Your FSW is more important than you may think!
Over the past couple of weeks I’ve seen two Database Availability Groups that had their File Share Witness go missing on them. In both cases the server which housed the FSW were replaced and the FSW role was not recreated.
I’m guessing you’re asking yourself “how that can ever happened?” The truth is it’s a lot easier than you think. With a 2 node cluster and File Share Witness you only need 2 nodes up to have a Quorum. This means if both of your DAG servers are up good, or if one DAG server a and the FSW is up you’re still good. The problem is that a well configured Exchange server is kind of “Set it and Forget it” and losing a FSW can have no impact on you at all when the Servers are both up. However when one of your servers go offline for whatever reason, the remaining server relies on the FSW to hold a quorum. IF you don’t have enough nodes to maintain a Quorum then all your Database will DISMOUNT!
This action is by design, there is a crazy thing that could happen to you called Split Brain where you would write different data to your 2 copies of the database that leaving it out of sync. Since you don’t that to happen you need the FSW. The FSW is a great thing that should not be forgotten!
One way to tell if your FSW is online is to use the Fail Over Clusters powershell tools. You can do that from your Exchange Server. I always do it from the Standard PowerShell console (I don’t know why) but the module will load into your EMS as well.
Depending on your status the output should look like one of these:
Or you could create a task to in your event log to email you if this error shows up in your Event Logs
Here’s a link on how to do that: https://technet.microsoft.com/en-us/library/cc732728.aspx