hadoop jobtracker start fail

Failed to operate on mapred.system.dir (hdfs://namenode/mapred/system) because of permissions.
2011-09-06 09:19:25,415 WARN org.apache.hadoop.mapred.JobTracker: This directory should be owned by the user 'mapred'
2011-09-06 09:19:25,416 WARN org.apache.hadoop.mapred.JobTracker: Bailing out ... 
org.apache.hadoop.security.AccessControlException: org.apache.hadoop.security.AccessControlException: 
Permission denied: user=mapred, access=WRITE, inode="/":hdfs:supergroup:drwxr-xr-x
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)

When I start Hadoop JobTracker, I got these exceptions. It shows the permission is not right.

After checking mapred-site.xml, i've set mapred.system.dir=/mapred/system. These setting is not local directory, but hdfs directory. So I need use following commands to create it:

$ sudo -u hdfs hadoop fs -mkdir /mapred/system 
$ sudo -u hdfs hadoop fs -chown mapred:hadoop /mapred/system

Then start JobTracker OK!