往Hadoop的HDFS写文件，出现下面问题
Stacktrace:] with root cause
org.apache.hadoop.ipc.RemoteException: org.apache.hadoop.hdfs.server.namenode.SafeModeException: Cannot create file/user/hadoop/building/001/44/01/17/010/building.txt. Name node is in safe mode.
The ratio of reported blocks 0.8333 has not reached the threshold 0.9990. Safe mode will be turned off automatically.
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.startFileInternal(FSNamesystem.java:1220)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.appendFile(FSNamesystem.java:1426)
原因：有一台datanode服务器没有启动，导致The ratio of reported blocks 0.8333小于threshold 0.9990，从而Safe mode will be turned off automatically，无法写文件。