The Hadoop in Real World team doesn’t need to play it safe:
When namenode is started or restarted, namenode will be in safemode for a period of time. At this time you will not be able to use your Hadoop cluster fully. Write operations to HDFS will fail and because of that your MapReduce jobs will also fail.
Read on for other reasons why the namenode might be stuck in safe mode and what you can do to fix it.