Stop throttling indexing numMergesInFlight= maxNumMerges= – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 6.8-8.9

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Briefly, this error occurs when Elasticsearch has finished throttling the indexing process due to the maximum number of merges (numMergesInFlight) being reached. This is a part of Elasticsearch’s internal mechanism to manage resources and prevent overloading. To resolve this issue, you can consider increasing the ‘’ setting to allow more merges at once. Alternatively, you can optimize your indexing process to reduce the number of merges required, such as by using bulk indexing or adjusting your index settings to better suit your data and query patterns.

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This guide will help you check for common problems that cause the log ” stop throttling indexing: numMergesInFlight={}; maxNumMerges={} ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: index.

Log Context

Log “stop throttling indexing: numMergesInFlight={}; maxNumMerges={}” classname is
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

        public synchronized void afterMerge(OnGoingMerge merge) {
            int maxNumMerges = mergeScheduler.getMaxMergeCount();
            if (numMergesInFlight.decrementAndGet() = engineConfig.getFlushMergesAfter().nanos()) {


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