Log Replicas will not be assigned to this node due to exceeding low disk watermark – How To Solve Related Issues


Log Replicas will not be assigned to this node due to exceeding low disk watermark – How To Solve Related Issues

Opster Team

Jan-20, Version: 1.7-8.0

 

Before you begin reading this guide, we recommend you try running the Elasticsearch Error Check-Up which can resolve issues that cause many errors.

This guide will help you check for common problems that cause the log ” Replicas will not be assigned to this node due to exceeding low disk watermark ” to appear. It’s important to understand the issues related to the log, so to get started, read the general overview on common issues and tips related to the Elasticsearch concepts: low disk watermark, allocation, cluster, monitor and node.

Advanced users might want to skip right to the common problems section in each concept or try running the Check-Up which analyses ES to pinpoint the cause of many errors and provides suitable actionable recommendations how to resolve them (free tool that requires no installation).

Quick Summary

The cause of this error is low disk space on a data node, and as a preventive measure, Elasticsearch throws this log message and takes some preventive measures explained further.

Explanation

Elasticsearch considers the available disk space before deciding whether to allocate new shards, relocate shards away or put all indices on read mode based on a different threshold of this error. The reason is Elasticsearch indices consists of different shards which are persisted on data nodes and low disk space can cause issues.
Relevant settings related to log:
cluster.routing.allocation.disk.watermark – have three thresholds of low, high, and flood_stage and can be changed dynamically, accepting absolute values as well as percentage values.

Permanent fixes

a). Delete unused indices.
b) Merge segments to reduce the size of the shard on the affected node, more info on opster’s Elasticsearch expert’s STOF answer
c) Attach external disk or increase the disk used by the data node.

Temp hack/fixes

a) Changed settings values to a higher threshold by dynamically update settings using 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""

}

}

b) Disable disk check by hitting below cluster update API

{

""transient"": {

""cluster.routing.allocation.disk.threshold_enabled"" : false

}

}

C) Even After all these fixes, Elasticsearch won’t bring indices in write mode for that this API needs to be hit.

PUT _all/_settings

{

""index.blocks.read_only_allow_delete"": null

}

Log context

Log “Low disk watermark [{}] exceeded on {}; replicas will not be assigned to this node” classname is DiskThresholdMonitor.java
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

                 diskThresholdSettings.getFreeBytesThresholdFloodStage(); usage);
        } else if (usage.getFreeBytes() 

 

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