Can anyone please explain me the significance of setting discovery.zen.minimum_master_nodes and how it avoids split brain.Also the formula mentioned to set this value is not clear .what would be the number of nodes in the formula .formula is (number of nodes/2 +1) so this number of nodes are total number of nodes in cluster or number of eligible master nodes in cluster?

The documentation says:

This setting must be set to a quorum of your master eligible nodes.

The way it works is that in the case of a network partition (your cluster split in two parts) the side with less nodes does not elect a master and instead becomes read-only.
If both sides would elect a master node, you would end up with two separate clusters and Elasticsearch doesn't know how to merge them together after the network split ends, because data might be conflicting between the two clusters.

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