exchange dag and witness practice

Question

dears,

i have exchange dag deployment over 2 datacenters. there is no third datacenter.

i need your advise with the best practice for preventing the cluster from coming down.

site1: 2 servers+ witness
site2: 2 servers

what will happen if site 1 goes down?
will the cluster be down? will everything be not operational?
let's start the discussion with the above before adding the additional points

thank you in advance

Answer 1

This is a typical example where best practices would suggest placing the file share witness in a third location, such as in Azure. You could go by using only one DC, which I have done several times for testing purposes. But it is not recommended as this will result in an unnecessary elevation of privileges.

https://learn.microsoft.com/en-us/exchange/high-availability/manage-ha/azure-vms-as-dag-witness-servers?view=exchserver-2019
https://practical365.com/exchange-server/domain-controller-azure-file-share-witness-database-availability-group/

Answer 2

Alternate file share witness doesn't make any sense when your DAG goes down, it will not contribute a vote to prevent the DAG from going down.
You don't need to configure which server to be alternate file share witness server at first as it should be set when you restore-DatabaseAvailabilityGroup on the existing datacenter. It doesn't have any "benefits", it's what you should configure when the number of servers in existing datacenter is even.
I assume the benefits of DAC mode is more worthy of discussion:https://learn.microsoft.com/en-us/exchange/high-availability/database-availability-groups/dac-mode?view=exchserver-2019

Answer 3

Hi,

Based on my knowledge, in this scenario, if Site 1 is down, then DAG would go down because it does not have a majority and quorum is lost causing the databases to dismount.

Enable the DAC mode if its not already enabled and configure the alternate file share witness in DAG properties which would help in using the Exchange powershell commands to perform the datacenter switchover.

This would be actually seizing the quorum to the secondary site. Alternate file share witness automatically will not provide its vote for the cluster rather its just a configuration which would be needed to perform the datacenter switchover.

https://learn.microsoft.com/en-us/exchange/high-availability/manage-ha/datacenter-switchovers?view=exchserver-2019

Answer 4

can you elaborate more please?
if i configure an alternative file share witness in site2, this woulldnt be considered as an additional vote and prevent the dag from being down?
if no what would be the behavior when dag go down? what would be the benefit of the alternative file share?

thank you