Hadoop NameNode 是 HDFS 文件系统的集中位置，它保存文件系统中所有文件的目录树，并跟踪文件数据在集群中的位置保持.简而言之，它保留与数据节点相关的元数据.当我们格式化 namenode 时，它​​会格式化与数据节点相关的元数据.这样做，数据节点上的所有信息都会丢失，并且可以重新用于新数据.

Hadoop NameNode is the centralized place of an HDFS file system which keeps the directory tree of all files in the file system, and tracks where across the cluster the file data is kept. In short, it keeps the metadata related to datanodes. When we format namenode it formats the meta-data related to data-nodes. By doing that, all the information on the datanodes are lost and they can be reused for new data.