问题描述：第一次启动输入jps都有，第二次没有datanode
日志如下：
查看日志如下: -- ::, INFO org.mortbay.log: Started HttpServer2$SelectChannelConnectorWithSafeStartup@0.0.0.0:
-- ::, INFO org.apache.hadoop.hdfs.server.datanode.DataNode: dnUserName = root
-- ::, INFO org.apache.hadoop.hdfs.server.datanode.DataNode: supergroup = supergroup
-- ::, INFO org.apache.hadoop.ipc.CallQueueManager: Using callQueue class java.util.concurrent.LinkedBlockingQueue
-- ::, INFO org.apache.hadoop.ipc.Server: Starting Socket Reader # for port
-- ::, INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Opened IPC server at /0.0.0.0:
-- ::, INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Refresh request received for nameservices: null
-- ::, INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Starting BPOfferServices for nameservices: <default>
-- ::, INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Block pool <registering> (Datanode Uuid unassigned) service to localhost/127.0.0.1: starting to offer service
-- ::, INFO org.apache.hadoop.ipc.Server: IPC Server Responder: starting
-- ::, INFO org.apache.hadoop.ipc.Server: IPC Server listener on : starting
-- ::, INFO org.apache.hadoop.hdfs.server.common.Storage: DataNode version: - and NameNode layout version: - -- ::, INFO org.apache.hadoop.hdfs.server.common.Storage: Lock on /home/hadoop/tmp/dfs/data/in_use.lock acquired by nodename @henry-ThinkPad-T400
-- ::, FATAL org.apache.hadoop.hdfs.server.datanode.DataNode: Initialization failed for Block pool <registering> (Datanode Uuid unassigned) service to localhost/127.0.0.1:. Exiting.
java.io.IOException: Incompatible clusterIDs in /home/hadoop/tmp/dfs/data: namenode clusterID = CID-19f887ba-2e8d-4c7e-ae01-e38a30581693; datanode clusterID = CID-14aac0b3-3c32-45db-adb8-b5fc494eaa3d
at org.apache.hadoop.hdfs.server.datanode.DataStorage.doTransition(DataStorage.java:)
at org.apache.hadoop.hdfs.server.datanode.DataStorage.addStorageLocations(DataStorage.java:)
at org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:)
at org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:)
at org.apache.hadoop.hdfs.server.datanode.DataNode.initStorage(DataNode.java:)
at org.apache.hadoop.hdfs.server.datanode.DataNode.initBlockPool(DataNode.java:)
at org.apache.hadoop.hdfs.server.datanode.BPOfferService.verifyAndSetNamespaceInfo(BPOfferService.java:)
at org.apache.hadoop.hdfs.server.datanode.BPServiceActor.connectToNNAndHandshake(BPServiceActor.java:)
at org.apache.hadoop.hdfs.server.datanode.BPServiceActor.run(BPServiceActor.java:)
at java.lang.Thread.run(Thread.java:)
-- ::, WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Ending block pool service for: Block pool <registering> (Datanode Uuid unassigned) service to localhost/127.0.0.1:
-- ::, INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Removed Block pool <registering> (Datanode Uuid unassigned)
-- ::, WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Exiting Datanode
-- ::, INFO org.apache.hadoop.util.ExitUtil: Exiting with status
-- ::, INFO org.apache.hadoop.hdfs.server.datanode.DataNode: SHUTDOWN_MSG:
/************************************************************
SHUTDOWN_MSG: Shutting down DataNode at henry-ThinkPad-T400/127.0.0.1
************************************************************/
问题分析：在第一次格式化dfs后，启动并使用了hadoop，后来又重新执行了格式化命令（hdfs namenode -format)，
这时namenode的clusterID会重新生成，而datanode的clusterID 保持不变。
问题总结：datanode的clusterID 和 namenode的clusterID 不匹配。
解决办法：根据日志中的路径，cd /home/hadoop/tmp/dfs 能看到 data和name两个文件夹，
将name/current下的VERSION中的clusterID复制到data/current下的VERSION中，覆盖掉原来的clusterID，让两个保持一致
然后重启，启动后执行jps，查看进程