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