hadoop2.6分布式部署时 livenodes等于1的原因
1.问题描述
在进行hadoop2.x版本的hdfs分布式部署时，遇到了一个奇怪的问题：
使用start-dfs.sh命令启动dfs之后，所有的datanode节点上均能看到datanode进程，然而在namenode的web UI上，显示live nodes数目为1.
2.问题分析
打开hadoop2.x/logs文件夹下的hadoop-root-datanode.log文件，发现里面报了一个很有趣的异常：
2015-12-20 22:55:21,374 ERROR org.apache.hadoop.hdfs.server.datanode.DataNode: Initialization failed for Block pool BP-362474484-127.0.1.1-1450617599362 (Datanode Uuid d3a052d5-7319-4bdf-98e1-6eea4637cb3d) service to 192.168.1.126/192.168.1.126:8020 Datanode denied communication with namenode because hostname cannot be resolved (ip=192.168.1.125, hostname=192.168.1.125): DatanodeRegistration(0.0.0.0, datanodeUuid=d3a052d5-7319-4bdf-98e1-6eea4637cb3d, infoPort=50075, ipcPort=50020, storageInfo=lv=-56;cid=CID-09307029-b7c7-4163-b2f1-b96f6c630758;nsid=1007280041;c=0)
at org.apache.hadoop.hdfs.server.blockmanagement.DatanodeManager.registerDatanode(DatanodeManager.java:889)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.registerDatanode(FSNamesystem.java:5048)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.registerDatanode(NameNodeRpcServer.java:1142)
at org.apache.hadoop.hdfs.protocolPB.DatanodeProtocolServerSideTranslatorPB.registerDatanode(DatanodeProtocolServerSideTranslatorPB.java:92)
at org.apache.hadoop.hdfs.protocol.proto.DatanodeProtocolProtos$DatanodeProtocolService$2.callBlockingMethod(DatanodeProtocolProtos.java:27329)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:619)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:962)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2040)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2036)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:422)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1656)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2034)可以看到，它说因为不能解析主机地址，datanode拒绝通信。
那么我们去查hadoop的文档，发现它的hdfs-default.xml里有一个值 dfs.namenode.datanode.registration.ip-hostname-check。它的描述是这样的：
If true (the default), then the namenode requires that a connecting datanode's address must be resolved to a hostname. If necessary, a reverse DNS lookup is performed. All attempts to register a datanode from an unresolvable address are rejected. It is recommended that this setting be left on to prevent accidental registration of datanodes listed by hostname in the excludes file during a DNS outage. Only set this to false in environments where there is no infrastructure to support reverse DNS lookup.
所以，当我们在配置datanode时，如果不是使用了主机名加dns解析或者hosts文件解析的方式，而是直接使用ip地址去配置slaves文件，那么就会产生这个错误。
3、修正方式
把以下内容加入到hdfs-site.xml中，并同步至所有节点即可。
<property>
<name>dfs.namenode.datanode.registration.ip-hostname-check</name>
<value>false</value>
</property>