What is the difference between force merge and freeze index on ILM actions?

[m-amano (Mizuki Amano)]

Is there any difference between force merge and freeze index action in ILM?

My entire ILM is under below.
In warm phase, the index is merged to a single segment which makes the index read only.

{
  "policy": {
    "phases": {
      "hot": {
        "min_age": "0ms",
        "actions": {
          "set_priority": {
            "priority": 100
          }
        }
      },
      "warm": {
        "min_age": "1d",
        "actions": {
          "shrink": {
            "number_of_shards": 1
          },
          "set_priority": {
            "priority": 50
          },
          "forcemerge": {
            "max_num_segments": 1
          }
        }
      },
      "delete": {
        "min_age": "90d",
        "actions": {
          "delete": {
            "delete_searchable_snapshot": true
          }
        }
      }
    }
  }
}

Now I have two questions.
Would it bring any better performance or any difference if I add cold phase with freeze action after warm Phase?
Is this freeze action not used in Elastic v8 anymore?

Index lifecycle actions | Elasticsearch Guide [8.1] | Elastic

[dakrone (Lee Hinman)]

Freeze was primarily used to lower the overhead that an index took in the memory of the cluster. With changes in later versions of Elasticsearch, it is no longer necessary (we reduced index overhead), so it was deprecated and will eventually be removed.

I would recommend not using it in your ILM policy, as it is also deprecated there.

Hope that helps.

[m-amano (Mizuki Amano)]

@dakrone

Thanks, I get it!