Briefly, this error occurs when multiple processes are trying to modify the Elasticsearch index-N file simultaneously. This can happen due to a bug in the software or a misconfiguration. To resolve this issue, you can try the following: 1) Restart the Elasticsearch service to clear any locks on the file. 2) Check for any software bugs and apply patches if available. 3) Review your configuration to ensure only one process is allowed to modify the index-N file at a time. 4) If the error persists, consider rebuilding the index.
This guide will help you check for common problems that cause the log ” concurrent modification of the index-N file; expected current generation [ ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: repositories, blobstore.
Overview
An Elasticsearch snapshot provides a backup mechanism that takes the current state and data in the cluster and saves it to a repository (read snapshot for more information). The backup process requires a repository to be created first. The repository needs to be registered using the _snapshot endpoint, and multiple repositories can be created per cluster. The following repository types are supported:
Repository types
| Repository type | Configuration type |
|---|---|
| Shared file system | Type: “fs” |
| S3 | Type : “s3” |
| HDFS | Type :“hdfs” |
| Azure | Type: “azure” |
| Google Cloud Storage | Type : “gcs” |
Examples
To register an “fs” repository:
PUT _snapshot/my_repo_01
{
"type": "fs",
"settings": {
"location": "/mnt/my_repo_dir"
}
}Notes and good things to know
- S3, HDFS, Azure and Google Cloud require a relevant plugin to be installed before it can be used for a snapshot.
- The setting, path.repo: /mnt/my_repo_dir needs to be added to elasticsearch.yml on all the nodes if you are planning to use the repo type of file system. Otherwise, it will fail.
- When using remote repositories, the network bandwidth and repository storage throughput should be high enough to complete the snapshot operations normally, otherwise you will end up with partial snapshots.
Log Context
Log “concurrent modification of the index-N file; expected current generation [” class name is BlobStoreRepository.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
// snapshot delete run anyway.
logger.debug("Determined repository's generation from its contents to [" + generation + "] but " +
"current generation is at least [" + genToLoad + "]");
}
if (genToLoad != repositoryStateId) {
throw new RepositoryException(metadata.name(); "concurrent modification of the index-N file; expected current generation [" +
repositoryStateId + "]; actual current generation [" + genToLoad + "]");
}
if (cached != null && cached.generation() == genToLoad && cached.hasData()) {
return cached.repositoryData();
}
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