Failure to start warm migration

Question

I am attempting to use a policy to transition indices to UltraWarm storage. However, each time the migration to UltraWarm begins, Kibana displays the error, "Failed to start warm migration" for the managed index. The complete error message is below. How can I find more details on the root cause of this issue / a fix for it. Thank you!  
  
{  
    "cause": "$$753f6f14e4f92c962243aec39d5a7c31$$$$10.212.32.199:9300$$$$indices:admin/ultrawarm/migration/warm$$",  
    "message": "Failed to start warm migration"  
}  
  
Edited by: JaredvH on Jul 21, 2020 3:05 PM

Answer

Have you tried migrating directly via the Ultrawarm API call? Does that also fail? Hitting the API directly might give you a more detailed error message   
Documentation of APIs here - https://docs.aws.amazon.com/elasticsearch-service/latest/developerguide/ultrawarm.html#ultrawarm-migrating

Answer

Great idea. Using the API directly showed me that the issue was that the index was in a "yellow" status, which was caused by having a combined number of shards + replicas greater than the total number of data nodes.

Failure to start warm migration

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