title: why map task always running on a single node
body: <p>I have a Fully-Distributed Hadoop cluster with 4 nodes.When I submit my job to Jobtracker which decide 12 map tasks will be cool for my job,something strange happens.The 12 map tasks always running on a single node instead of running on the entire cluster.Before I ask the question ,I have already done the things below:</p>
  
  <ol>
  <li>Try different Job</li>
  <li>Run start-balance.sh to rebalance the cluster</li>
  </ol>
  
  <p>But it does not work,so I hope someone can tell me why and how to fix it.</p>