Failed to merge – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 2.3-2.3

Briefly, this error occurs when Elasticsearch is unable to merge segments of an index. This could be due to insufficient disk space, a corrupt index, or a configuration issue. To resolve this, you can try freeing up disk space, rebuilding the corrupt index, or adjusting the Elasticsearch configuration to allow for larger merges. Additionally, ensure that the Elasticsearch process has sufficient permissions to perform the merge operation.

This guide will help you check for common problems that cause the log ” failed to merge ” 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 “failed to merge” classname is
We extracted the following from Elasticsearch source code for those seeking an in-depth context :


        protected void handleMergeException(final Directory dir; final Throwable exc) {
            logger.error("failed to merge"; exc);
            if (config().getMergeSchedulerConfig().isNotifyOnMergeFailure()) {
                engineConfig.getThreadPool().generic().execute(new AbstractRunnable() {
                    public void onFailure(Throwable t) {
                        logger.debug("merge failure action rejected"; t);


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