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

Opster Team

Aug-23, Version: 6.8-8.9

Briefly, this error occurs when Elasticsearch is experiencing high indexing load and the number of merges in flight exceeds the maximum limit. This is a built-in mechanism to prevent the system from overloading. To resolve this issue, you can consider the following: 1) Increase the system resources (CPU, RAM, Disk I/O) if possible. 2) Optimize your indexing process by using bulk requests, reducing the index refresh rate, or increasing the index buffer size. 3) Review your shard size and number, as large or numerous shards can cause high merge pressure.

This guide will help you check for common problems that cause the log ” now 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 “now throttling indexing: numMergesInFlight={}; maxNumMerges={}” classname is
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

        public synchronized void beforeMerge(OnGoingMerge merge) {
            int maxNumMerges = mergeScheduler.getMaxMergeCount();
            if (numMergesInFlight.incrementAndGet() > maxNumMerges) {
                if (isThrottling.getAndSet(true) == false) {
          "now throttling indexing: numMergesInFlight={}; maxNumMerges={}"; numMergesInFlight; maxNumMerges);


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