Before you read this guide, we recommend you run the Elasticsearch Error Check-Up which detects issues in ES causing log errors (free tool that requires no installation).
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. It’s important to understand the issues related to the log, so to get started, read the general overview on common issues and tips related to the Elasticsearch concepts: repositories, blobstore.
Advanced users might want to skip right to the common problems section in each concept or try running the Check-Up which analyses ES to discover the cause of many errors and provides suitable actionable recommendations how to resolve them (join over 700 users who use this free tool).
Repository in Elasticsearch
What it is:
An Elasticsearch snapshot provides a backup mechanism that takes the current state and data in the cluster and saves it to a repository (read the Glossary term 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 a repository of type fs:
PUT _snapshot/my_repo_01
{
"type": "fs",
"settings": {
"location": "/mnt/my_repo_dir"
}
}Notes and Common Problems:
- S3, HDFS , Azure and Google Cloud requires 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 in case you are planning to use 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 [“classname 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.v1() == genToLoad) {
return repositoryDataFromCachedEntry(cached);
}
