I'm trying to understand how search queries are devided between primary and replica shards. I don't clearly underatsnad why ones are said that one of benefit of replica is increasing search performance. But as I understand primary shard can serve search queries as replica does. If so let's assume that we have ES cluster with 2 nodes and queries are read-only. Will index with 10 shard work with such performanse as index with 5 shards and 1 replica?
First you need to understand what is primary shard and replica shard.
Primary shard is where you first write request goes and then replicates to its replica shards, based on the replication factor Elasticsearch will create n number of replica shard for a primary shard.
Now one document is always a part of a single primary shard but its copies are present in all the replica shards.
Now when you search you can search either in its primary shard(just one main copy) or any one of the replica shards. And these copies can be present on different nodes in the cluster(Elasticsearch is a distributed system). And to improve the performance Elasticsearch can query depends on the load and various other factors any shard which contains the copy, this explains why having replicas increase search performance and why you can search from the replicas.