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Thursday, May 3, 2012

Failover techniques for Database Mirroring using SQL SERVER 2008


Fail-over techniques in database mirroring.There are different techniques for database mirroring.
  1. Automatic fail over
  2. Manual fail over
  3. Forced fail over


The following table summarizes which forms of failover are supported under each of the operating modes. 

High performance
High-safety mode without a witness
High-safety mode with a witness
Automatic failover
No
No
Yes
Manual failover
No
Yes
Yes
Forced service
Yes
Yes
No

Automatic Fail-Over:
To test database mirroring auto fail over solution you need to restart database engine and your Principal and Mirror servers will be reversed but you need to set your mirroring solution as (Principal – Witness - Mirror). You can view how we can test auto fail over solution look at this link.


Manual Fail-Over:
During a manual failover, the principal and mirror server roles are swapped for the database on which the failover occurs. The mirror database becomes the principal database and the principal database becomes the mirror. For example, the following table shows the how a manual failover swaps the roles of two mirroring partners: System


Forced Fail-Over:
We are now looking into Forced Service for database mirroring. This option is available against high safety but automatic failover is not available.  In the event of failure of the principal, the database service becomes unavailable. You need manual intervention to make the database service available.


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