I encountered an issue this week where my cluster went yellow and I had a large number of unassigned replicas. Relocate worked only for a very small number of the shards. Allocation/explain returned the message below. I tried setting the index from 1 replica to 0 replica then back to 1 to try to re-initialize the replica but it did not resolve the issue. It seemed like the replica shards were orphaned in some manner. Any feedback for what might have caused this and how to resolve would be great. Running v 5.1.1 on AWS.
"details" : "existing allocation of replica to [{nodeNameRemoved}{ODEM6_OxSmmlCaVfir-Tmw}{w1b-HhGPQgi47Zfri7h7LA}{removed:8193}{availability_zone=us-east-1b, instance_type=removed, external_ip=removed, tags=Data}] cancelled, sync id match found on node [{nodeNameRemoved}...