Index.blocks.read_only vs Frozen indices

[Karthik_Ramachandran]

Trying to clarify between read-only and frozen indices.

We have a indice setting index.blocks.read_only, and also have Freezing option. Freezing helps with heap, but trying to understand not freezing and setting only the aforementioned setting to true, and performing forcemerge.

Could someone help understand the differences w.r.t impact against cluster performance. What if we set the indice setting to read_only, force_merge (NO Freeze) vs force_merge+Freeze.

References
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/indices-forcemerge.html
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/best_practices.html

Creating frozen indices with the Elasticsearch Freeze index API

[Christian_Dahlqvist]

Forcemerging down to a single segment can, as discussed in this webinar, dramatically reduce the amount of heap used. Freezing the index basically reduces the heap used even further to close to 0. Querying frozen indices require them to be opened, queried and then closed again which is considerably slower. Exactly how much depends a lot on the data and query patterns so I would recommend you test.