The NameNode is the most critical piece of the system of an HDFS file system. The NameNode manages the entire HDFS file system metadata (i.e owners of files, file permission, no of blocks, block locations, size etc.) and maintained it in main memory. Clients first contact point is the NameNode for file metadata and then perform actual file I/O directly with the DataNodes. If something goes wrong with the NameNode, then whatever metadata was there in main memory would get lost permanently.
Hadoop High Availability
Hadoop HDFS High Availability
Prior to Hadoop 2.x (Hadoop 1.x), the NameNode was a single point of failure (SPOF) in an HDFS cluster. Each cluster had a single NameNode, and if that machine or process became unavailable, the cluster as a whole would be unavailable until the NameNode was either restarted or brought up on a separate machine.
This reduced the total availability of the HDFS cluster in two major ways: