Many a times I was asked If the cluster Quorum is configured properly. Their has been questions around Quorum configuration and number of  nodes in DAG ( Database Availability Group) , number of nodes in Cluster and the cluster quorum configuration type.  This Blog post discuss more on Cluster configuration.

Exchange 2010 supports one these quorum models, depending on number of nodes. DAG with an even number of Nodes use the failover cluster's Node and File Share Majority quorum mode.The quorum configuration in a failover cluster determines the number of failures that the cluster can sustain. If an additional failure occurs, the cluster must stop running.

IMPORTANT: In most situations, use the quorum configuration that the cluster identifies as appropriate for your cluster. Change the quorum configuration only if you have determined that the change is appropriate for your cluster.  You can easily let cluster determine the type of quorum to be used by running exchange cmdlet Set-Databaseavailabilitygroup …… with appropriate parameters.

Following example shows how cluster quorum is automatically determined based on number of nodes.On Adding 5 nodes to the creates the FSW on adding  2 and 4 node , switch to Node majority on adding  1,3,5 node to the cluster.

A- Create a DAG Object and Add first Node to Cluster

Add-databaseavailabilitygroupserver “Contoso-DAG” –Mailboxserver  Server1                                                                                                   

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2 – Adding  Second Mailbox Server to the DAG and Node 2 to cluster

Add-databaseavailabilitygroupserver “Contoso-DAG” –mailboxserver Server2

Adding second node to DAG from Exchange management shell or GUI configures the FSW Automatically , because now we have even nodes 

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3 – Adding Mailbox Server 3 to the DAG and node 3 to cluster

Add-databaseavailabilitygroupserver “Contoso-DAG” –mailboxserver Server3

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D – Adding fourth Mailbox Server to the DAG [ * Even number of nodes ] / Node 4 to the cluster configures FSW resource automatically

Add-databaseavailabilitygroupserver “Contoso-DAG” –mailboxserver Server4

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FSW resource is added back to the cluster automatically

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E – Adding Fifth node to the DAG . , quorum will be set to Node Majority , based on number of nodes in cluster .[ *Odd number ]

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This is how cluster and DAG looks at the end of adding 5 nodes .

   * sometimes you may change the Quorum configuration manual. to fix your quorum model , run following cmdlet . Exchange  internally calls use the cluster API, which validate quorum model based on number of nodes in Cluster.

Set-databaseavailabilitygroup yourdagname –witnessserver “servername”  -witnessdirectory “c:\witness”

  This functionality is documented here, : http://technet.microsoft.com/en-us/library/cc770620(WS.10).aspx

- Manju