Unable to set index.merge.scheduler.max_thread_count

Elastic Stack Elasticsearch
1

According to official document said, if we unfortunately still use spin disk, we should set this value to 1.
I used to be set this in elasticsearch.yml but not luck in v5.4

I know this setting has been move to index level, what I confused is, if this is per index setting? What happened if I create two indices and set this value to different in each indices?

Can anyone help to guide?
Thanks.

2

Then they will have different merge settings.

3

So merge scheduler is per shard? since each shard is actually a Lucene writer?
If a node with 2 indices, index A set max_thread_count=1, index B set max_thread_count=2. We will have 3 threads running when doing merge right?
I thought this is node setting.