Elasticsearch IndexShardRestoreFailedException: Index shard restore failed - Common Causes & Fixes

Brief Explanation

The "IndexShardRestoreFailedException: Index shard restore failed" error occurs during the process of restoring an index from a snapshot. This error indicates that Elasticsearch encountered an issue while attempting to restore one or more shards of the specified index.

Impact

This error can have significant impact on data availability and cluster operations:

• The affected index may be partially or completely unavailable.
• Data loss may occur if the restore process cannot be completed successfully.
• Cluster performance may be affected if multiple restore attempts are made.

Common Causes

1. Corrupted snapshot data
2. Incompatible versions between the snapshot and the current Elasticsearch cluster
3. Insufficient disk space on the target node
4. Network issues during the restore process
5. Misconfigured snapshot repository settings

Troubleshooting and Resolution Steps

1. Check the Elasticsearch logs for detailed error messages related to the restore failure.

2. Verify the integrity of the snapshot:

   POST /_snapshot/my_backup/_verify/snapshot_name
   

3. Ensure that the Elasticsearch versions are compatible between the snapshot and the current cluster.

4. Check available disk space on the target nodes and free up space if necessary.

5. Verify network connectivity between nodes and the snapshot repository.

6. Review snapshot repository settings and ensure they are correctly configured.

7. If the issue persists, try restoring the index to a new name:

   POST /_snapshot/my_backup/snapshot_name/_restore
   {
     "indices": "old_index_name",
     "rename_pattern": "old_index_name",
     "rename_replacement": "new_index_name"
   }
   

8. If partial restore is possible, consider restoring individual shards:

   POST /_snapshot/my_backup/snapshot_name/_restore
   {
     "indices": "index_name",
     "partial": true,
     "include_global_state": false
   }
   

9. If all else fails, consider recreating the index from the original data source if available.

Best Practices

• Regularly test your backup and restore processes to ensure they work as expected.
• Keep your Elasticsearch cluster and snapshot repository on compatible versions.
• Monitor disk space and network health to prevent restore failures.
• Use the _verify API to check snapshot integrity before attempting a restore.

Frequently Asked Questions

Q: Can I restore a snapshot from an older version of Elasticsearch to a newer version?
A: Generally, you can restore snapshots to the same or newer versions of Elasticsearch, but not to older versions. Always check the compatibility matrix in the Elasticsearch documentation for specific version details.

Q: How can I identify which specific shard failed during the restore process?
A: Check the Elasticsearch logs for detailed error messages. They usually contain information about the specific index and shard number that failed to restore.

Q: Is it possible to restore only a subset of indices from a snapshot?
A: Yes, you can specify which indices to restore using the indices parameter in the restore API call. This allows you to selectively restore specific indices from a snapshot.

Q: What should I do if the snapshot repository is no longer accessible?
A: If the original repository is inaccessible, you may need to recreate the index from the original data source. Always ensure you have multiple backup locations for critical data.

Q: Can a failed shard restore affect other shards or indices in the cluster?
A: While a failed shard restore primarily affects the specific index being restored, it can indirectly impact cluster performance if multiple restore attempts are made or if the cluster is under heavy load during the restore process.