Briefly, this error occurs when Elasticsearch is unable to access or read the metadata associated with a specific snapshot. This could be due to issues with the snapshot repository, such as incorrect configuration, permission issues, or network connectivity problems. To resolve this, you can verify the repository configuration, ensure the Elasticsearch process has the necessary permissions to access the repository, and check network connectivity. Additionally, the snapshot metadata could be corrupted, in which case you may need to delete and recreate the snapshot.
This guide will help you check for common problems that cause the log ” cannot read metadata for snapshot [{}] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: blobstore, metadata, repositories, repository-azure and snapshot.
Overview
Metadata in Elasticsearch refers to additional information stored for each document. This is achieved using the specific metadata fields available in Elasticsearch. The default behavior of some of these metadata fields can be customized during mapping creation.
Examples
Using _meta meta-field for storing application-specific information with the mapping:
PUT /my_index?pretty
{
"mappings": {
"_meta": {
"domain": "security",
"release_information": {
"date": "18-01-2020",
"version": "7.5"
}
}
}
}Notes
- In version 2.x, Elasticsearch had a total 13 meta fields available, which are: _index, _uid, _type, _id, _source, _size, _all, _field_names, _timestamp, _ttl, _parent, _routing, _meta
- In version 5.x, _timestamp and _ttl meta fields were removed.
- In version 6.x, the _parent meta field was removed.
- In version 7.x, _uid and _all meta fields were removed.
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 “cannot read metadata for snapshot [{}]” classname is BlobStoreRepository.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :
metaData = readSnapshotMetaData(snapshotId; snapshot.version(); indices; true);
} else {
metaData = readSnapshotMetaData(snapshotId; null; indices; true);
}
} catch (IOException | SnapshotException ex) {
logger.warn("cannot read metadata for snapshot [{}]"; ex; snapshotId);
}
try {
// Delete snapshot file first so we wouldn't end up with partially deleted snapshot that looks OK
if (snapshot != null) {
snapshotFormat(snapshot.version()).delete(snapshotsBlobContainer; snapshotId.getSnapshot());
