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I have been studying Cluster voting and AlwaysOn failover. The topic of split-brain comes up and sounds terrible. This following from Microsoft seems to indicate that only a person with SA, doing something stupid can cause it.

When more than one subset of voting nodes is able to establish a quorum on its own, that is known as a split-brain scenario. In such a scenario, the nodes in the separate quorums may behave differently, and in conflict with one another.

The split-brain scenario is only possible when a system administrator manually performs a forced quorum operation, or in very rare circumstances, a forced failover; explicitly subdividing the quorum node set. Source

There are a lot of variables, and I wonder if something like changing the Dynamic quorum (is on by default) could create a scenario where a split-brain scenario exist.

Can auto failover be configured so a split-brain scenario is possible?

I want to know, so that if it is possible I won't configure that way.

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  • Are you really using FCIs in conjunction with AGs? Apr 6 '18 at 17:52
  • @AaronBertrand I don't think I understand the question. windows FCM is required for AGs. Apr 6 '18 at 18:11
  • WSFC = Windows Server Failover Clustering. This is a requirement for AGs. FCI = SQL Server Failover Cluster Instance. This is not a requirement for AGs. "Split brain" is something that typically affects FCIs. And I haven't seen any real world cases of this in years. I think with AGs people are just careful to use the listener to point clients at the group rather than directly at any specific node in the WSFC. Apr 6 '18 at 18:20
  • I am using WSFC, my question is about it. Apr 6 '18 at 18:23
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This following from Microsoft seems to indicate that only a person with SA, doing something stupid can cause it.

Yes, or doing completely unsupported things like using DSFR on a fileshare witness. If you're staying in supported land and aren't going around forcing stuff up, the worst case is the entire cluster will be offline until an admin can intervene.

There are a lot of variables, and I wonder if something like changing the Dynamic quorum (is on by default) could create a scenario where a split-brain scenario exist.

I wouldn't turn this off, it's actually your friend. This stops the need from having to change the node weight per node every time something happens in the cluster. BTW, you can't turn dynamic witness off.

There are extremely rare times that I would even consider turning dynamic quorum off and in those times it's because we've looked at the way we want the cluster to work and we specifically want it to fail if certain things happen. It's not something that you'd want to do without great testing and consideration.

Can auto failover be configured so a split-brain scenario is possible?

Absolutely not.

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