查看日志如下:
2014-12-22 12:08:27,264 INFO org.mortbay.log: Started HttpServer2$SelectChannelConnectorWithSafeStartup@0.0.0.0:50075
2014-12-22 12:08:27,692 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: dnUserName = root
2014-12-22 12:08:27,692 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: supergroup = supergroup
2014-12-22 12:08:32,865 INFO org.apache.hadoop.ipc.CallQueueManager:
Using callQueue class java.util.concurrent.LinkedBlockingQueue
2014-12-22 12:08:32,889 INFO org.apache.hadoop.ipc.Server: Starting Socket Reader #1 for port 50020
2014-12-22 12:08:32,931 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Opened IPC server at /0.0.0.0:50020
2014-12-22 12:08:32,945 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Refresh request received for nameservices: null
2014-12-22 12:08:32,968 INFO
org.apache.hadoop.hdfs.server.datanode.DataNode: Starting
BPOfferServices for nameservices: <default>
2014-12-22 12:08:32,992 INFO
org.apache.hadoop.hdfs.server.datanode.DataNode: Block pool
<registering> (Datanode Uuid unassigned) service to
localhost/127.0.0.1:8020 starting to offer service
2014-12-22 12:08:33,001 INFO org.apache.hadoop.ipc.Server: IPC Server Responder: starting
2014-12-22 12:08:33,003 INFO org.apache.hadoop.ipc.Server: IPC Server listener on 50020: starting
2014-12-22 12:08:33,536 INFO org.apache.hadoop.hdfs.server.common.Storage: DataNode version: -56 and NameNode layout version: -60
2014-12-22 12:08:33,699 INFO
org.apache.hadoop.hdfs.server.common.Storage: Lock on
/home/hadoop/tmp/dfs/data/in_use.lock acquired by nodename
17247@henry-ThinkPad-T400
2014-12-22 12:08:33,706 FATAL
org.apache.hadoop.hdfs.server.datanode.DataNode: Initialization failed
for Block pool <registering> (Datanode Uuid unassigned) service to
localhost/127.0.0.1:8020. 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:646)
at org.apache.hadoop.hdfs.server.datanode.DataStorage.addStorageLocations(DataStorage.java:320)
at org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:403)
at org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:422)
at org.apache.hadoop.hdfs.server.datanode.DataNode.initStorage(DataNode.java:1311)
at org.apache.hadoop.hdfs.server.datanode.DataNode.initBlockPool(DataNode.java:1276)
at org.apache.hadoop.hdfs.server.datanode.BPOfferService.verifyAndSetNamespaceInfo(BPOfferService.java:314)
at org.apache.hadoop.hdfs.server.datanode.BPServiceActor.connectToNNAndHandshake(BPServiceActor.java:220)
at org.apache.hadoop.hdfs.server.datanode.BPServiceActor.run(BPServiceActor.java:828)
at java.lang.Thread.run(Thread.java:662)
2014-12-22 12:08:33,716 WARN
org.apache.hadoop.hdfs.server.datanode.DataNode: Ending block pool
service for: Block pool <registering> (Datanode Uuid unassigned)
service to localhost/127.0.0.1:8020
2014-12-22 12:08:33,718 INFO
org.apache.hadoop.hdfs.server.datanode.DataNode: Removed Block pool
<registering> (Datanode Uuid unassigned)
2014-12-22 12:08:35,718 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Exiting Datanode
2014-12-22 12:08:35,720 INFO org.apache.hadoop.util.ExitUtil: Exiting with status 0
2014-12-22 12:08:35,722 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: SHUTDOWN_MSG:
/************************************************************
SHUTDOWN_MSG: Shutting down DataNode at henry-ThinkPad-T400/127.0.0.1
************************************************************/
从日志上看，加粗的部分说明了问题
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
20131 SecondaryNameNode
20449 NodeManager
19776 NameNode
21123 Jps
19918 DataNode
20305 ResourceManager
出现该问题的原因：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