I've a ES cluster of 4 nodes with 3 indices each having 10 shards and 2
replicas. After indexing some data into it and running some performance
tests, I realised 4 nodes may not be enough and added an additional node.
In principle, every index should've (uniformly) re-distributed its shards.
But, to my surprise, 2 indices have redistributed only 2 shards to this new
node, while it should be 6 shards. And the third index has not transferred
even a single shard to the new node. I dont think this is an expected
behaviour from ES.