Hadoop hostname: Unknown host

本来下想在一台虚拟机上，搭建一个hadoop的测试hadoop，用于调试和阅读hadoop源代码，发现在虚拟机上执行：

$hostname -i

hostname: Unknown host

这个是因为没有设置/etc/hosts文件导致的，如果linux的环境是这样的，那么将会在启动datanode和tasktracker的时候失败，

  datanode和namenode将会抛出如下的异常：

2015-01-28 12:36:36,506 INFO org.apache.hadoop.metrics2.impl.MetricsConfig: loaded properties from hadoop-metrics2.properties
2015-01-28 12:36:36,535 INFO org.apache.hadoop.metrics2.impl.MetricsSourceAdapter: MBean for source MetricsSystem,sub=Stats registered.
2015-01-28 12:36:36,540 ERROR org.apache.hadoop.metrics2.impl.MetricsSystemImpl: Error getting localhost name. Using 'localhost'...
java.net.UnknownHostException: PC_PAT1: PC_PAT1: Name or service not known
        at java.net.InetAddress.getLocalHost(InetAddress.java:1473)
        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.getHostname(MetricsSystemImpl.java:463)
        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.configureSystem(MetricsSystemImpl.java:394)
        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.configure(MetricsSystemImpl.java:390)
        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.start(MetricsSystemImpl.java:152)
        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.init(MetricsSystemImpl.java:133)
        at org.apache.hadoop.metrics2.lib.DefaultMetricsSystem.init(DefaultMetricsSystem.java:40)
        at org.apache.hadoop.metrics2.lib.DefaultMetricsSystem.initialize(DefaultMetricsSystem.java:50)
        at org.apache.hadoop.hdfs.server.datanode.DataNode.instantiateDataNode(DataNode.java:1520)
        at org.apache.hadoop.hdfs.server.datanode.DataNode.createDataNode(DataNode.java:1539)
        at org.apache.hadoop.hdfs.server.datanode.DataNode.secureMain(DataNode.java:1665)
        at org.apache.hadoop.hdfs.server.datanode.DataNode.main(DataNode.java:1682)
Caused by: java.net.UnknownHostException: PC_PAT1: Name or service not known
        at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
        at java.net.InetAddress$1.lookupAllHostAddr(InetAddress.java:901)
        at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1293)
        at java.net.InetAddress.getLocalHost(InetAddress.java:1469)
        ... 11 more

解决方案是在/etc/hosts文件中添加如下红色的内容，即172.18.140.24 PC_PAT1

hadoop@PC_PAT1:~/hadoop-1.0.3> less /etc/hosts
#
# hosts         This file describes a number of hostname-to-address
#               mappings for the TCP/IP subsystem.  It is mostly
#               used at boot time, when no name servers are running.
#               On small systems, this file can be used instead of a
#               "named" name server.
# Syntax:
#    
# IP-Address  Full-Qualified-Hostname  Short-Hostname
#

#127.0.0.1      localhost
#127.0.0.1      localhost.localdomain localhost cpyftest-2
127.0.0.1       localhost localhost.localdomain localhost4 localhost4.localdomain4
172.18.140.24   PC_PAT1
# special IPv6 addresses
::1             localhost ipv6-localhost ipv6-loopback

fe00::0         ipv6-localnet

ff00::0         ipv6-mcastprefix
ff02::1         ipv6-allnodes
ff02::2         ipv6-allrouters
ff02::3         ipv6-allhosts

添加后，hostname –i 将会得到正确的结果：

将hostname unkown的问题解决后，重启hadoop集群，可以发现，datanode/tasktracker , namenode/jobtracker都启动了：

ERROR org.apache.hadoop.hdfs.server.namenode.NameNode: java.io.IOException: Incomplete HDFS URI, no host: hdfs://data_181.uc:9000

着实郁闷到了，原来在Hadoop中，主机名不要包含下划线“_”，但可以有横线“-”。

Incomplete HDFS URI