The format also includes the initial fileset and control structures necessary to describe it. The fileset is the mountable entity. A fileset contains files and directories. JFS also uses inodes to represent other file system objects, such as the map that describes the allocation state and location on disk of each inode in the fileset. Directories map user-specified names to the inodes allocated for files and directories and form the traditional name hierarchy.
Files contain user data, and there are no restrictions or formats implied in the data.
That is, user data is treated, by JFS, as an uninterpreted byte stream. Extent-based addressing structures rooted in the inode are used for mapping file data to disk. Together, the aggregate superblock and disk allocation map, file descriptor and inode map, inodes, directories, and addressing structures represent JFS control structures or meta-data. Logs JFS logs are maintained in each aggregate and used to record information about operations on meta-data. The log has a format that also is set by the file system creation utility.