I'm at the early stages of implementing a high availibilty solution using SQL 2008.

With regard to database mirroring I believe that under the syncronous model, transactions are not committed on the principal server until they are committed on the mirror server. If the principal server fails then failover can either be automated using a witness instance of performed manually.

In this mode, what happens should the mirror server fail?

Would the principal be unable to commit transactions? or does the mirror break?

I'm concerned that the principal is at the mercy of the mirror.

2 Answers 2


Cool - just note that if you are using a witness, a "quorum" between principal, witness, and mirror is required. If you lose the mirror and the witness (or connectivity to both), the principal database will not be available.

  • thank you, i had planned on using a witness but haven't got that far with it yet, if I had two witnesses and a mirror (which is what i'm planning), would I need to lose all three before the principal becomes unavailable? Jul 23, 2010 at 16:23
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    Hi Chris - I believe you can only have one witness per mirrored pair of databases. technet.microsoft.com/en-us/library/ms189902.aspx Jul 23, 2010 at 16:46
  • Oh well that won't work then. Thanks for the info and the link, i'll have a read. Jul 23, 2010 at 17:03

Found the answer.

The loss of a partner has the following effect:

  If the mirror server instance becomes unavailable, the principal continues.


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