错误1.由linux下允许打开的最大文件数量引起
错误消息：
java.io.IOException: background merge hit exception: _0:C500->_0 _1:C500->_0 _2:C500->_..... [optimize]
at org.apache.lucene.index.IndexWriter.optimize(IndexWriter.java:2310)
at org.apache.lucene.index.IndexWriter.optimize(IndexWriter.java:2249)
at org.apache.lucene.index.IndexWriter.optimize(IndexWriter.java:2219)
at org.apache.nutch.indexer.lucene.LuceneWriter.close(LuceneWriter.java:237)
at org.apache.nutch.indexer.IndexerOutputFormat$1.close(IndexerOutputFormat.java:48)
at org.apache.Hadoop.mapred.ReduceTask.runOldReducer(ReduceTask.java:474)
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:411)
at org.apache.hadoop.mapred.Child.main(Child.java:170)
Caused by: java.io.FileNotFoundException: /var/lib/crawlzilla/nutch-crawler/mapred/local/index/_682243155/_6a.frq (Too many open files)
at java.io.RandomAccessFile.open(Native Method)
at java.io.RandomAccessFile.(RandomAccessFile.java:212)
at org.apache.lucene.store.SimpleFSDirectory$SimpleFSIndexInput$Descriptor.(SimpleFSDirectory.java:76)
at org.apache.lucene.store.SimpleFSDirectory$SimpleFSIndexInput.(SimpleFSDirectory.java:97)
at org.apache.lucene.store.NIOFSDirectory$NIOFSIndexInput.(NIOFSDirectory.java:87)
at org.apache.lucene.store.NIOFSDirectory.openInput(NIOFSDirectory.java:67)
at org.apache.lucene.index.SegmentReader$CoreReaders.(SegmentReader.java:129)
at org.apache.lucene.index.SegmentReader.get(SegmentReader.java:576)
at org.apache.lucene.index.IndexWriter$ReaderPool.get(IndexWriter.java:609)
at org.apache.lucene.index.IndexWriter.mergeMiddle(IndexWriter.java:4239)
at org.apache.lucene.index.IndexWriter.merge(IndexWriter.java:3917)
at org.apache.lucene.index.ConcurrentMergeScheduler.doMerge(ConcurrentMergeScheduler.java:231)
at org.apache.lucene.index.ConcurrentMergeScheduler$MergeThread.run(ConcurrentMergeScheduler.java:288)
原因及解决方法：
Java程序运行在Unix/Linux环境下，并且该Java程序需要对文件做大量的操作，则会产生这样的异常。
Unix/Linux环境下有文件句柄的限制，可以使用ulimit -n查看当前环境允许打开的文件句柄数量，默认为1024。但是在我们的Java程序并发接近于或者多余1024而同时又在频繁的读写文件，所以会出
现以上异常，解决方式是按实际需要增大对文件句柄的限制数。
命令：
ulimit –n 32768
错误2.硬盘空间不足
错误消息：
Error: java.io.IOException: No space left on device
at java.io.FileOutputStream.writeBytes(Native Method)
at java.io.FileOutputStream.write(FileOutputStream.java:260)
at org.apache.hadoop.fs.RawLocalFileSystem$LocalFSFileOutputStream.write(RawLocalFileSystem.java:190)
at java.io.BufferedOutputStream.write(BufferedOutputStream.java:105)
at org.apache.hadoop.fs.FSDataOutputStream$PositionCache.write(FSDataOutputStream.java:49)
at java.io.DataOutputStream.write(DataOutputStream.java:90)
at org.apache.hadoop.mapred.IFileOutputStream.write(IFileOutputStream.java:84)
at org.apache.hadoop.fs.FSDataOutputStream$PositionCache.write(FSDataOutputStream.java:49)
at java.io.DataOutputStream.write(DataOutputStream.java:90)
at org.apache.hadoop.mapred.IFile$Writer.append(IFile.java:218)
at org.apache.hadoop.mapred.Merger.writeFile(Merger.java:157)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$LocalFSMerger.run(ReduceTask.java:2454)
原因及解决方法：
当硬盘不足时nutch会等待可用空间，如果是分布式的话可以增加一个运行节点。
错误3.namenode ID冲突导致HDFS不能启动
错误消息：
2010-07-21 10:12:11,987 ERROR org.apache.Hadoop.hdfs.server.datanode.DataNode: java.io.IOException: Incompatible namespaceIDs in
/home/admin/joe.wangh/hadoop/data/dfs.data.dir: namenode namespaceID = 898136669; datanode namespaceID = 2127444065
at org.apache.hadoop.hdfs.server.datanode.DataStorage.doTransition(DataStorage.java:233)
at org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:148)
at org.apache.hadoop.hdfs.server.datanode.DataNode.startDataNode(DataNode.java:288)
at org.apache.hadoop.hdfs.server.datanode.DataNode.<init>(DataNode.java:206)
at org.apache.hadoop.hdfs.server.datanode.DataNode.makeInstance(DataNode.java:1239)
at org.apache.hadoop.hdfs.server.datanode.DataNode.instantiateDataNode(DataNode.java:1194)
at org.apache.hadoop.hdfs.server.datanode.DataNode.createDataNode(DataNode.java:1202)
at org.apache.hadoop.hdfs.server.datanode.DataNode.main(DataNode.java:1324)
......
原因及解决方法：
每次namenode format会重新创建一个namenodeId,而tmp/dfs/data下包含了上次format下的id,namenode format清空了namenode下的数据,但是没有晴空datanode下的数据,导致启动时失败,所要做的就
是每次fotmat前,清空tmp一下的所有目录。
解决方法1:
停掉hadoop集群，然后将本地的datanode的数据文件夹删除 。文件夹位置在conf/hdfs-site.xml中配置，默认的话放在/usr/local/hadoop-datastore/hadoop-hadoop(这个是该机器的登录
名)/dfs/data ,但我的默认放在 /tmp/hadoop-dev(我的hadoop启动的用户名) /dfs/data
解决方法2（推荐）:
停止datanode节点，找到<dfs.data.dir>/current/VERSION，把里面的namenode id改为错误信息里提示的，我这例子里是898136669
错误4.配置文件出错
错误消息：
[Fatal Error] hadoop-site.xml:15:7: The content of elements must consist of well
-formed character data or markup.
Exception in thread "main" java.lang.RuntimeException: org.xml.sax.SAXParseException:
The content of elements must consist of well-formed character data or mark up.
原因及解决方法
nutch-site.xml等配置文件其中一个标签</property>前面多了一个尖括号。