Exchange – Database Availability Group – FSW on DC

I needed to use DC as File Share Witness for my LAB and in one small environment. Microsoft doesn´t recommend that, but if you have only limited number of servers, you dont have a choice.


Really basics, full article can be found on MS Technet)

  • FSW must be configured every time you create Database Availability Group for Exchange 2010 and Exchange 2013 (if you do not specify, Exchange will configure FSW on first CAS server without mailbox role installed). Besides the other parameters you should specify the WitnessServer and WitnessDirectory parameters
New-DatabaseAvailabilityGroup -Name DAG1 -WitnessServer CAS1 -WitnessDirectory D:\DAG1_FSW
    • FSW is used to maintain quorum (node majority for DAG application) when even number of nodes in the DAG
    • FSW is only actively used, when there is even number of servers in the DAG. One case is that you have configured even number of servers by design or you have conffigured odd number of servers by design and one of those is broken. Otherwise Witness directory on Witness server is empty
    • Alternate FSW must be also configured, if you enable Datacentre Activation Coordination
Set-DatabaseAvailabilityGroup <identity> -DatacenterActivationMode DAGOnly -AlternateWitnessServer <FQDN or NetBIOS name of the server> -AlternateWitnessDirectory <Path>
  • More then one DAG can have FSW on the same server, but directory must be unique


To configure FSW on DC there are more steps to perform before configuration of FSW:

  • Add domain controller to Exchange Trusted Subsystem security group
  • Add Exchange Trusted Subsystem to Buildin\Administrators
  • Create Directory on the DC and share the directory with the share name of the DAG
  • Set sharing permissions so that virtual account for DAG will have Full Control


If set some of the point incorrectly, you will get the result, that DAG cannot access FSW and availability of DAG is limited

Get-DatabaseAvailabilityGroup -Status

Result is shown in Picture:


If this happens to you, fix incorrectly set steps and re-enable FSW:

Set-DatabaseAvailabilityGroup <identity> -WitnessServer <FQDN or NetBios name of the server>