obtaining the value of 'index.merge.scheduler.max_merge_count' from index-name/_settings?include_defaults=true error

[kkewwei]

ES_VERSION: 7.3.1
OS version: Linux
Description of the problem including expected versus actual behavior:
The paramater value of index.merge.scheduler.max_merge_count obtaining from two different api are different:

1. The first way:
   curl -XGET 'http://localhost:9200/test1/_settings?pretty&include_defaults=true'
   the result:

{
  "test1": {
    "settings": {
      "index": {
        "provided_name": "test1",
        "merge": {
          "scheduler": {
            "max_thread_count": "1"
          }
        }
      }
    },
    "defaults": {
      "index": {
        "merge": {
          "scheduler": {
            "auto_throttle": "true",
            "max_merge_count": "9"
          }
        }
      }
    }
  }
}

2. The second way:

PUT test1/_settings
{
"index":{"merge.scheduler.max_merge_count":"7"}
}

The log is like this:
[2020-01-19T17:30:28,079][INFO ][o.e.c.s.IndexScopedSettings] [node-1] updating [index.merge.scheduler.max_merge_count] from [6] to [7]

The value of index.merge.scheduler.max_merge_count from the first way is 9(default value), the value from the second value is 6.

Steps to reproduce:

1. put the template:

PUT _template/test1
{
  "order": 0,
  "version": 2,
  "index_patterns": [
    "test1"
  ],
  "settings": {
    "index": {
      "merge": {
        "scheduler": {
          "max_thread_count": "1"
        }
      }
    }
  }
}

2. creating index test1
3. obtaining by the first way and second way.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Settings)

[gaobinlong]

The defaut value of index.merge.scheduler.max_merge_count is the value of index.merge.scheduler.max_thread_count plus 5.

elasticsearch/server/src/main/java/org/elasticsearch/index/MergeSchedulerConfig.java

Line 60 in ee43115

public static final Setting<Integer> MAX_MERGE_COUNT_SETTING =

When max_thread_count is changed, the value of max_merge_count is also changed, we can find the change from the log when we update max_thread_count to 1 by update settings API:

[index] updating [index.merge.scheduler.max_thread_count] from [4] to [1]
[index] updating [index.merge.scheduler.max_merge_count] from [9] to [6]

I think the updated value of max_merge_count is not contained in the settings of IndexMetaData, so we cannot see the real value by GET index settings API.
Should we add index.merge.scheduler.max_merge_count in the settings of IndexMetaData if it has been changed associated with index.merge.scheduler.max_thread_count ?

[kkewwei]

yes, It may be better to put index.merge.scheduler.max_merge_count to IndexMetaData after updating index.merge.scheduler.max_thread_count.

[gwbrown]

I believe this is a duplicate of #47890, as this setting depends on the value of another setting. I'm going to close this issue as a duplicate and add this setting to that issue with a link to this issue.