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:
Hadoop ABCD 