本文记录了一次Hadoop集群部署过程中遇到的问题：datanode无法正常连接到namenode，导致Web界面无法正确显示。通过检查和修改hosts文件，最终解决了连接问题。
hadoop 启动时namenode和datanode可以启动，使用jps命令也可以看到进程，但是在浏览器中输入master:50070却没有显示datanode
查看datanode的log日志：
2016-05-06 13:22:39,899 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 1 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:40,901 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 2 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:41,903 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 3 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:42,905 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 4 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:43,907 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 5 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:44,908 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 6 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:45,910 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 7 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:49,236 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 8 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:50,238 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 9 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:40,901 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 2 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:41,903 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 3 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:42,905 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 4 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:43,907 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 5 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:44,908 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 6 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:45,910 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 7 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:49,236 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 8 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:50,238 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:9000. Already tried 9 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1 SECONDS)
2016-05-06 13:22:50,240 INFO org.apache.hadoop.ipc.RPC: Server at master/192.168.200.128:9000 not available yet, Zzzzz...
发现连接不上namenode。
所以在master（也就是namenode）中配置etc/hosts文件，将120.0.0.1 master注释掉，然后添加192.168.200.131 master（如果不注释可能会优先匹配），重新hadoop namenode -format 然后启动就可以了。
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