Exchange 2010 File share witness

Posted on 2009-12-20
Last Modified: 2012-05-08
I've read in numerous articles that a File share witness vote is used for achieving majority node set whenever there is an even number of members in the Exchange 2010 DAG.

So let's say I have 2 Sites - Site A and Site B and a 6 node DAG.
Site A - has 4 mailbox servers
Site B - has 2 mailbox servers.
Suppose Site B fails.
By MNS, (6/2 + 1) = 4 which is the majority (more than half). So quorum is not lost and
the cluster continues to function on Site A. In this case we would not need the FSW?
So then it may not be correct to say that the file share witness is needed when there is an even number of members in the DAG ?

Question by:anarine
    LVL 14

    Accepted Solution

    It that situation, the site B nodes will NEVER stay active..

    FSW is in Site A, and communication between sites is lost.
    Site A can see 4 of 6 nodes + FSW (5/7), keeps mailstores mounted
    Site B can only see 2 of 6 nodes (2/7), dsmounts any stores

    FSW in Site B, and communication between sites is lost.
    Site A can see 4 of 6 nodes, no FSW (4/7), keeps mailstores mounted
    Site B can see 2 of 6 nodes + FSW (3/7), dsmounts any stores

    You basically need to split the nodes evenly if you want to have it automatically handle communication failures. The File Share Witness is only there as a tie breaker so you don't end up with a "split-breain" setup where both have 50% and both are active.

    Author Comment

    Ok so in a 6 node DAG stretched across 2 sites what would be the recommended configuration to allow
    both sites to continue to function when the communication link is down between those sites ?
    ie. how many nodes and fsw in each site ?
    LVL 1

    Expert Comment

    I'm working on a similar situation, 2 nodes, 2 datacenters. I'm considering placing my DAG Witness at a 3rd datacenter connected via VPN. The liklihood of BOTH servers having network failure to this 3rd site (lets call it the DAG Site) is nearly impossible. So, this should satisfy the requirement that both nodes are able to communicate with the Witness at all times (unless node is down).



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