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From the documentation Understanding cluster and pool quorum - Dynamic witness:

Dynamic witness significantly reduces the risk that the cluster will go down because of witness failure.

Assuming 3 node + 1 file share witness cluster, since there is an even number of votes, dynamic witness will 0 out the file share vote.

How does the dynamic witness concept significantly reduce the risk that the cluster will go down because of witness failure?

Had the file share witness not 0 out, then what was the risk? If it did go down, there would still be 3 of 4 - quorum majority and cluster will survive.

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  • Commentary removed. Anyone thinking the answer is to be found in the documentation is invited to post an answer so we can all vote on it.
    – Paul White
    Feb 8, 2023 at 9:55

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It's been a while but, from what I remember, the cluster scenario you're suggesting would likely not work in a setup without dynamic witness since the number of votes would ultimately be even (3 members + 1 witness), and the vote majority quorum mechanism requires an odd number of votes. What dynamic witness does is allow you to have any number of cluster members, and automatically assign or remove the witness vote.

How it protects against witness failures: without it, you would have to resort to using a quorum disk, like it was in the old days - which would be a single point of failure for the entire cluster, or manually add or remove the witness itself from the cluster configuration to ensure the number of voters is kept at an odd value - this latest one is somewhat relieved by the dynamic quorum option, as (as long as the nodes disappear one by one) it can reduce the total number of votes.

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