今天把hadoop环境给搭起来了，在master上面启动hadoop后，jps查看进程，SecondaryNameNode NameNode Jps DataNode JobTracker TaskTracker都没问题。但是去
http://master:50070/dfshealth.jsp查 看，live nodes为0。所以我到slave1和slave2上面去执行jps查看进程，结果都没有datanode进程，也就是说datanode没有启动成 功，所以我就到logs里面去执行less hadoop-hadoop-datanode-slave2.log，发现报错的内容如下：
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 $Proxy4.getProtocolVersion(Unknown Source)
at org.apache.hadoop.ipc.RPC.getProxy(RPC.java:359)
at org.apache.hadoop.ipc.RPC.getProxy(RPC.java:346)
at org.apache.hadoop.ipc.RPC.getProxy(RPC.java:383)
at org.apache.hadoop.ipc.RPC.waitForProxy(RPC.java:314)
at org.apache.hadoop.ipc.RPC.waitForProxy(RPC.java:291)
at org.apache.hadoop.hdfs.server.datanode.DataNode.startDataNode(DataNode.java:269)
at org.apache.hadoop.hdfs.server.datanode.DataNode.(DataNode.java:216)
at org.apache.hadoop.hdfs.server.datanode.DataNode.makeInstance(DataNode.java:1283)
at org.apache.hadoop.hdfs.server.datanode.DataNode.instantiateDataNode(DataNode.java:1238)
at org.apache.hadoop.hdfs.server.datanode.DataNode.createDataNode(DataNode.java:1246)
at org.apache.hadoop.hdfs.server.datanode.DataNode.main(DataNode.java:1368)
Caused by: java.net.NoRouteToHostException: No route to host
at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)
at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:692)
at org.apache.hadoop.net.SocketIOWithTimeout.connect(SocketIOWithTimeout.java:206)
at org.apache.hadoop.net.NetUtils.connect(NetUtils.java:404)
at org.apache.hadoop.ipc.Client$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由此文件可以看出，它无法与matser建立连接，所以我想应该是master的防火墙没有关闭，于是关闭master的防火墙，问题就解决了。
