Safe mode is indicative of the administrative mode and this is used for maintenance purposes. For the hadoop HDFS cluster, this is a read-only mode and this mode forbids any modifications or changes to the blocks or file system present within the HDFS. So, basically, at the start of the Namenode, the following occurs:
- The file system namespace is loaded from the last saved FsImage into the main memory and the log files are edited.
- After each merge, the log files are edited which in turn creates a new file system namespace.
- All the block location data is received as block reports.
When the HDFS is started up, Namenode goes into safe mode by default until the data nodes have reported that most of the blocks are functioning.