一、登录Cloudera Manager (http://192.168.201.128:7180/cmf/login)时，无法访问web页面
针对此问题网上有较多的解决方案（e.g. https://www.cnblogs.com/zlslch/p/7078119.html)， 如果还不能解决你的问题，请看下面的解决方案。
登录MySQL数据库（或利用Navicat），会发现有一个mysql数据库（下图所示），在mysql数据库中有一个user表，将User="root"的两条记录进行删除
select * from user; delete from user where User='root';
再次登录http://192.168.201.128:7180/cmf/login，发现登录成功！
二、利用Navicat连接MySql数据库时，错误信息：Can't connect to MySQL server on 'xxxxx'(10038)
解决方案：
查看网络的端口信息：netstat -ntpl，下图状态为正常状态（不是请进行如下操作），如果没有netstat，在CentOS 7下利用yum -y install net-tools进行安装。
查看防火墙的状态，发现3306的端口是丢弃状态：
iptables -vnL
这里要清除防火墙中链中的规则
iptables -F
再次连接MySql数据库，发现连接成功！
三、无法启动NameNode，查看日志发现如下错误...
Exception in thread "main" org.apache.hadoop.ipc.RemoteException(org.apache.hadoop.hdfs.server.namenode.SafeModeException): Cannot delete /tmp/hadoop-yarn/staging/hadoop/.staging/job_1490689337938_0001. Name node is in safe mode.
The reported blocks 48 needs additional 5 blocks to reach the threshold 0.9990 of total blocks 53.
The number of live datanodes 2 has reached the minimum number 0. Safe mode will be turned off automatically once the thresholds have been reached.
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.checkNameNodeSafeMode(FSNamesystem.java:1327)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.delete(FSNamesystem.java:3713)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.delete(NameNodeRpcServer.java:953)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.delete(ClientNamenodeProtocolServerSideTranslatorPB.java:611)
at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:616)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:982)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2049)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2045)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:422)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1698)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2045)
什么是安全模式？
安全模式是HDFS所处的一种特殊状态，在这种状态下，文件系统只接受读数据请求，而不接受删除、修改等变更请求。在NameNode主节点启动时，HDFS首先进入安全模式，DataNode在启动的时候会向namenode汇报可用的block等状态，当整个系统达到安全标准时，HDFS自动离开安全模式。如果HDFS出于安全模式下，则文件block不能进行任何的副本复制操作，因此达到最小的副本数量要求是基于datanode启动时的状态来判定的，启动时不会再做任何复制（从而达到最小副本数量要求）原博文：https://blog.csdn.net/bingduanlbd/article/details/51900512。
1、集群升级维护时手动进入安全模式
hadoop dfsadmin -safemode enter
2、退出安全模式：
hadoop dfsadmin -safemode leave
3、返回安全模式是否开启的信息
hadoop dfsadmin -safemode get
因此，当发现namenode处于安全模式，无法启动时，可以使用hadoop dfsadmin -safemode leave退出安全模式，重启namenode解决问题！
四、INFO hdfs.DFSClient: Exception in createBlockOutputStream java.net.NoRouteToHostException: No route to host
16/07/27 01:29:26 INFO hdfs.DFSClient: Exception in createBlockOutputStream
java.net.NoRouteToHostException: No route to host
at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)
at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:717)
at org.apache.hadoop.net.SocketIOWithTimeout.connect(SocketIOWithTimeout.java:206)
at org.apache.hadoop.net.NetUtils.connect(NetUtils.java:531)
at org.apache.hadoop.hdfs.DFSOutputStream.createSocketForPipeline(DFSOutputStream.java:1537)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.createBlockOutputStream(DFSOutputStream.java:1313)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.nextBlockOutputStream(DFSOutputStream.java:1266)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:449)
16/07/27 01:29:26 INFO hdfs.DFSClient: Abandoning BP-555863411-172.16.95.100-1469590594354:blk_1073741825_1001
16/07/27 01:29:26 INFO hdfs.DFSClient: Excluding datanode DatanodeInfoWithStorage[172.16.95.101:50010,DS-ee00e1f8-5143-4f06-9ef8-b0f862fce649,DISK]
16/07/27 01:29:26 INFO hdfs.DFSClient: Exception in createBlockOutputStream
java.net.NoRouteToHostException: No route to host
at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)
at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:717)
at org.apache.hadoop.net.SocketIOWithTimeout.connect(SocketIOWithTimeout.java:206)
at org.apache.hadoop.net.NetUtils.connect(NetUtils.java:531)
at org.apache.hadoop.hdfs.DFSOutputStream.createSocketForPipeline(DFSOutputStream.java:1537)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.createBlockOutputStream(DFSOutputStream.java:1313)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.nextBlockOutputStream(DFSOutputStream.java:1266)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:449)
16/07/27 01:29:26 INFO hdfs.DFSClient: Abandoning BP-555863411-172.16.95.100-1469590594354:blk_1073741826_1002
16/07/27 01:29:26 INFO hdfs.DFSClient: Excluding datanode DatanodeInfoWithStorage[172.16.95.102:50010,DS-eea51eda-0a07-4583-9eee-acd7fc645859,DISK]
16/07/27 01:29:26 WARN hdfs.DFSClient: DataStreamer Exception
org.apache.hadoop.ipc.RemoteException(java.io.IOException): File /wc/mytemp/123._COPYING_ could only be replicated to 0 nodes instead of minReplication (=1). There are 2 datanode(s) running and 2 node(s) are excluded in this operation.
at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.chooseTarget4NewBlock(BlockManager.java:1547)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getNewBlockTargets(FSNamesystem.java:3107)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getAdditionalBlock(FSNamesystem.java:3031)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.addBlock(NameNodeRpcServer.java:724)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.addBlock(ClientNamenodeProtocolServerSideTranslatorPB.java:492)
at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:616)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:969)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2049)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2045)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:422)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1657)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2043)
at org.apache.hadoop.ipc.Client.call(Client.java:1475)
at org.apache.hadoop.ipc.Client.call(Client.java:1412)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Invoker.invoke(ProtobufRpcEngine.java:229)
at com.sun.proxy.$Proxy9.addBlock(Unknown Source)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolTranslatorPB.addBlock(ClientNamenodeProtocolTranslatorPB.java:418)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:191)
at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:102)
at com.sun.proxy.$Proxy10.addBlock(Unknown Source)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.locateFollowingBlock(DFSOutputStream.java:1459)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.nextBlockOutputStream(DFSOutputStream.java:1255)
at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:449)
put: File /wc/mytemp/123._COPYING_ could only be replicated to 0 nodes instead of minReplication (=1). There are 2 datanode(s) running and 2 node(s) are excluded in this operation.
[hadoop@master bin]$ service firewall
The service command supports only basic LSB actions (start, stop, restart, try-restart, reload, force-reload, status). For other actions, please try to use systemctl.
- 检查防火墙是否关闭，如果没有关闭将所有节点的防火墙进行关闭。在CentOS 6中命令 service iptables stop，在CentOS 7中命令 service firewalld stop
- 检查所有的主机，/etc/selinux/config下的SELINUX，设置SELINUX=disabled。
- 再检测上述问题即可解决。为了防止防火墙开机重启，执行命令systemctl disable firewalld.service
五、hadoop 运行出错，发现是ClusterId不一致问题
- 进入/dfs/nn/current，利用cat VERSION，查看确认各节点的clusterID是否一致
- 如果不一致，将主节点的clusterID进行拷贝，并修改各不一致的子节点的clusterID，保存退出，即可解决问题！
六、解决SecureCRT等软件连接Linux速度缓慢问题，(有时出现 The semaphore timeout period has expired)
编辑sshd_config文件 ---->vi /etc/ssh/sshd_config
在文件ssh_config中添加如下代码，并保存退出，重启service sshd restart 或重启 reboot 即可。
两种情况类似一块儿处理了.......，避免再出现问题
UseDns no ClientAliveInterval 60
七、登录MySQL5.7时出现ERROR 1045 (28000): Access denied for user 'root'@'localhost'
- 使用vi /etc/my.cnf，打开mysql配置文件，在文件中[mysqld]加入代码 skip-grant-tables, 退出并保存。
- 使用service mysql restart, 重启MySQL服务
- 然后再次进入到终端当中，敲入
mysql -u root -p命令然后回车，当需要输入密码时，直接按enter键，便可以不用密码登录到数据库当中 update mysql.user set authentication_string=password('123456') where user='root';- 再次进入到之前的配置文件中，将代码 skip-grant-tables进行删除即可。
- 如果实在不行，请参考https://www.cnblogs.com/yanqr/p/9753445.html
