fore_merge 如何做到 segments 均衡？

选哪些段集进行合并的决定因素是MergePolicy所影响的，上述参数是TieredMergePolicy,看了下force_merge采用的MergePolicy 
ElasticsearchMergePolicy(包装的最顶层还是TieredMergePolicy) 所以以上参数还是适用于force_merge,我记得 TieredMergePolicy
有个maxMergedSegmentBytes 默认是（5GB/2）  和 deletesPctAllowed 默认33% 这两个参数会限制大段合并，对应这里应该是 max_merged_segment 和 deletes_pct_allowed 你可以自己研究下。
 

你说得没错，
max_merged_segment只针对系统自己的merge有效，对手工的force merge操作无效。
 
我不知倒你系统是啥方面的压力，需要“需要手动 force merge 减轻压力”？
还得要“segments 均衡”？

不建议在仍有写入的情况下做force_merge，原因见ES force merge的官方文档：
 

Force merge should only be called against an index after you have finished writing to it. Force merge can cause very large (>5GB) segments to be produced, and if you continue to write to such an index then the automatic merge policy will never consider these segments for future merges until they mostly consist of deleted documents. This can cause very large segments to remain in the index which can result in increased disk usage and worse search performance.

ES官方不建议在有写入的情况下做forcemerge：
 

Force merge should only be called against an index after you have finished writing to it. Force merge can cause very large (>5GB) segments to be produced, and if you continue to write to such an index then the automatic merge policy will never consider these segments for future merges until they mostly consist of deleted documents. This can cause very large segments to remain in the index which can result in increased disk usage and worse search performance.