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Questions on DAG quorum and setup across Sites

Posted on 2013-02-01
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Last Modified: 2013-02-03
I have 3 questions:

1) When you are figuring out quorum for a cluster and you are using the N/2 +1 formula, is N just the number of servers in the DAG or do you include the FSW also. So if I have a 4 node DAG, is my quorum 4/2 +1 or 3 ?

2) Is it possible to stretch a DAG across two Sites with users in each Site and still have both Sites operate if the link between the Sites goes down? I would think this is something you would want to avoid because of split brain syndrome and the possiblity of the same database mounting in both Sites. Or,  is it better in this situation to use 2 DAGS.

3) Assuming you go with 2 DAGS, beyond more administrative work, is there any other concerns to be aware of.

Links to articles are welcome but I would really like an answer to each question from someone with experience in these situations. Sometimes articles are not clear and make you even more confused.
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Question by:osiexchange
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Akhater earned 500 total points
ID: 38846252
1)   n is the number of DAG nodes within the DAG. With DAGs, if you have an odd number of DAG nodes in the same DAG (Cluster), you have an odd number of votes so you don’t have a witness.  If you have an even number of DAGs nodes, you will have a file share witness in case half of your nodes go down, you have a witness who will act as that extra +1 number.
read this http://www.shudnow.net/2010/06/30/exchange-2010-database-activation-coordination-dac/

Keep it simple in your head, think of it as a tennis match it is always played 2 out of 3 , 3 out of 5 etc.. in a way there is always a winner. same goes here you should always end up with an odd number of nodes so that, when you vote, you always have a winner.

So if you have 4 nodes that's even it can't work, you add the share witness and that's 5 now to have a winner you need 3 out of 5

2) no it is not possible in 2010, if the link goes down the site with least nodes will lose quorum and will go down. if your link is not reliable then 2 dags is the best way to go

3) no concerns at all
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by:osiexchange
ID: 38847116
Thanks for that explanation. Is there any good reason to NOT have a FSW? I would think you should always have one just in case you lose more than one node. I suppose if you have 10 servers in your DAG, you probably would be OK.
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Expert Comment

by:Akhater
ID: 38847126
it is not an option if you have or not, the system will automatically do it

if you have an even number of mailbox servers then you are using a share witness, if you have an odd number then the sharewitness is not used
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Author Comment

by:osiexchange
ID: 38847672
But don't you need to create a FSW first before it can be used. I am not talking about how it will be used. I am talking about whether to create it at all. I believe this is a manual process when you first set up the DAG.
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Expert Comment

by:Akhater
ID: 38848077
When you setup the dag it will ask you for the share witness in all cases. Then exchange will decide to use it or not depending on the number of nodes
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