Why does Elasticsearch choose to relocate shards?

I've read https://www.elastic.co/blog/every-shard-deserves-a-home but can't see how any of the cases there apply.

ES seems to be spending all its time moving shards around, sometimes apparently taking hours to move multi-gigabyte shards, occupying both the two-things-at-once that it seems to like doing, thus other, smaller, shards remain "yellow" for hours waiting for a chance to run.

How do I find out what's going on and why?

What version are you on?


It seems very odd that it would be continually moving things.

How much disk space do you have on your nodes?

  1. 165G, 81% used
  2. 165G, 71% used
  3. 165G, 53% used

It might be moving them from the first node due to watermarks - https://www.elastic.co/guide/en/elasticsearch/reference/6.4/disk-allocator.html

What do the logs show?

I assumed it wouldn't be that because the disk usage is below 85%. But ... the largest shards it moves around are 7.9G, which is more than 4% of 165G, and 81% + 4.7% > 85% so perhaps that is indeed what's going on, although I've never seen actual usage above 85%.

Thank you for pointing me at the logs - I deleted all the historical logs from the node that was at 81% and it's now at 71% ... the logs themselves are full of errors which I supposed I'd better clean up before trying to find anything in there about relocating shards.

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