本人是hadoop新手，在一次应用中配置出现了问题：就是hadoop 的map阶段正常，但是reduce却卡在18%哪里，一直要将近一个小时才能完成，查看日志如下：
2011-10-03 09:45:58,330 INFO org.apache.hadoop.mapred.JobInProgress: Choosing rack-local task task_201110022127_0003_m_000011
2011-10-03
09:46:01,334 INFO org.apache.hadoop.mapred.JobInProgress: Task
'attempt_201110022127_0003_m_000011_0' has completed
task_201110022127_0003_m_000011 successfully.
2011-10-03 09:46:01,334
INFO org.apache.hadoop.mapred.ResourceEstimator:
completedMapsUpdates:9 completedMapsInputSize:437327225
completedMapsOutputSize:193
2011-10-03 09:46:01,737 INFO
org.apache.hadoop.mapred.JobInProgress: Task
'attempt_201110022127_0003_m_000006_0' has completed
task_201110022127_0003_m_000006 successfully.
2011-10-03 09:46:01,737
INFO org.apache.hadoop.mapred.ResourceEstimator:
completedMapsUpdates:10 completedMapsInputSize:504436090
completedMapsOutputSize:215
2011-10-03 09:46:01,738 INFO
org.apache.hadoop.mapred.JobInProgress: Task
'attempt_201110022127_0003_m_000007_0' has completed
task_201110022127_0003_m_000007 successfully.
2011-10-03 09:46:01,738
INFO org.apache.hadoop.mapred.ResourceEstimator:
completedMapsUpdates:11 completedMapsInputSize:571544955
completedMapsOutputSize:237
2011-10-03 09:46:04,007 INFO
org.apache.hadoop.mapred.JobInProgress: Task
'attempt_201110022127_0003_m_000009_0' has completed
task_201110022127_0003_m_000009 successfully.
2011-10-03 09:46:04,007
INFO org.apache.hadoop.mapred.ResourceEstimator:
completedMapsUpdates:12 completedMapsInputSize:593329451
completedMapsOutputSize:258
2011-10-03 09:46:04,008 INFO
org.apache.hadoop.mapred.JobInProgress: Task
'attempt_201110022127_0003_m_000008_0' has completed
task_201110022127_0003_m_000008 successfully.
2011-10-03 09:46:04,008
INFO org.apache.hadoop.mapred.ResourceEstimator:
completedMapsUpdates:13 completedMapsInputSize:615113993
completedMapsOutputSize:279
2011-10-03 09:46:13,349 INFO org.apache.hadoop.mapred.JobInProgress: Failed fetch notification #1 for task
attempt_201110022127_0003_m_000000_0
2011-10-03 09:48:49,450 INFO org.apache.hadoop.mapred.JobInProgress: Failed fetch notification #2 for task
attempt_201110022127_0003_m_000000_0
2011-10-03 09:53:52,659 INFO org.apache.hadoop.mapred.JobInProgress: Failed fetch notification #3 for task
attempt_201110022127_0003_m_000000_0
2011-10-03 09:53:52,659 INFO org.apache.hadoop.mapred.JobInProgress: Too many fetch-failures
for output of task: attempt_201110022127_0003_m_000000_0 ... killing it
2011-10-03
09:53:52,659 INFO org.apache.hadoop.mapred.TaskInProgress: Error from
attempt_201110022127_0003_m_000000_0: Too many fetch-failures
2011-10-03
09:53:52,661 INFO org.apache.hadoop.mapred.JobTracker: Adding task
'attempt_201110022127_0003_m_000000_1' to tip
task_201110022127_0003_m_000000, for tracker
'tracker_ubuntu-server:127.0.0.1/127.0.0.1:49740'
2011-10-03 09:53:52,661 INFO org.apache.hadoop.mapred.JobInProgress: Choosing rack-local task task_201110022127_0003_m_000000
2011-10-03
09:53:53,107 INFO org.apache.hadoop.mapred.JobTracker: Removed
completed task 'attempt_201110022127_0003_m_000000_0' from
'tracker_fangfei-desktop:127.0.0.1/127.0.0.1:54181'
2011-10-03
09:53:58,264 INFO org.apache.hadoop.mapred.JobTracker: Adding task
'attempt_201110022127_0003_m_000000_2' to tip
task_201110022127_0003_m_000000, for tracker
'tracker_huangzhongyuan-desktop:127.0.0.1/127.0.0.1:48184'
2011-10-03 09:53:58,264 INFO org.apache.hadoop.mapred.JobInProgress: Choosing rack-local task task_201110022127_0003_m_000000
2011-10-03
09:54:01,668 INFO org.apache.hadoop.mapred.JobInProgress: Task
'attempt_201110022127_0003_m_000000_1' has completed
task_201110022127_0003_m_000000 successfully.
2011-10-03 09:54:01,668
INFO org.apache.hadoop.mapred.ResourceEstimator:
completedMapsUpdates:14 completedMapsInputSize:682222858
completedMapsOutputSize:301
2011-10-03 09:54:07,282 INFO
org.apache.hadoop.mapred.JobTracker: Adding task
(cleanup)'attempt_201110022127_0003_m_000000_2' to tip
task_201110022127_0003_m_000000, for tracker
'tracker_huangzhongyuan-desktop:127.0.0.1/127.0.0.1:48184'
2011-10-03
09:54:10,285 INFO org.apache.hadoop.mapred.JobTracker: Removed
completed task 'attempt_201110022127_0003_m_000000_2' from
'tracker_huangzhongyuan
在网上google了很久没找到答案，后来有的说把/etc/hosts中的127.0.0.1的主机映射去掉，试了也不行。知道第二天上午google
的时候才受到一点启发，突然想到，这个问题是发生在reduce阶段，而提示的消息应该是取不到map阶段的结果，既然在Failed fetch notification #1 for task
attempt_201110022127_0003_m_000000_0
中有取不到的任务分块的名字，说明namenode正常工作，namenode通知reduce节点进行reduce操作，而它却取不到，只能说明它没法
和那些节点通信，又由于我在配置hadoop的时候用的是主机的名字，不是ip，所以想到解决办法应该是把各个datanode节点的映射互相加到
/etc/hosts中。试了一下，果然正确。所以在此记录。
解决办法
：把各个datanode节点的映射都加到每个datanode节点的/etc/hosts中
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