今天启动hadoop时，发现datanode启动不了，查看日志发现出现以下的错误：
java.io.IOException: File /opt/hadoop/tmp/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:1271)
at org.apache.hadoop.hdfs.server.namenode.NameNode.addBlock(NameNode.java:422)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:508)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:959)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:955)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:953)
at org.apache.hadoop.ipc.Client.call(Client.java:740)
at org.apache.hadoop.ipc.RPC$Invoker.invoke(RPC.java:220)
at com.sun.proxy.$Proxy2.addBlock(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:82)
at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:59)
at com.sun.proxy.$Proxy2.addBlock(Unknown Source)
at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.locateFollowingBlock(DFSClient.java:2937)
at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.nextBlockOutputStream(DFSClient.java:2819)
at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.access$2000(DFSClient.java:2102)
at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream$DataStreamer.run(DFSClient.java:2288)
发现原因：原来是前天格式化了hdfs，每次格式化时（namenode format）会重新创建一个namenodeId,而dfs.data.dir参数配置的目录中包含的是上次format创建的id,和dfs.name.dir参数配置的目录中的id不一致。namenode format清空了namenode下的数据,但是没有清空datanode下的数据,导致启动时失败。
解决方法：我是重新创建dfs.data.dir指定的文件夹，然后修改成该文件夹，如原本hdfs-site.xml中dfs.data.dir的参数是/opt/hadoop/data，我在/opt/hadoop下创建了data1的文件夹，然后将dfs.data.dir的参数改为：/opt/hadoop/data1，最后重新格式化hdfs (hadoop namenode -format)，问题解决.