File system metadata is stored in two different constructs: the fsimage and the edit log. The fsimage is a file that represents a point-in-time snapshot of the filesystem’s metadata. However, while the fsimage file format is very efficient to read. Thus, rather than writing a new fsimage every time the namespace is modified, the NameNode instead records the modifying operation in the edit log for durability. This way, if the NameNode crashes, it can restore its state by first loading the fsimage then replaying all the operations (also called edits or transactions) in the edit log to catch up to the most recent state of the file system. Edit log modifications must be written to a majority of Journal Nodes. This will allow the system to tolerate the failure of a single machine. ... View more