NameNode is the heart of HDFS. NameNode maintains the metadata of HDFS – files, list of blocks, directories, permissions etc. The metadata is persisted on a file named FSIMAGE. During the start up of NameNode, the FSIMAGE file will be read and loaded into memory.
Any ongoing changes to the files, directories in FSIMAGE will be written to memory and to a temporary log file. NameNode does not save the ongoing changes to FSIMAGE directly and this is because FSIMAGE file could be big for a big HDFS and updating a big file at runtime will be quite expensive and slow.
Read on to learn what the secondary NameNode does. As a hint, it’s not a secondary NameNode in the sense of high availability. If you’re a new Hadoop administrator, the name can be deceiving, letting you think you have high availability when you really don’t.