项目将近尾声，上线一切顺利，在开发过程中遇到了不少的问题，趁着空闲时间对项目中遇到的常见问题做一个总结，当作一个笔记，问题如下：
- java.io.IOException: Could not obtain block: blk_194219614024901469_1100 file=/user/hive/warehouse/src_20180124_log/src_20180124_log
出现这种情况大多是结点断了，没有连接上。检查配置，重新启动服务即可。
2. java.lang.OutOfMemoryError: Java heap space
出现这种异常，明显是jvm内存不够得原因，要修改所有的datanode的jvm内存大小。Java -Xms1024m -Xmx4096m
一般jvm的最大内存使用应该为总内存大小的一半，我们使用的8G内存，所以设置为4096m，这一值可能依旧不是最优的值。
3. IO写操作出现问题
0-1246359584298, infoPort=50075, ipcPort=50020):Got exception while serving blk_-5911099437886836280_1292 to /172.16.100.165:
java.net.SocketTimeoutException: 480000 millis timeout while waiting for channel to be ready for write. ch : java.nio.channels.SocketChannel[connected local=/
172.16.100.165:5001remote=/172.16.100.165:50930]atorg.apache.hadoop.net.SocketIOWithTimeout.waitForIO(SocketIOWithTimeout.java:185)at org.apache.hadoop.net.SocketOutputStream.waitForWritable(SocketOutputStream.java:159)
atorg.apache.hadoop.net.SocketOutputStream.transferToFully(SocketOutputStream.java:198)at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendChunks(BlockSender.java:293)
at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendBlock(BlockSender.java:387)
at org.apache.hadoop.hdfs.server.datanode.DataXceiver.readBlock(DataXceiver.java:179)
at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:94)
at java.lang.Thread.run(Thread.java:619)
It seems there are many reasons that it can timeout, the example given in
HADOOP-3831 is a slow reading client.
连接超时，解决办法：在hadoop-site.xml中设置dfs.datanode.socket.write.timeout=0。
4.解决hadoop OutOfMemoryError问题：
<property>
<name>mapred.child.java.opts</name>
<value>-Xmx800M -server</value>
</property>
或者：hadoop jar jarfile [main class] -D mapred.child.java.opts=-Xmx800M
5. Hadoop java.io.IOException: Job failed! at org.apache.hadoop.mapred.
JobClient.runJob(JobClient.java:1232) while indexing.
when i use nutch1.0,get this error:
Hadoop java.io.IOException: Job failed! at org.apache.hadoop.mapred.JobClient.
runJob(JobClient.java:1232) while indexing.
解决办法：可以删除conf/log4j.properties，然后可以看到详细的错误报告。如出现的是out of memory，解决办法是在给运行主类org.apache.nutch.crawl.Crawl加上参数：-Xms64m -Xmx512m。
6. Namenode in safe mode
解决方法：执行命令 bin/hadoop dfsadmin -safemode leave
7. java.net.NoRouteToHostException: No route to host
解决方法：sudo /etc/init.d/iptables stop
8．更改namenode后，在hive中运行select 依旧指向之前的namenode地址
解决办法：将metastore中的之前出现的namenode地址全部更换为现有的namenode地址
9. ERROR metadata.Hive (Hive.java:getPartitions(499)) - javax.jdo.JDODataStoreException: Required table missing : ""PARTITIONS"" in Catalog "" Schema "". JPOX requires this table to perform its persistence operations. Either your MetaData is incorrect, or you need to enable "org.jpox.autoCreateTables"
原因：就是因为在 hive-default.xml 里把 org.jpox.fixedDatastore 设置成 true 了，应该把值设为false。
10.INFO hdfs.DFSClient: Exception in createBlockOutputStream java.io.IOException:Bad connect ack with firstBadLink 192.168.1.11:50010
> INFO hdfs.DFSClient: Abandoning block blk_-8575812198227241296_1001
> INFO hdfs.DFSClient: Exception in createBlockOutputStream java.io.IOException:
Bad connect ack with firstBadLink 192.168.1.16:50010
> INFO hdfs.DFSClient: Abandoning block blk_-2932256218448902464_1001
> INFO hdfs.DFSClient: Exception in createBlockOutputStream java.io.IOException:
Bad connect ack with firstBadLink 192.168.1.11:50010
> INFO hdfs.DFSClient: Abandoning block blk_-1014449966480421244_1001号的
> INFO hdfs.DFSClient: Exception in createBlockOutputStream java.io.IOException:
Bad connect ack with firstBadLink 192.168.1.16:50010
> INFO hdfs.DFSClient: Abandoning block blk_7193173823538206978_1001
> WARN hdfs.DFSClient: DataStreamer Exception: java.io.IOException: Unable
to create new block.
at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.nextBlockOutputStream(DFSClient.java:2731)
>at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.access$2000(DFSClient.java:1996
at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream$DataStreamer.run(DFSClient.java:2182)
> WARN hdfs.DFSClient: Error Recovery for block blk_7193173823538206978_1001
bad datanode[2] nodes == null
> WARN hdfs.DFSClient: Could not get block locations. Source file "/user/umer/8GB_input"
- Aborting...
> put: Bad connect ack with firstBadLink 192.168.1.16:50010
解决方法：
1) '/etc/init.d/iptables stop' -->stopped firewall
2) SELINUX=disabled in '/etc/selinux/config' file.-->disabled selinux
11．某次正常运行mapreduce实例时,抛出错误
java.io.IOException: All datanodes xxx.xxx.xxx.xxx:xxx are bad. Aborting…
atorg.apache.hadoop.dfs.DFSClient$DFSOutputStream.processDatanodeError(DFSClient.java:2158)at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.access$1400(DFSClient.java:1735
at org.apache.hadoop.dfs.DFSClient$DFSOutputStream$DataStreamer.run(DFSClient.java:1889)
java.io.IOException: Could not get block locations. Aborting…at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.processDatanodeError(DFSClient.java:2143)
at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.access$1400(DFSClient.java:1735)
at org.apache.hadoop.dfs.DFSClient$DFSOutputStream$DataStreamer.run(DFSClient.java:1889)
问题原因是linux机器打开了过多的文件导致。解决方法：用命令ulimit -n可以发现linux默认的文件打开数目为1024，修改/ect/security/limit.conf，增加hadoop soft 65535，再重新运行程序（最好所有的datanode都修改），问题解决。
12. bin/hadoop jps后报如下异常：
Exception in thread "main" java.lang.NullPointerException
atsun.jvmstat.perfdata.monitor.protocol.local.LocalVmManager.activeVms(LocalVmManager.java:127)atsun.jvmstat.perfdata.monitor.protocol.local.MonitoredHostProvider.activeVms(MonitoredHostProvider.java:133)at sun.tools.jps.Jps.main(Jps.java:45)
解决方法：系统根目录/tmp文件夹被删除了。重新建立/tmp文件夹即可。
13. bin/hive中出现 unable to create log directory /tmp/
解决办法：系统根目录/tmp文件夹被删除了。重新建立/tmp文件夹即可。
14．MySQL报错
[root@localhost mysql]# ./bin/mysqladmin -u root password '123456'
./bin/mysqladmin: connect to server at 'localhost' failed
error: 'Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)'
Check that mysqld is running and that the socket: '/tmp/mysql.sock' exists!
解决不能通过mysql .sock连接MySQL问题 这个问题主要提示是，不能通过 '/tmp/mysql .sock'连到服务器，而php标准配置正是用过'/tmp/mysql .sock'，但是一些mysql 安装方法 将 mysql .sock放在/var/lib/mysql .sock或者其他的什么地方，可以通过修改/etc/my.cnf文件来修正它，打开文件，可以看到如下的配置：
[mysqld]
socket=/var/lib/mysql.sock
改一下就好了，但也会引起其他的问题，如mysql 程序连不上了，再加一点：
[mysql]
socket=/tmp/mysql.sock
或者还可以通过修改my.ini中的配置来使用其他的mysql.sock来连，或者用这样的方法:
ln -s /var/lib/mysql/mysql.sock /tmp/mysql.sock
成功了,就是这样ln -s /var/lib/mysql/mysql.sock /tmp/mysql.sock
15. NameNode不能切换
less $BEH_HOME/logs/hadoop/hadoop-hadoop-zkfc-hadoop001.log
日志文件中有错误提醒：Unable to create SSH session
com.jcraft.jsch,JSchException:java.io.FileNotFoundException:~/.ssh/id_rsa (No such file or directory)
解决办法：vim $HADOOP_HOME/etc/hadoop/HDFS-site.xml
修改以下参数：
<property>
<name>dfs.ha.fencing.ssh.private-key-files</name>
<value>/home/hadoop/.ssh/id_rsa</value>
<final>true</final>
<description/>
</property>