Updating max merge at once explicit from to – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 2.3-2.3

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Briefly, this error occurs when there’s an attempt to update the maximum number of segments that can be merged at once in Elasticsearch. This could be due to an incorrect setting or a system limitation. To resolve this, you can adjust the index settings to a suitable value for your system’s capacity. Alternatively, you could optimize your indexing process to reduce the number of segments created, or increase your system resources if possible. Regularly monitoring and managing your Elasticsearch cluster can also help prevent such issues.

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This guide will help you check for common problems that cause the log ” updating [max_merge_at_once_explicit] from [{}] to [{}] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: index, merge and shard.

Log Context

Log “updating [max_merge_at_once_explicit] from [{}] to [{}]” classname is MergePolicyConfig.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :


        final int oldMaxMergeAtOnceExplicit = mergePolicy.getMaxMergeAtOnceExplicit();
        final int maxMergeAtOnceExplicit = settings.getAsInt(INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE_EXPLICIT; oldMaxMergeAtOnceExplicit);
        if (maxMergeAtOnceExplicit != oldMaxMergeAtOnceExplicit) {
            logger.info("updating [max_merge_at_once_explicit] from [{}] to [{}]"; oldMaxMergeAtOnceExplicit; maxMergeAtOnceExplicit);

        final double oldMaxMergedSegmentMB = mergePolicy.getMaxMergedSegmentMB();
        final ByteSizeValue maxMergedSegment = settings.getAsBytesSize(INDEX_MERGE_POLICY_MAX_MERGED_SEGMENT; null);


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