other and therefore cannot synchronize their databases. Until the original principal comes back online and can be used as the new mirror, the database mirroring session is suspended.
Quick Check
What are the requirements to issue the ALTER DATABASE statement at the mirror to initiate a failover of the database mirroring session To initiate a failover from the mirror, the witness must be turned off or connected to the mirror, and the principal must be inaccessible.
Quick Check Answer
PRACTICE
Failing Over a Database Mirror
In this exercise, you practice the manual failover of the AdventureWorks database from the principal to the mirror.
17
Implementing Database Mirroring
NOTE
Use High Performance or High Protection operating mode
In this exercise, to avoid issues with getting a High Availability operating mode in the proper state to accomplish a manual failover at the mirror, first configure a database mirroring session in either High Performance or High Protection operating mode without a witness server.
1. Stop the instance that is hosting the principal database. (Right-click the instance in Object Explorer, choose Stop, and then click Yes to confirm that you want to stop the instance.) 2. Connect to the instance that is hosting the mirror database. 3. Execute the ALTER DATABASE command to initiate the failover to the mirror:
ALTER DATABASE AdventureWorks SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS
Lesson Summary
You can initiate a manual failover at the mirror only when the principal is inaccessible and the witness is either off or connected to the mirror. To accomplish the failover of a database mirroring session, you need to issue an ALTER DATABASE command at the mirror database using the option of FORCE_SERVICE_ALLOW_DATA_LOSS. You can force a failover from the principal if you need to perform maintenance on the principal.
Lesson Review
The following questions are intended to reinforce key information presented in this lesson. The questions are also available on the companion CD if you prefer to review them in electronic form.
NOTE
Answers
Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.
1. Which command, when executed from the mirror, will fail over a database mirroring session A. ALTER DATABASE SET PARTNER OFF B. ALTER DATABASE SET WITNESS OFF C. ALTER DATABASE SET PARTNER FAILOVER D. ALTER DATABASE SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS
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