The indices.breaker.fielddata.limit setting in Elasticsearch controls the maximum amount of memory that can be used for fielddata. It is part of the fielddata circuit breaker, which helps prevent out-of-memory errors by limiting the amount of data that can be loaded into memory for fielddata-based operations.
- Default value: 40% of JVM heap
- Possible values: Percentage of JVM heap (e.g., 40%) or absolute value in bytes (e.g., 10gb)
- Recommendation: The default is suitable for most cases, but it may need adjustment based on your specific use case and available resources
This setting is crucial for maintaining stability in Elasticsearch clusters, especially when dealing with large amounts of fielddata. It prevents Elasticsearch from consuming excessive memory, which could lead to performance degradation or node failures.
Example
To change the fielddata limit to 50% of the JVM heap:
indices.breaker.fielddata.limit: 50%
You might want to increase this limit if you have a lot of memory available and your use case involves heavy fielddata usage, such as with aggregations on high-cardinality fields. However, be cautious as setting it too high can lead to out-of-memory errors.
Common Issues and Misuses
- Setting the limit too low can result in frequent circuit breaker trips, impacting query performance
- Setting the limit too high may lead to out-of-memory errors if fielddata consumption is not properly managed
- Misunderstanding the relationship between this setting and actual fielddata usage
Do's and Don'ts
Do's:
- Monitor fielddata usage and circuit breaker trips to fine-tune this setting
- Consider using doc values instead of fielddata when possible for better performance
- Adjust this setting in conjunction with other memory-related settings for optimal performance
Don'ts:
- Don't set this value arbitrarily high without understanding your cluster's memory usage patterns
- Don't ignore circuit breaker trips; they are indicators that you may need to optimize your queries or index structure
- Don't rely solely on this setting to manage memory; also consider query optimization and index design
Frequently Asked Questions
Q: How does indices.breaker.fielddata.limit relate to actual fielddata usage?
A: This setting defines the maximum amount of memory that can be used for fielddata. Actual usage may be lower, but it prevents fielddata from exceeding this limit to avoid out-of-memory errors.
Q: Can increasing indices.breaker.fielddata.limit improve query performance?
A: While it may allow more data to be loaded into memory, increasing this limit doesn't directly improve performance. It can prevent circuit breaker trips, but optimal performance comes from proper index design and query optimization.
Q: How often should I adjust the indices.breaker.fielddata.limit setting?
A: You should adjust it when you consistently see circuit breaker trips or if you've significantly changed your cluster's resources or query patterns. Regular monitoring will help determine if adjustments are necessary.
Q: What happens when the fielddata limit is reached?
A: When the limit is reached, Elasticsearch will trigger a circuit breaker exception, preventing the operation that would exceed the limit from completing. This helps protect the node from potential out-of-memory errors.
Q: Is it better to use a percentage or an absolute value for indices.breaker.fielddata.limit?
A: Using a percentage is generally more flexible as it automatically adjusts with changes to the JVM heap size. However, an absolute value might be preferable if you want to ensure a specific amount of memory is always available for fielddata regardless of heap size changes.
