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

Opster Team

Aug-23, Version: 1-2.9

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Briefly, this error occurs when the OpenSearch engine is trying to merge segments during indexing but the number of merges exceeds the maximum limit, causing the system to throttle. This can be due to heavy indexing operations or insufficient resources. To resolve this, you can consider reducing the indexing rate, increasing the system resources, or adjusting the ‘’ setting to a higher value. Also, ensure your shards are properly sized and distributed to avoid overloading a single node. Regularly monitor your system to prevent such issues.

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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 OpenSearch concepts: index.

Log Context

Log “stop throttling indexing: numMergesInFlight={}; maxNumMerges={}” classname is
We extracted the following from OpenSearch 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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