A file system–internal data structure that assures
all the data on
disk is properly organized and accessible. Essentially, it is data
about the data.
Almost every file system has its own structure
of metadata, which is part of why the file systems show different
performance characteristics. It is extremely important to maintain
metadata intact, because otherwise all data on the file system
could become inaccessible.
Inodes contain various information about a file, including size, number of links, pointers to the disk blocks where the file contents are actually stored, and date and time of creation, modification, and access.
In the context of a file system, a journal is an on-disk structure containing a kind of log in which the file system stores what it is about to change in the file system's metadata. Journaling greatly reduces the recovery time of a Linux system because it obsoletes the lengthy search process that checks the entire file system at system start-up. Instead, only the journal is replayed.