The index.merge.policy.segments_per_tier setting in Elasticsearch controls the number of segments that should exist per tier of the merge process. This setting is part of the merge policy configuration and affects how aggressively Elasticsearch merges segments during indexing.
- Default value: 10
- Possible values: Any positive integer
- Recommendations: The default value is suitable for most use cases. Adjusting this setting should be done cautiously and with performance testing.
This setting determines how many segments Elasticsearch aims to have in each tier of the merge process. A lower value results in more aggressive merging, potentially improving search performance but increasing indexing overhead. A higher value leads to less aggressive merging, which can improve indexing speed but may impact search performance due to a larger number of segments.
Example
To change the index.merge.policy.segments_per_tier setting for an index:
PUT /my_index/_settings
{
"index": {
"merge.policy.segments_per_tier": 8
}
}
Reason for change: You might lower this value if you prioritize search performance over indexing speed, especially for indices that are searched frequently but updated less often.
Effect: More aggressive merging, potentially slower indexing but faster searches due to fewer segments.
Common Issues and Misuses
- Setting the value too low can lead to excessive merging, significantly impacting indexing performance.
- Setting the value too high can result in too many small segments, which may degrade search performance.
- Changing this setting without understanding its relationship with other merge policy settings can lead to unexpected behavior.
Do's and Don'ts
Do's:
- Monitor indexing and search performance when adjusting this setting.
- Consider the nature of your index (write-heavy vs. read-heavy) when optimizing.
- Test changes in a non-production environment before applying to production.
Don'ts:
- Don't set this value extremely low (e.g., 1 or 2) as it can cause excessive merging.
- Don't change this setting in isolation; consider its interaction with other merge policy settings.
- Don't expect immediate results; the effects of changing merge policies may take time to manifest.
Frequently Asked Questions
Q: How does index.merge.policy.segments_per_tier affect indexing performance?
A: A lower value leads to more aggressive merging, which can slow down indexing as more resources are spent on merging segments. Conversely, a higher value can improve indexing speed by reducing merge frequency.
Q: Can changing index.merge.policy.segments_per_tier improve search performance?
A: Yes, potentially. Fewer segments (achieved by a lower value) can lead to faster searches as there are fewer segments to query. However, this comes at the cost of more intensive merging during indexing.
Q: Is it safe to change index.merge.policy.segments_per_tier on a production index?
A: While it's possible, it's recommended to test such changes in a non-production environment first. Changes to merge policies can have significant impacts on performance and resource utilization.
Q: How does index.merge.policy.segments_per_tier interact with other merge policy settings?
A: This setting works in conjunction with others like max_merge_at_once and floor_segment. The overall merge behavior is a result of the interplay between these settings, so changing one may require adjusting others for optimal performance.
Q: How often should I adjust index.merge.policy.segments_per_tier?
A: It's not typically necessary to adjust this setting frequently. Only consider changes if you have specific performance issues related to indexing or searching, and always based on careful monitoring and testing.
