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Where to locate the Witness server when using Mirroring for DR Expand / Collapse
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Posted Saturday, May 11, 2013 3:39 AM
Grasshopper

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Hi,

I am looking into setting up mirroring for DR purposes. What I want to know is the correct location for the Witness server at our DR site? The primary server will be in our office and the target server will be at our DR site.

Thanks in advance.

Russell



Post #1451819
Posted Sunday, May 12, 2013 12:40 AM


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Ideally the witness should be on a third site, if not then place it at the primary site. If you put it at the DR site and you have a network interruption you could end up with unwanted failovers or the principal suddenly going offline.

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Post #1451882
Posted Sunday, May 12, 2013 3:05 AM
Grasshopper

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Thanks for the info Perry.

So in the event of a power outage at the primary site where the secondary server will lose communication with both the primay and witness server will it automatically failover?



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Posted Sunday, May 12, 2013 3:37 AM
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Russell101 (5/12/2013)
Thanks for the info Perry.

So in the event of a power outage at the primary site where the secondary server will lose communication with both the primay and witness server will it automatically failover?


No. "If the mirror server is disconnected from the witness and also from the principal server, automatic failover is not possible, regardless of the state of the principal server." http://msdn.microsoft.com/en-us/library/ms175191%28v=sql.105%29.aspx
You should make a good DR plan for different type of outage, and now eg. when perform a manual failover in case of failure.
Post #1451893
Posted Sunday, May 12, 2013 3:59 AM
Grasshopper

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The systems that I am looking to mirror currently are the management database for our Blackberry Enterprise Server and the configuration database for Active Directory Federation Services. In the event of power loss at the primary site I would like the databases to automatically failover to the DR site. As from what I can tell to manually failover the severs involved need to be able to connect to each other and in the event of a powerloss at the primary site I would then not be able to fail them over.

So given the above do need to place the witness at the DR site after all?




Post #1451895
Posted Sunday, May 12, 2013 4:21 AM
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Russell101 (5/12/2013)
...
So given the above do need to place the witness at the DR site after all?


But then if DR site goes down: "Loss of the mirror and the witness

Assume you have configured database mirroring with a witness. When the mirror is unavailable, the principal runs exposed. While the mirror is unavailable, if the witness is also lost, the principal becomes isolated and can’t service the clients. Even though the principal database is running, it is not available to the clients. If you attempt to connect to the database, you get the message “Database <dbname> is enabled for database mirroring, but neither the partner nor witness server instances are available: the database cannot be opened.”

http://technet.microsoft.com/en-us/library/cc917681.aspx#EIAA

So without a third site it depends, I think you should place the witness at the more reliable site.
Post #1451896
Posted Sunday, May 12, 2013 4:46 AM


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Russell101 (5/12/2013)
So given the above do need to place the witness at the DR site after all?

No, for the reasons that have been specified.


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Post #1451898
Posted Sunday, May 12, 2013 6:16 AM
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So if I have the primary and witness servers at our main location and we suffer a power outage will the secondary server automatically takeover?




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Posted Sunday, May 12, 2013 6:45 AM


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Russell101 (5/12/2013)
So if I have the primary and witness servers at our main location and we suffer a power outage will the secondary server automatically takeover?


No, more detail on failover scenarios is provided here


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