So I found an interesting bit of info in a post and wanted to know more information
"The best answer to those is don't disable merging as I mentioned.
Otherwise yes merges use IO, if you disable them then they won't happen and a force merge is required."
found here
Elasticsearch indexing performance: throttle merging - #6 by warkolm?
If we are disabling refresh during reindexing should we be doing a force merge when the reindexing is completed before we re-enable refresh rate?
seems we are having high merges on our cluster after this last re-index of one index which seems to be slowing the cluster down and leaving Kibana in accessible
index reindexed to 400 shards, after reindex we add 2 replicas and set refresh to 1s
in an attempt to resolve this we increased the refresh rate to higher than 30s
Cluster is V5 yes we know v8 is out but we cant upgrade yet due to changes related to parent child and our application searches return much slower in newer version 7
135 plus data nodes mix of i3 and r5 instance types
3 master r6 instance types
2 coordination nodes with Kibana