Elasticsearch OpenSearch Disk Watermark

By Opster Team

Updated: Mar 30, 2023

| 1 min read

In addition to reading this guide, we recommend you run the Elasticsearch Health Check-Up. It will detect issues and improve your Elasticsearch performance by analyzing your shard sizes, threadpools, memory, snapshots, disk watermarks and more.

The Elasticsearch Check-Up is free and requires no installation.

Aside from reading this guide, we recommend you try AutoOps for OpenSearch. It diagnoses problems by analyzing hundreds of metrics collected by a lightweight agent and offers guidance for resolving them.

If you’d like to learn how you can reduce the cost of your cluster by adjusting your disk watermarks, run Opster’s Cost Insight tool.


There are various “watermark” thresholds on your OpenSearch cluster. As the disk fills up on a node, the first threshold to be crossed will be the “low disk watermark”. The second threshold will then be the “high disk watermark threshold”. Finally, the “disk flood stage” will be reached. Once this threshold is passed, the cluster will then block writing to ALL indices that have one shard (primary or replica) on the node which has passed the watermark. Reads (searches) will still be possible.

Relevant settings

cluster.routing.allocation.disk.watermark have three thresholds of watermarks, it accepts absolute values as well as percentage values. The three watermarks are:

  1. Low disk watermark
  2. High disk watermark
  3. Flood stage disk watermark

Permanent fixes

1. Delete unused indices. 

2. Merge segments to reduce the size of the shard on the affected node

3. Attach external disk or increase the disk used by the data node

Temporary hacks/fixes

1. Changed these settings values to a higher threshold by dynamically update settings using below update cluster API.

PUT _cluster/settings :


  “transient”: {

    “cluster.routing.allocation.disk.watermark.low”: “100gb”, –>adjust according to your situations

    “cluster.routing.allocation.disk.watermark.high”: “50gb”,

    “cluster.routing.allocation.disk.watermark.flood_stage”: “10gb”,

    “cluster.info.update.interval”: “1m”



2. Disable disk check by hitting below cluster update API


    “transient”: {

       “cluster.routing.allocation.disk.threshold_enabled” : false



Even After all these fixes, OpenSearch won’t bring indices in write mode for that this API needs to be activated

PUT _all/_settings


     “index.blocks.read_only_allow_delete”: null


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