我设置了2个节点（virtaul机器）hadoop集群设置。成功启动dfs和mapred deamons后，我运行hadoop演示示例，程序在这个终端显示后变慢：

Number of Maps = 4 Samples per Map = 10000
Wrote input for Map #0
Wrote input for Map #1
Wrote input for Map #2
Wrote input for Map #3
Starting Job`enter code here`
13/06/10 21:36:43 INFO mapred.FileInputFormat: Total input paths to process : 4
13/06/10 21:36:43 INFO mapred.FileInputFormat: Total input paths to process : 4
13/06/10 21:36:43 INFO mapred.JobClient: Running job: job_201306101254_0005
13/06/10 21:36:44 INFO mapred.JobClient:  map 0% reduce 0%
13/06/10 21:36:49 INFO mapred.JobClient:  map 75% reduce 0%
13/06/10 21:36:50 INFO mapred.JobClient:  map 100% reduce 0%

所以基本上map任务是正确完成的，我看了尝试日志，特别是reduce任务的尝试日志，确认reduce任务无法读取另一个slave生成的mapoutput**，错误如下：

****13/06/11 01:55:45 WARN mapred.JobClient: Error reading task outputhttp://hadoop-desk.localdomain:50060/tasklog?plaintext=true&taskid=attempt_201306110154_0001_m_000000_0&filter=stdout

13/06/11 01:55:45 WARN mapred.JobClient: Error reading task outputhttp://hadoop-desk.localdomain:50060/tasklog?plaintext=true&taskid=attempt_201306110154_0001_m_000000_0&filter=stderr
13/06/11 01:55:49 INFO mapred.JobClient:  map 75% reduce 16%****

因此，生成这个mapoutput的map任务被认为是失败的，并且在不同的slave（运行reduce的slave）上重新调度，使得整个程序变慢。我认为原因是因为ubuntu的etc/hosts：

127.0.0.1   localhost
127.0.1.1   hadoop-desk.localdomain hadoop-desk
192.168.196.128 master
192.168.196.129 slave

# The following lines are desirable for IPv6 capable hosts

::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts

即使在删除localhost之后，我也会遇到同样的错误，这一行

127.0.0.1   localhost

我去掉了这条线

127.0.1.1   hadoop-desk.localdomain hadoop-desk

然后我得到一个错误：

Number of Maps = 4 Samples per Map = 10000
org.apache.hadoop.ipc.RemoteException: org.apache.hadoop.dfs.SafeModeException: Cannot delete /user/hadoop-user/test-mini-mr. Name node is in safe mode.
The ratio of reported blocks 0.0000 has not reached the threshold 0.9990. Safe mode will be turned off automatically.
    at org.apache.hadoop.dfs.FSNamesystem.deleteInternal(FSNamesystem.java:1494)
    at org.apache.hadoop.dfs.FSNamesystem.delete(FSNamesystem.java:1466)
    at org.apache.hadoop.dfs.NameNode.delete(NameNode.java:425)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:452)
    at org.apache.hadoop.ipc.Server$Handler.run(Server.java:888)

    at org.apache.hadoop.ipc.Client.call(Client.java:715)
    at org.apache.hadoop.ipc.RPC$Invoker.invoke(RPC.java:216)
    at org.apache.hadoop.dfs.$Proxy0.delete(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:82)
    at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:59)
    at org.apache.hadoop.dfs.$Proxy0.delete(Unknown Source)
    at org.apache.hadoop.dfs.DFSClient.delete(DFSClient.java:529)
    at org.apache.hadoop.dfs.DistributedFileSystem.delete(DistributedFileSystem.java:192)
    at org.apache.hadoop.examples.PiEstimator.launch(PiEstimator.java:188)
    at org.apache.hadoop.examples.PiEstimator.run(PiEstimator.java:245)
    at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
    at org.apache.hadoop.examples.PiEstimator.main(PiEstimator.java:252)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.hadoop.util.ProgramDriver$ProgramDescription.invoke(ProgramDriver.java:68)
    at org.apache.hadoop.util.ProgramDriver.driver(ProgramDriver.java:139)
    at org.apache.hadoop.examples.ExampleDriver.main(ExampleDriver.java:53)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.hadoop.util.RunJar.main(RunJar.java:155)
    at org.apache.hadoop.mapred.JobShell.run(JobShell.java:54)
    at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
    at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
    at org.apache.hadoop.mapred.JobShell.main(JobShell.java:68)