在对HDFS格式化，执行hadoop namenode -format命令时
在对HDFS格式化，执行hadoop namenode -format命令时，出现未知的主机名的问题，异常信息如下所示：
[shirdrn@localhost bin]$ hadoop namenode -format
11/06/2207:33:31INFOnamenode.NameNode:STARTUP_MSG:
/************************************************************
STARTUP_MSG:StartingNameNode
STARTUP_MSG:host=java.net.UnknownHostException:localhost.localdomain:localhost.localdomain
STARTUP_MSG:args=[-format]
STARTUP_MSG:version=0.20.0
STARTUP_MSG:build=https://svn.apache.org/repos/asf/hadoop/core/branches/branch-0.20-r763504;compiledby'ndaley'onThuApr905:18:40UTC2009
************************************************************/
Re-formatfilesystemin/tmp/hadoop/hadoop-shirdrn/dfs/name?(YorN)Y
11/06/2207:33:36INFOnamenode.FSNamesystem:fsOwner=shirdrn,shirdrn
11/06/2207:33:36INFOnamenode.FSNamesystem:supergroup=supergroup
11/06/2207:33:36INFOnamenode.FSNamesystem:isPermissionEnabled=true
11/06/2207:33:36INFOmetrics.MetricsUtil:UnabletoobtainhostName
java.net.UnknownHostException:localhost.localdomain:localhost.localdomain
atjava.net.InetAddress.getLocalHost(InetAddress.java:1353)
atorg.apache.hadoop.metrics.MetricsUtil.getHostName(MetricsUtil.java:91)
atorg.apache.hadoop.metrics.MetricsUtil.createRecord(MetricsUtil.java:80)
atorg.apache.hadoop.hdfs.server.namenode.FSDirectory.initialize(FSDirectory.java:73)
atorg.apache.hadoop.hdfs.server.namenode.FSDirectory.<init>(FSDirectory.java:68)
atorg.apache.hadoop.hdfs.server.namenode.FSNamesystem.<init>(FSNamesystem.java:370)
atorg.apache.hadoop.hdfs.server.namenode.NameNode.format(NameNode.java:853)
atorg.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:947)
atorg.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:964)
11/06/2207:33:36INFOcommon.Storage:Imagefileofsize97savedin0seconds.
11/06/2207:33:36INFOcommon.Storage:Storagedirectory/tmp/hadoop/hadoop-shirdrn/dfs/namehasbeensuccessfullyformatted.
11/06/2207:33:36INFOnamenode.NameNode:SHUTDOWN_MSG:
/************************************************************
SHUTDOWN_MSG:ShuttingdownNameNodeatjava.net.UnknownHostException:localhost.localdomain:localhost.localdomain
************************************************************/我们通过执行hostname命令可以看到：
[shirdrn@localhost bin]# hostnamelocalhost.localdomain
也就是说，Hadoop在格式化HDFS的时候，通过hostname命令获取到的主机名是localhost.localdomain，然后在/etc/hosts文件中进行映射的时候，没有找到，看下我的/etc/hosts内容：
[root@localhost bin]# cat /etc/hosts
#Donotremovethefollowingline,orvariousprograms
#thatrequirenetworkfunctionalitywillfail.
127.0.0.1localhostlocalhost
192.168.1.103 localhost localhost也就说，通过localhost.localdomain根本无法映射到一个IP地址，所以报错了。
此时，我们查看一下/etc/sysconfig/network文件：
NETWORKING=yes
NETWORKING_IPV6=yes
HOSTNAME=localhost.localdomain可见，执行hostname获取到这里配置的HOSTNAME的值。
解决办法：
修改/etc/sysconfig/network中HOSTNAME的值为localhost，或者自己指定的主机名，保证localhost在/etc/hosts文件中映射为正确的IP地址，然后重新启动网络服务：
[root@localhost bin]# /etc/rc.d/init.d/network restart
Shuttingdowninterfaceeth0:[OK]
Shuttingdownloopbackinterface:[OK]
Bringinguploopbackinterface:[OK]
Bringingupinterfaceeth0:
DeterminingIPinformationforeth0...done.
[ OK ]这时，再执行格式化HDFS命令，以及启动HDFS集群就正常了：
[shirdrn@localhost bin]$ hadoop namenode -format
11/06/2208:02:37INFOnamenode.NameNode:STARTUP_MSG:
/************************************************************
STARTUP_MSG:StartingNameNode
STARTUP_MSG:host=localhost/127.0.0.1
STARTUP_MSG:args=[-format]
STARTUP_MSG:version=0.20.0
STARTUP_MSG:build=https://svn.apache.org/repos/asf/hadoop/core/branches/branch-0.20-r763504;compiledby'ndaley'onThuApr905:18:40UTC2009
************************************************************/
11/06/2208:02:37INFOnamenode.FSNamesystem:fsOwner=shirdrn,shirdrn
11/06/2208:02:37INFOnamenode.FSNamesystem:supergroup=supergroup
11/06/2208:02:37INFOnamenode.FSNamesystem:isPermissionEnabled=true
11/06/2208:02:37INFOcommon.Storage:Imagefileofsize97savedin0seconds.
11/06/2208:02:37INFOcommon.Storage:Storagedirectory/tmp/hadoop/hadoop-shirdrn/dfs/namehasbeensuccessfullyformatted.
11/06/2208:02:37INFOnamenode.NameNode:SHUTDOWN_MSG:
/************************************************************
SHUTDOWN_MSG:ShuttingdownNameNodeatlocalhost/127.0.0.1
************************************************************/[shirdrn@localhostbin]$start-all.sh
startingnamenode,loggingto/home/shirdrn/eclipse/eclipse-3.5.2/hadoop/hadoop-0.20.0/logs/hadoop-shirdrn-namenode-localhost.out
localhost:startingdatanode,loggingto/home/shirdrn/eclipse/eclipse-3.5.2/hadoop/hadoop-0.20.0/logs/hadoop-shirdrn-datanode-localhost.out
localhost:startingsecondarynamenode,loggingto/home/shirdrn/eclipse/eclipse-3.5.2/hadoop/hadoop-0.20.0/logs/hadoop-shirdrn-secondarynamenode-localhost.out
startingjobtracker,loggingto/home/shirdrn/eclipse/eclipse-3.5.2/hadoop/hadoop-0.20.0/logs/hadoop-shirdrn-jobtracker-localhost.out
localhost: starting tasktracker, logging to /home/shirdrn/eclipse/eclipse-3.5.2/hadoop/hadoop-0.20.0/logs/hadoop-shirdrn-tasktracker-localhost.out[shirdrn@localhostbin]$jps
8192TaskTracker
7905DataNode
7806NameNode
8065JobTracker
8002SecondaryNameNode
8234Jps