Elasticsearch Read-Only Index Error - Common Causes & Fixes

Brief Explanation

The "Read-only index error" in Elasticsearch occurs when an index is set to read-only mode, preventing any write operations from being performed on that index. This is often a protective measure triggered by the cluster to prevent data corruption or loss.

Impact

This error can significantly impact the functionality of your Elasticsearch cluster:

• Write operations (indexing, updates, deletions) on the affected index will fail
• Applications relying on write operations to this index may experience errors or data inconsistencies
• Overall cluster performance may degrade if multiple indices are affected

Common Causes

1. Low disk space: When available disk space falls below a certain threshold
2. Cluster health issues: Red or yellow cluster status
3. Manual setting: Accidentally setting the index to read-only through API calls
4. Index settings: Misconfigured index settings related to read-only status

Troubleshooting and Resolution Steps

1. Check cluster health:

   GET _cluster/health
   

2. Identify read-only indices:

   GET _all/_settings/index.blocks.read_only_allow_delete
   

3. Check disk space:

   GET _cat/allocation?v
   

4. If disk space is low:

   • Free up disk space by deleting unnecessary data or adding more storage
   • Adjust the low watermark threshold if necessary

5. Remove the read-only block:

   PUT /index_name/_settings
   {
     "index.blocks.read_only_allow_delete": null
   }
   

6. If the issue persists, check and adjust cluster-wide settings:

   PUT _cluster/settings
   {
     "persistent": {
       "cluster.routing.allocation.disk.threshold_enabled": true,
       "cluster.routing.allocation.disk.watermark.low": "85%",
       "cluster.routing.allocation.disk.watermark.high": "90%",
       "cluster.routing.allocation.disk.watermark.flood_stage": "95%"
     }
   }
   

Best Practices

• Regularly monitor disk space and cluster health
• Implement proactive alerts for low disk space
• Use index lifecycle management (ILM) to automate index management
• Ensure proper capacity planning for your Elasticsearch cluster

Frequently Asked Questions

Q: Can I still read data from a read-only index?
A: Yes, read operations are still allowed on a read-only index. Only write operations are blocked.

Q: How can I prevent read-only index errors in the future?
A: Implement proper monitoring, set up alerts for low disk space, use ILM policies, and ensure adequate storage capacity for your cluster.

Q: Will removing the read-only block solve all my issues?
A: Not necessarily. If the underlying cause (e.g., low disk space) isn't addressed, the index may become read-only again.

Q: Can a single index becoming read-only affect the entire cluster?
A: While it doesn't directly affect other indices, it can be symptomatic of cluster-wide issues like low disk space, which could impact overall cluster health.

Q: How does Elasticsearch determine when to set an index to read-only?
A: Elasticsearch uses configurable watermarks for disk usage. When these thresholds are crossed, it starts setting indices to read-only to prevent data corruption.