title: Why do my mappers appear to repeat the "last merge-pass" many times?
body: <p>In the task logs of my mappers, I'm seeing behavior like this, which is confusing me:</p>
  
  <pre><code>INFO org.apache.hadoop.mapred.Merger: Merging 6 sorted segments
  INFO org.apache.hadoop.io.compress.CodecPool: Got brand-new decompressor
  INFO org.apache.hadoop.io.compress.CodecPool: Got brand-new decompressor
  INFO org.apache.hadoop.mapred.Merger: Down to the last merge-pass, with 6 segments left of total size: 38872249 bytes
  INFO org.apache.hadoop.mapred.Merger: Merging 6 sorted segments
  INFO org.apache.hadoop.mapred.Merger: Down to the last merge-pass, with 6 segments left of total size: 38636106 bytes
  INFO org.apache.hadoop.mapred.Merger: Merging 6 sorted segments
  INFO org.apache.hadoop.mapred.Merger: Down to the last merge-pass, with 6 segments left of total size: 39490573 bytes
  INFO org.apache.hadoop.mapred.Merger: Merging 6 sorted segments
  ....
  repeat 20+ times
  ....
  INFO org.apache.hadoop.mapred.Merger: Down to the last merge-pass, with 6 segments left of total size: 37234005 bytes
  INFO org.apache.hadoop.mapred.Task: Task:attempt_201209251414_0326_m_000052_0 is done. And is in the process of commiting
  INFO org.apache.hadoop.mapred.Task: Task 'attempt_201209251414_0326_m_000052_0' done.\
  </code></pre>
  
  <p>How is it that I'm having 25+ "last" merge passes? This process takes forever, so I'm trying to figure out if I've configured something incorrectly.</p>