我用的是hadoop 1.2.1

遇到的问题是：

hadoop中datanode无法启动，报Caused by: java.net.NoRouteToHostException: No route to host

2013-06-11 02:22:13,637 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: STARTUP_MSG:

/************************************************************

STARTUP_MSG: Starting DataNode

STARTUP_MSG:   host = slave2/192.168.137.112

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

************************************************************/

2013-06-11 02:22:14,931 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.137.110:9000. Already tried 0 time(s).

2013-06-11 02:22:15,932 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.137.110:9000. Already tried 1 time(s).

2013-06-11 02:22:16,934 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.137.110:9000. Already tried 2 time(s).

2013-06-11 02:22:17,952 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.137.110:9000. Already tried 3 time(s).

2013-06-11 02:22:18,954 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.137.110:9000. Already tried 4 time(s).

2013-06-11 02:22:19,957 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.137.110:9000. Already tried 5 time(s).

2013-06-11 02:22:20,959 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.137.110:9000. Already tried 6 time(s).
2013-06-11 02:22:23,978 ERROR org.apache.hadoop.hdfs.server.datanode.DataNode: java.io.IOException: Call to master/192.168.137.110:9000 failed on local exception: java.net.NoRouteToHostException: No route to host

        at org.apache.hadoop.ipc.Client.wrapException(Client.java:775)

        at org.apache.hadoop.ipc.Client.call(Client.java:743)

        at org.apache.hadoop.ipc.RPC$Invoker.invoke(RPC.java:220)

        at $Proxy4.getProtocolVersion(Unknown Source)

        at org.apache.hadoop.ipc.RPC.getProxy(RPC.java:359)

        at org.apache.hadoop.ipc.RPC.getProxy(RPC.java:346)

        at org.apache.hadoop.ipc.RPC.getProxy(RPC.java:383)

        at org.apache.hadoop.ipc.RPC.waitForProxy(RPC.java:314)

        at org.apache.hadoop.ipc.RPC.waitForProxy(RPC.java:291)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.startDataNode(DataNode.java:269)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.<init>(DataNode.java:216)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.makeInstance(DataNode.java:1283)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.instantiateDataNode(DataNode.java:1238)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.createDataNode(DataNode.java:1246)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.main(DataNode.java:1368)
Caused by: java.net.NoRouteToHostException: No route to host

        at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)

        at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:692)

        at org.apache.hadoop.net.SocketIOWithTimeout.connect(SocketIOWithTimeout.java:206)

        at org.apache.hadoop.net.NetUtils.connect(NetUtils.java:404)

        at org.apache.hadoop.ipc.Client$Connection.setupIOstreams(Client.java:304)

        at org.apache.hadoop.ipc.Client$Connection.access$1700(Client.java:176)

        at org.apache.hadoop.ipc.Client.getConnection(Client.java:860)

        at org.apache.hadoop.ipc.Client.call(Client.java:720)

        ... 13 more

切换root用户将所有机器上的防火墙都关闭。

重新启动。成功。

一定要从新启动，是因为没有启动，我一直就没有配置好！