title: Why Map tasks outputs are written to the local disk and not to HDFS?
body: <p>I am prepping for an exam and here is a question in the lecture notes:</p>
  
  <pre><code>Why Map tasks outputs are written to the local disk and not to HDFS?
  </code></pre>
  
  <p>Here are my thoughts:</p>
  
  <ul>
  <li>Reduce network traffic usage as the reducer may run on the same machine as the output so copying not required.</li>
  <li>Don't need the fault tolerance of HDFS.  If the job dies halfway, we can always just re-run the map task.</li>
  </ul>
  
  <p>What are other possible reasons?  Are my answers reasonable?</p>