Sql-server – Dynamic quorum and Dynamic witness in Always On AG 2016

If you want to be able to sustain a failover of two nodes within the failover cluster, then you'll need to ensure that you have five voters for quorum.

You currently have three nodes, and even if you were to add a disk witness, you'd still be at the same point: whether you have three or four possible quorum votes, you'd still need to have three votes for quorum (you need to have more than half of the votes for quorum).

I've never heard the terminology of a "witness server" before, and I think that person is just trying to give a logical name to simply adding another node to your cluster.

In other words, if you now have a four node cluster with a disk witness, that'll be a total of five voters. Five voters will allow you to sustain two nodes to fail (provided your disk witness continues to be functioning properly).

With that being said, you also need to answer a few questions for yourself:

• Does you really need three separate instances in this cluster? What is the driving force behind that?

• Can one of your possible owning nodes for the FCI be able to sustain all three instances of SQL Server? (think about it, there could be a lot of serious contention there)

• Do all of these instances need to be in the same cluster?

And I do want to echo what @AaronBertrand has said above in his comment on your question. There is no such thing as an active / active [ / n [ / n ... ] ] cluster for SQL Server. It's extremely misleading to think of it like that, and that terminology has traditionally caused a lot of confusion (not to mention, the basic problem of a wrong title for something).

I agree with @Spörri but to add to his answer : You have 3 nodes, and then you shut one down leaving you with 2. With 2 nodes(votes), there is no sure way the cluster can maintain a failure.If both nodes vote for themselves it will mess things up. So to avoid this, it sets the one node to 0 votes. That way, only one node can vote. This does however mean that if the active node fails, you will have the entire cluster fail because the inactive node is not participating in voting at this point in time.

The easy resolution for this, is to add a file share or disk witness. This will resolve a situation like this and is recommended for a fail over configuration. With Dynamic quorum, having all 3 nodes active, the witness will be set to 0, as you already have 3 votes, a odd number. However when a node is shutdown/fails etc, the witness will gain a vote, allowing you to still have a odd number voting, meaning that you can loose 2 full nodes before your cluster is in trouble, whereas with your current configuration, once one node goes down your cluster is at risk of failing.

See this table for more info : Understanding cluster and pool quorum