java.io.IOException: File /home/hadoop/mapred/system/jobtracker.info could only be replicated to 0 n

我又再次在关闭hadoop前format了，然后又出现这个错误。

java.io.IOException: File /home/hadoop/mapred/system/jobtracker.info could only be replicated to 0 nodes, instead of 1
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getAdditionalBlock(FSNamesystem.java:1558)
at org.apache.hadoop.hdfs.server.namenode.NameNode.addBlock(NameNode.java:696)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:563)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1388)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1384)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1121)

at org.apache.hadoop.ipc.Server$Handler.run(Server.java:1382)

查看datanode的log可以看见：

2012-08-15 07:55:44,922 ERROR org.apache.hadoop.hdfs.server.datanode.DataNode: java.io.IOException: Incompatible namespaceIDs
in /home/hadoop/hdfs/data: namenode namespaceID = 1505547525; datanode namespaceID = 709954036
at org.apache.hadoop.hdfs.server.datanode.DataStorage.doTransition(DataStorage.java:232)
at org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:147)
at org.apache.hadoop.hdfs.server.datanode.DataNode.startDataNode(DataNode.java:385)
at org.apache.hadoop.hdfs.server.datanode.DataNode.<init>(DataNode.java:299)
at org.apache.hadoop.hdfs.server.datanode.DataNode.makeInstance(DataNode.java:1582)
at org.apache.hadoop.hdfs.server.datanode.DataNode.instantiateDataNode(DataNode.java:1521)
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)

解决方法（前面安装那篇提到过）：

    关闭hadoop，找到datanode下面文件系统的VERSION文件，将其中的namespaceID改为上面红字的namenode namespaceID，然后启动hadoop即可。