Failed to flush index – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 6.8-8.9

Briefly, this error occurs when Elasticsearch is unable to flush an index due to issues like insufficient disk space, heavy indexing load, or a faulty shard. To resolve this, you can try freeing up disk space, reducing the indexing load, or relocating the faulty shard. Additionally, check the Elasticsearch logs for more specific error details to guide your troubleshooting process.

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 Elasticsearch concepts: index, flush, shard.

Log Context

Log “failed to flush index” classname is
We extracted the following from Elasticsearch 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() {


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