Exchange 2016 File Share Witness (clarification question)

Question

I've read several articles related to maintaining quorum and node/file based quorum. The question that seems to allude me is as follows.

Exchange 2016 Single DAG 4 Node with File Share Witness:

2 nodes in SITE 1 --- 2 nodes in SITE 2 --- 1 file share witness in SITE 3

Question is. If SITE 1 and SITE 3 go offline, does Exchange in SITE 3 keep running and swap automatically to the NODE majority? Or, does Exchange completely go offline because SITE 2 cannot access the other 2 nodes in SITE 1 OR the File Share Witness in SITE 3?

Hope that makes sense. Thank you for any clarification that can be provided.

CWT

Answer 1

The answer is, it depends.. :)

If you suddenly lose both Site 1 and Site 3 ( Wan Failure for example),
then Site 2 would go down because it does not have a majority and quorum is lost causing the databases to dismount.

If however, a node was lost in Site 1, then an hour later another node in Site 1 was lost , then later the FSW witness went down, Site 2 has a chance to stay up assuming dynamic quorum was working as expected and was able to gracefully shrink quorum
Note this enabled by default, so dont change it :)

https://practical365.com/exchange-server/windows-server-2012-dynamic-quorum/

Answer 2

That is what I thought you might say.

Followup 1
If the worse case happens and SITE 1 and SITE 3 are POOF gone at the same time for whatever reason and the SITE 2 Exchange databases dismount. Whats the resolution? Can the offline databases in SITE 2 be brought back online quickly/easily?

Followup 2
To avoid the scenario depicted above where databases dismount in SITE 2, would one potential fix for that be to have an alternate witness server configured in a 4th site provided SITE 1 & SITE 2 had a direct link to that 4th sites FSW server? Have never used an alternate, but for HA it seems like it might not be a bad idea to avoid the circumstances we are discussing. Is my understanding accurate?

Thank you so much for your assistance. Very much appreciated.

CWT

Answer 3

Hi @CWT :

So lets go through each one:

1) In this scenario you would need to seize quorum and mount the databases in Site 2. If you have DAC Mode enabled, you can use the built-in PS commands to do that and essentially its a datacenter switchover. If DAC mode is not enabled, the cluster Commands are needed:
https://learn.microsoft.com/en-us/exchange/high-availability/manage-ha/datacenter-switchovers?view=exchserver-2019

Easily is a loaded word :) Enable DAC mode if you haven't already and you can use the Exchange PS commands to do this in the event it ever happens.
Here is the thing, you have done the right thing by having 2 peer datacenters and a FSW in a third site, but its important you know what to do in the hopefully rare time you lose the connection to two sites at the same time. If that did happen, it would probably be a WAN loss more than anything - and even if you did lose both sites, you would have to then determine how long it may be down and whether its worth seizing quorum in the standing data center.

2) The alternate witness is a misnomer actually. Its not needed and its really nothing more than a configuration setting. You are essentially telling the DAG that if I need to do a datacenter switchover ( see 1 above), then use this as the FSW. Its not a backup nor can it automatically kick in when needed in a 4th site. You actually would never use the alternate witness except in a datacenter switchover. So, to answer your question, it can't be used to provide any sort of high availability.

Answer 4

AD-7937,

I can't thank you enough for taking some time to go through these scenarios with me as it definitely cleared up my confusion based on what I have read. We definitely have our DAC enabled (DatacenterActivationMode :DagOnly), so we are covered there. To your point, it really comes down to knowing how to seize control in another data center (switchover) using the commands in the link should such a scenario ever occur.

Only issue I have now is not having the ability to accept your second response as an actual answer although it is :o). Thanks again sir and have a great weekend.

CWT

Answer 5

Last question.

If you have a SITE 1 and SITE 2 DAG and each site has 2 nodes, I understand that quorum cannot be established if either site goes offline and they lose communication with each other, but provided there is no outage between the two sites, rebooting/rebuilding a File Share Witness server doesn't impact anything. Exchange mailboxes/databases would only be impacted if either site goes offline or WAN link breaks there communication.

Do you agree with that statement?

CWT