Failed to flush index – How to solve this OpenSearch error

Opster Team

Aug-23, Version: 1-2.9

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Briefly, this error occurs when OpenSearch is unable to write or “flush” data to an index. This could be due to a variety of reasons such as insufficient disk space, a locked index, or a network connectivity issue. To resolve this, you can try freeing up disk space, unlocking the index, or checking your network connection. Additionally, ensure that your OpenSearch cluster has enough resources to handle the data load. If the problem persists, consider checking the OpenSearch logs for more detailed error information.

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This guide will help you check for common problems that cause the log ” failed to flush index ” to appear. To understand the issues related to this log, read the explanation below about the following OpenSearch concepts: flush, index, shard.

Log Context

Log “failed to flush index” classname is
We extracted the following from OpenSearch source code for those seeking an in-depth context :

                    logger.debug("submitting async flush request");
                    final AbstractRunnable flush = new AbstractRunnable() {
                        public void onFailure(final Exception e) {
                            if (state != IndexShardState.CLOSED) {
                                logger.warn("failed to flush index"; e);

                        protected void doRun() throws IOException {


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