Merge Rate Limiting

[harshitakaushik-dev]

Description

This PR adds rate limiting capabilities for forced merges in OpenSearch. It introduces both index-level and cluster-level settings to control the maximum throughput of forced merge operations, helping to prevent these resource-intensive operations from overwhelming system resources.

The implementation includes:

1. A new index-level setting index.merge.scheduler.max_force_merge_mb_per_sec to control forced merge rate at the index level
2. A new cluster-level setting cluster.merge.scheduler.max_force_merge_mb_per_sec as a fallback when index-level setting is not specified
3. Clear precedence rules where index-level settings take priority over cluster-level settings
4. Dynamic configuration support allowing settings to be updated without node restarts

This enhancement gives administrators fine-grained control over forced merge operations, allowing them to balance merge performance against other system operations.

Related Issues

Resolves #[19308]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[harshitakaushik-dev]

@gbbafna requesting code review.

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❌ Gradle check result for 9f3babb: FAILURE

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❌ Gradle check result for 1c0f321: FAILURE

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✅ Gradle check result for 1c0f321: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 73.68421% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.98%. Comparing base (cb65261) to head (1c0f321).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...in/java/org/opensearch/indices/IndicesService.java	28.57%	5 Missing ⚠️
...c/main/java/org/opensearch/index/IndexService.java	0.00%	3 Missing ⚠️
...ava/org/opensearch/index/MergeSchedulerConfig.java	81.81%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19309      +/-   ##
============================================
- Coverage     73.00%   72.98%   -0.02%     
+ Complexity    70483    70453      -30     
============================================
  Files          5717     5717              
  Lines        323021   323057      +36     
  Branches      46790    46793       +3     
============================================
- Hits         235826   235796      -30     
- Misses        68207    68247      +40     
- Partials      18988    19014      +26     

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