Elasticsearch Threshold

Elasticsearch Threshold

Last Update: March 2020

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Threshold in Elasticsearch

What it is

Elasticsearch uses several parameters to enable it to manage hard disk storage across the cluster

What it’s used for
  • Elasticsearch will actively try to relocate shards away from nodes which exceed the disk watermark high threshold.
  • Elasticsearch will NOT locate new shards or relocate shards on to nodes which exceed the disk watermark low threshold.
  • Elasticsearch will prevent all writes to an index which has any shard on a node that exceeds the disk.watermark.flood_stage threshold.
  • The info update interval is the time it will take Elasticsearch to re-check the disk usage
Examples
PUT _cluster/settings
{
  "transient": {
   
    "cluster.routing.allocation.disk.watermark.low": "85%",
    "cluster.routing.allocation.disk.watermark.high": "90%",
    "cluster.routing.allocation.disk.watermark.flood_stage": "95%",
    "cluster.info.update.interval": "1m"
  }
}
Notes and good things to know:
  • ou can use absolute values eg.”100gb” or percentages eg. “90%”, but you cannot mix the two on the same cluster. 
  • In general, it is recommended to use percentages, since this will work in cases where disks are resized.
  • You can put the cluster settings on the elasticsearch.yml on each node,  but it is recommended to use the PUT _cluster/settings API because it is easier to manage, and ensures that the settings are coherent across the cluster.
  • Elasticsearch comes with sensible defaults for these settings, so think twice before modifying them.  If you find you are spending a lot of time fine-tuning these settings, then it is probably time to invest in new disk space.
  • In the event of the flood_stage.the threshold being exceeded, once you delete data, Elasticsearch should detect automatically that the block can be released (bearing in mind the update interval which could be, for instance, a minute).  However if you want to accelerate this process, you can unblock an index manually, with the following call 
PUT /my_index/_settings
{
  "index.blocks.read_only_allow_delete": null
}
Common problems

Inappropriate cluster settings (if the disk watermark.low is too low) can make it impossible for Elasticsearch to allocate shards on the cluster.  In particular, bear in mind that these parameters work in combination with other cluster settings (for example shard allocation awareness) which cause further restraints on how elasticsearch can allocate shards.


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