Is there any performance gain from making an elasticsearch index read-only

Question

Though a trivial easy task, am curious as to whether or not this has a positive performance benefit.

curl -XPUT 'localhost:9200/my_index/_settings' -d '
{
    "index" : {
        "blocks" {
            "read_only" : "true"
    } } }
'

Answer 1

I assume you mean 'search performance' (search time) since you try to make your index read-only.

Without any changes to index (add/delete), the search performance (search time) should be the same whether it's set to read-only or not.

In both cases, a shard may be optimized to have only one segment. There is no overhead to maintain/search increasing number of segments inside a shard. Also, there is no need to merge segments and refresh/flush as well.

Please refer to ElasticSearch official document: Dynamically updatable indices to understand how an index update is done by ElasticSearch.

Also, according to this discussion in Github, Aaron Mildenstein mentioned:

It has no improvement on performance whatever.

Technically speaking, once an index has been optimized to 1 segment per shard, and had bloom filters disabled, there's not really anything else at the software level that will improve performance in any way.