Updating max merge size from mb to – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 1.7-1.7

Briefly, this error occurs when Elasticsearch is automatically adjusting the maximum size of a single segment in an index, which is part of its internal management of storage. This is not necessarily an error, but more of an informational message. However, if you’re experiencing performance issues, you may want to manually configure the merge policy settings. You can do this by adjusting the ‘index.merge.policy.max_merge_at_once_explicit’ and ‘index.merge.policy.max_merged_segment’ settings in your Elasticsearch configuration. Always ensure to set these values considering your hardware capabilities to avoid performance degradation.

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

Log Context

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


            double oldMaxMergeSizeMB = mergePolicy.getMaxMergeMB();
            ByteSizeValue maxMergeSize = settings.getAsBytesSize(INDEX_MERGE_POLICY_MAX_MERGE_SIZE; null);
            if (maxMergeSize != null && maxMergeSize.mbFrac() != oldMaxMergeSizeMB) {
                logger.info("updating max_merge_size from [{}mb] to [{}]"; oldMaxMergeSizeMB; maxMergeSize);

            int oldMaxMergeDocs = mergePolicy.getMaxMergeDocs();
            int maxMergeDocs = settings.getAsInt(INDEX_MERGE_POLICY_MAX_MERGE_DOCS; oldMaxMergeDocs);


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