How and why does elasticsearch enter a bad state on a long query

Elastic Stack Elasticsearch
1

So after running a really expensive query, elasticsearch got derped and a lot of the shards became unassigned.

They remained unassigned and I had to force assign these shards.

  1. Does this result in data loss
  2. if so, why does it result in data loss? Shouldn't elasticsearch have everything on disk for each shard. Why can't elasticsearch recover this?
2

What version are you on?

3

version 2.4

4

Force assigning primaries can cause data loss. Did you look into why the unassigned shards happened though - check your logs.

See https://www.elastic.co/guide/en/elasticsearch/reference/2.4/cluster-reroute.html for more