最近在搭建hadoop 集群的过程中， 格式化namenode 时出现了一个错误就是:
Re-format filesystem in Storage Directory /home/hadoop/hadoop/tmp/dfs/name ? (Y or N) Y
18/05/28 03:56:36 WARN net.DNS: Unable to determine local hostname -falling back to "localhost"
java.net.UnknownHostException: slave4: slave4: Name or service not known
at java.net.InetAddress.getLocalHost(InetAddress.java:1505)
at org.apache.hadoop.net.DNS.resolveLocalHostname(DNS.java:264)
at org.apache.hadoop.net.DNS.<clinit>(DNS.java:57)
at org.apache.hadoop.hdfs.server.namenode.NNStorage.newBlockPoolID(NNStorage.java:982)
at org.apache.hadoop.hdfs.server.namenode.NNStorage.newNamespaceInfo(NNStorage.java:591)
at org.apache.hadoop.hdfs.server.namenode.FSImage.format(FSImage.java:157)
at org.apache.hadoop.hdfs.server.namenode.NameNode.format(NameNode.java:992)
at org.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:1434)
at org.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:1559)
Caused by: java.net.UnknownHostException: slave4: Name or service not known
at java.net.Inet4AddressImpl.lookupAllHostAddr(Native Method)
at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:928)
at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1323)
at java.net.InetAddress.getLocalHost(InetAddress.java:1500)
... 8 more
18/05/28 03:56:36 WARN net.DNS: Unable to determine address of the host-falling back to "localhost" address
java.net.UnknownHostException: slave4: slave4: Name or service not known
at java.net.InetAddress.getLocalHost(InetAddress.java:1505)
at org.apache.hadoop.net.DNS.resolveLocalHostIPAddress(DNS.java:287)
at org.apache.hadoop.net.DNS.<clinit>(DNS.java:58)
at org.apache.hadoop.hdfs.server.namenode.NNStorage.newBlockPoolID(NNStorage.java:982)
at org.apache.hadoop.hdfs.server.namenode.NNStorage.newNamespaceInfo(NNStorage.java:591)
at org.apache.hadoop.hdfs.server.namenode.FSImage.format(FSImage.java:157)
at org.apache.hadoop.hdfs.server.namenode.NameNode.format(NameNode.java:992)
at org.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:1434)
at org.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:1559)
Caused by: java.net.UnknownHostException: slave4: Name or service not known
at java.net.Inet4AddressImpl.lookupAllHostAddr(Native Method)
at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:928)
at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1323)
at java.net.InetAddress.getLocalHost(InetAddress.java:1500)
... 8 more
并没有出现期望的 successfully formatted 提示
想了一下，知道是主机名应该有错误，可是排查错误 hostname 也是正确的， 就查看slaves 配置文件发现没有改过来，还有就是hosts 文件的映射要提前改过来， ip 主机名要对应，否则会提示这种错误。 改过来后重新格式化正确。
格式化成功，我们启动一下 集群的节点。
命令: hadoop-daemons.sh start namenode
记住: 环境变量需要提前配置好的，否则使用起来比较麻烦的。