报错日志：

2013-07-05 18:17:31,885 INFO org.apache.hadoop.mapred.JobTracker: STARTUP_MSG:
/************************************************************
STARTUP_MSG: Starting JobTracker
STARTUP_MSG:   host = namenode/192.168.202.50
STARTUP_MSG:   args = []
STARTUP_MSG:   version = 0.20.2
STARTUP_MSG:   build = https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.20 -r 911707; compiled by 'chrisdo' on Fri Feb 19 08:07:34 UTC 2010
************************************************************/
2013-07-05 18:17:32,046 INFO org.apache.hadoop.mapred.JobTracker: Scheduler configured with (memSizeForMapSlotOnJT, memSizeForReduceSlotOnJT, limitMaxMemForMapTasks, limitMaxMemForReduceTasks) (-1, -1, -1, -1)
2013-07-05 18:17:32,055 FATAL org.apache.hadoop.mapred.JobTracker: java.lang.RuntimeException: java.lang.RuntimeException: java.lang.ClassNotFoundException: org.apache.hadoop.mapred.CapacityTaskScheduler
       at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:841)
       at org.apache.hadoop.mapred.JobTracker.<init>(JobTracker.java:1573)
       at org.apache.hadoop.mapred.JobTracker.startTracker(JobTracker.java:183)
       at org.apache.hadoop.mapred.JobTracker.startTracker(JobTracker.java:175)
       at org.apache.hadoop.mapred.JobTracker.main(JobTracker.java:3702)
Caused by: java.lang.RuntimeException: java.lang.ClassNotFoundException: org.apache.hadoop.mapred.CapacityTaskScheduler
       at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:809)
       at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:833)
       ... 4 more
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.mapred.CapacityTaskScheduler
       at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
       at java.security.AccessController.doPrivileged(Native Method)
       at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
       at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
       at java.lang.Class.forName0(Native Method)
       at java.lang.Class.forName(Class.java:247)
       at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:762)
       at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:807)
       ... 5 more
2013-07-05 18:17:32,056 INFO org.apache.hadoop.mapred.JobTracker: SHUTDOWN_MSG:
/************************************************************
SHUTDOWN_MSG: Shutting down JobTracker at namenode/192.168.202.50
************************************************************/

解决方法：

cp /usr/local/hadoop-0.20.2/contrib/capacity-scheduler/hadoop-0.20.2-capacity-scheduler.jar /usr/local/hadoop-0.20.2/lib/

（如果存在FairScheduler 要先删除）