title: Why are pending map tasks not starting?
body: <p>I have a particular pig job that usually takes around 12 minutes to run but a couple of times a day it gets stuck in an odd state. The job has 6 maps and 1 reduce task. It successfully completes the first map task and then it stays "on hold" for hours (forever!). The job tracker indicates there are 5 maps and 1 reduce pending, however no tasks are running. No tasks show any errors. The map tasks will simply not start but no errors occur. </p>
  
  <p>How can I figure out why are the tasks not starting?</p>
  
  <p>I have tried increasing the jobs priority and I have verified that there are available map and reduce slots. The task log for the one map that ran looks completely normal. Is there some other log somewhere that would tell me what is going on?</p>
  
  <p>I am using Cloudera CDH4.5 with MR1. The small cluster has 5 nodes.</p>