Clone index does not keep "number_of_replicas" setting, nor it respects index templates

[pmishev]

Elasticsearch version (bin/elasticsearch --version): 7.12.1

Description of the problem including expected versus actual behavior:

There could as well be 2 unrelated problems:

1. When cloning an index, number_of_replicas of the target index is not kept the same as the source index.

2. If there is an index template that sets the number_of_replicas to 0, this is being ignored as well, although the target index name matches the template pattern

Steps to reproduce:

PUT _index_template/template_1
{
  "index_patterns": ["mytest*"],
  "template": {
    "settings": {
      "number_of_replicas" : 0
    }
  }
}
PUT mytest1
GET mytest1

Observe that number_of_replicas = 0, as expected.

PUT /mytest1/_settings
{
  "settings": {
    "index.blocks.write": true
  }
}
POST /mytest1/_clone/mytest2
GET mytest2

Observe that number_of_replicas = 1, so the value was neither copied over from the source index, nor the index template was respected.

[DaveCTurner]

This behaviour is by design, and the note in the docs that says "Cloning doesn’t copy index metadata..." is intended to describe it, but I agree it could be clearer. Especially we should note that index templates have no impact on cloning. Marking this as a docs bug.

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[pmishev]

In the PR you did you added Notes the clone API doesn't copy index settings., but I don't think that's the case. All other index settings I had in my index were copied over (e.g analysis, max_ngram_diff, blocks, etc.).

At the moment I'm handling copying the number_of_replicas manually app-side, but I can't be sure if there aren't any other settings that are not being copied over as well. Perhaps you should make it more explicit exactly what is copied and what is not.

[DaveCTurner]

Sorry, it seems I've misread the code, I have just discovered a whole load of complications and it seems to vary by version. Let me investigate further what it's supposed to be doing.