有好几次，启动Hadoop和HBase之后，执行jps命令，已经看到有HMaster的进程，
但是进入到HBase的shell，执行一个命令，会出现下面的错误：
ERROR: org.apache.hadoop.hbase.MasterNotRunningException: Retried 7 times
进入到logs目录查看master的日志：发现一直显示下面的内容：
2013-04-13 17:13:17,374 INFO org.apache.hadoop.hbase.util.FSUtils: Waiting for dfs to exit safe mode... 2013-04-13 17:13:27,377 INFO org.apache.hadoop.hbase.util.FSUtils: Waiting for dfs to exit safe mode... 2013-04-13 17:13:37,386 INFO org.apache.hadoop.hbase.util.FSUtils: Waiting for dfs to exit safe mode... 2013-04-13 17:13:47,393 INFO org.apache.hadoop.hbase.util.FSUtils: Waiting for dfs to exit safe mode... 2013-04-13 17:13:57,395 INFO org.apache.hadoop.hbase.util.FSUtils: Waiting for dfs to exit safe mode... 2013-04-13 17:14:07,409 INFO org.apache.hadoop.hbase.util.FSUtils: Waiting for dfs to exit safe mode...
原来是Hadoop在刚启动的时候，还处在安全模式造成的。
[coder@h1 hadoop-0.20.2]$ bin/hadoop dfsadmin -safemode get
Safe mode is ON
[coder@h1 hadoop-0.20.2]$
可等Hadoop退出安全模式后再执行HBase命令，或者手动退出Hadoop的安全模式
[coder@h1 hadoop-0.20.2]$ bin/hadoop dfsadmin -safemode leave
Safe mode is OFF
[coder@h1 hadoop-0.20.2]$
现在再执行HBase的命令就没有问题了：
[coder@h1 hbase-0.90.5]$ bin/hbase shell HBase Shell; enter 'help<RETURN>' for list of supported commands. Type "exit<RETURN>" to leave the HBase Shell Version 0.90.5, r1212209, Fri Dec 9 05:40:36 UTC 2011 hbase(main):001:0> list TABLE student user 2 row(s) in 0.7530 seconds hbase(main):002:0>
