Elasticsearch IndexShardNotRecoveringException: Index shard not recovering

Brief Explanation

The IndexShardNotRecoveringException occurs when Elasticsearch attempts to perform an operation on a shard that is expected to be in the recovering state, but it is not. This error indicates a problem with the shard recovery process, which is crucial for maintaining data consistency and availability in the cluster.

Common Causes

1. Network issues between nodes
2. Insufficient disk space on data nodes
3. Corrupted shard data
4. Misconfigured cluster settings
5. Node failures during shard allocation

Troubleshooting and Resolution Steps

1. Check cluster health:

   GET _cluster/health
   GET _cat/indices?v
   GET _cat/shards?v
   

2. Verify node status and connectivity:

   GET _cat/nodes?v
   

3. Inspect shard allocation:

   GET _cluster/allocation/explain
   

4. Review logs for error messages related to shard recovery.

5. Ensure sufficient disk space on all data nodes.

6. Attempt to manually allocate the unassigned shards:

   POST /_cluster/reroute
   {
     "commands": [
       {
         "allocate_empty_primary": {
           "index": "your_index_name",
           "shard": 0,
           "node": "target_node_name",
           "accept_data_loss": true
         }
       }
     ]
   }
   

7. If the issue persists, consider restoring from a snapshot or rebuilding the index.

Additional Information and Best Practices

• Regularly monitor cluster health and shard allocation.
• Implement proper disk space monitoring and alerting.
• Use shard allocation filtering to control shard distribution.
• Maintain up-to-date backups and test recovery procedures.
• Consider using cross-cluster replication for critical indices.

Frequently Asked Questions

Q1: Can this error occur during normal cluster operations?

A1: While rare, it can occur during normal operations, especially if there are sudden node failures or network issues.

Q2: How does this error affect cluster performance?

A2: It can lead to reduced performance and data availability as the affected shards are not accessible.

Q3: Is data loss possible with this error?

A3: Data loss is possible if the error is not addressed promptly and properly. Always ensure you have recent backups.

Q4: Can increasing the index.recovery.max_bytes_per_sec setting help?

A4: In some cases, yes. This setting controls the recovery rate and can be adjusted to optimize shard recovery speed.

Q5: How can I prevent this error from occurring?

A5: Regular maintenance, proper resource allocation, and monitoring cluster health can help prevent this error. Also, ensure your cluster is properly sized for your workload.