Can I put my File Share Witness on a DFS share?

I get asked this question all the time. People are concerned about losing their file share witness, so like many of their other shares, they want to leverage DFS for some additional availability. This is a very bad idea and is not supported.

Microsoft recently publish a great blog article that describes exactly why this is not supported.

https://blogs.msdn.microsoft.com/clustering/2018/04/13/failover-cluster-file-share-witness-and-dfs/

Much of this article would also apply to people who ask if they can use a DataKeeper replicated volume resource as a Disk Share. It makes sense, you can use a DataKeeper volume resource in place of a Physical Disk resource for any other workload, so why not a Disk Witness?

This issue is the same as the DFS issue, in the event of a loss of communication between the two servers there is nothing to guarantee that the volume wouldn’t come online on both servers, causing a potential split-brain condition. The Physical Disk resource overcomes this issue by using SCSI reservations, ensuring the disk is only accessible by one cluster node at a time.

The good news is that Microsoft already blocks you from trying to us a replicated DataKeeper Volume resource and coming in Windows Server 2019 it looks like they will also block you from using a DFS share as a File Share Witness.

Taken from the Failover Clustering and Network Load Balancing Team Blog Post “Failover Cluster File Share Witness and DFS

 

Can I put my File Share Witness on a DFS share?

2 thoughts on “Can I put my File Share Witness on a DFS share?

  1. Steve H says:

    Hi Dave,

    The article says “Microsoft does not support running a File Share Witness on certain DFS shares”, and “As long as there is only one link on the DFS share (meaning DFS-N only used as a namespace), you should be good.”

    I take it from that, that if a normal single link share and no replication IS supported. What’s your take on that?

    Seems the Win 2019 logic just checks to see if the DFS share has multiple links. If so it will fail. Otherwise it will pass?

    Steve

    1. daveberm says:

      Good question. Based upon what you quoted from the article I would agree with your assumption. I’m not sure of the behavior of 2019, I would have to explore that a bit. Are you telling me that it checks whether or not a share is part of DFS-N during the creation of the FSW and will fail if it has multiple links, but pass if only a single link? Again, great question, I’d have to do a little research, but do let us know what you experience. Next time I talk to someone on the cluster team at Microsoft I’ll see if I can get some clarification.

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