Adding segment to be upgraded – How to solve this OpenSearch error

Opster Team

Aug-23, Version: 1-2.9

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Briefly, this error occurs when OpenSearch identifies a segment in the index that is created with an older version and needs to be upgraded to the current version. This is not necessarily a problem, but an informational message. However, if it’s causing issues, you can resolve it by reindexing your data to the current version, or by using the “_upgrade” API to upgrade the segments manually. Also, ensure that your OpenSearch cluster is running on the same version to avoid such discrepancies.

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

Log Context

Log “Adding segment {} to be upgraded” classname is
We extracted the following from OpenSearch source code for those seeking an in-depth context :

                if (shouldUpgrade(info)) {

                    // TODO: Use IndexUpgradeMergePolicy instead. We should be comparing codecs;
                    // for now we just assume every minor upgrade has a new format.
                    logger.debug("Adding segment {} to be upgraded";;
                    spec.add(new OneMerge(Collections.singletonList(info)));

                // TODO: we could check IndexWriter.getMergingSegments and avoid adding merges that IW will just reject?


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