title: Why is TieredMergePolicy is better than others?
body: <p>I'm Studying ElasticSearch and Apache Lucene.</p>
  <p>Lately, I found that Apache Lucene has a few Merge Policy but ElasticSearch uses TieredMergePolicy instead of other merge policy such as LogMergePolicy and LogByteSizeMergePolicy.</p>
  <p>So I've been searching for information about TieredMergePolicy. I found the algorithm but I couldn't see why TieredMergePolicy is better than others(I mean in general case, not special case)</p>
  <p>why is it important to pick similar size segments when segment merging and how it affect the overall performance?</p>
  <p>please help me.</p>