Use of master-eligible nodes that are not acting as master?

From the documentation:
https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-node.html#master-node

Indexing and searching your data is CPU-, memory-, and I/O-intensive work which can put pressure on a node’s resources. To ensure that your master node is stable and not under pressure, it is a good idea in a bigger cluster to split the roles between dedicated master-eligible nodes and dedicated data nodes.

Our cluster currently consists of 13 nodes, where all nodes are data nodes and nodes 1-4 are master-eligible nodes. We're having performance issues so I'm thinking that maybe it's time to remove the data role from the master-eligible nodes (according to the quoted text above).

Question: If we have four master-eligible nodes that are not data nodes, what use will we have of the three master-eligible nodes that are not acting as the master? Will they add value to the cluster during this time or only be of value as fail-over if the current master goes down and a new master needs to be elected? Will they basically be idle all of the time when another node is acting as master?

Although the extra master nodes won't necessarily be using all their resources at all times, that doesn't mean they are not valuable. There is a great deal of value in having enough redundancy in your systems to cope with inevitable outages.

Four master-eligible nodes are no more useful than three, however, because both configurations can tolerate the loss of just one master-eligible node. I would recommend sticking with three. You could also try having just two dedicated master-eligible nodes together with a voting-only master-eligible node which could reasonably be one of your data nodes.

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