| | | | | | The file system is a major part of any operating system. It is a repository for programs and data and constitutes a large part of the "personality" of an operating system. | | | | | | | | | The term file system has two meanings in the Linux operating system. First there is the concept of a rooted directory hierarchy containing files and subdirectories. There is no notion of separate drive letters, only different disk partitions mounted as directories somewhere under the root directory structure. The second meaning of a file system refers to the specific formats for representing files and directories on a disk or in memory. For example, the format for storing files in MS-DOS differs from the format for storing files in MacOS. These constitute different file systems. Linux can work with over 20 of these file system formats, including MSDOS, HPFS (OS/2), ISO9660 (CD-ROM), VFAT (Windows95), and the Linux native EXT2 file system. Any of these file systems can be mounted as directories within a Linux system's directory structure. Linux even includes a process file system, which resides in memory and allows access to system information through a file interface. | | | | | | | | | The earliest versions of Linux only supported the ability to read from and write to the Minix file system. In 1992 the first extended file system (EXT) was developed specifically for Linux. About a year later, an improved version of this file system, named EXT2, was written to overcome the performance deficiencies of EXT. Since then, EXT2 has served as the Linux native file system. EXT2 was designed to be extensible, so that future versions of the system could always be compatible with older versions. But EXT2 is not compatible with EXT. The most frequently used file systems are EXT2 for storing the main Linux system, a DOS or VFAT system for accessing Windows files, and ISO9660 to read CD-ROMs. | | | | | | | | | When EXT2 was written, an architectural change was made in the way the Linux kernel interacted with file systems. The code to handle specific file systems was separated from the operating system through a layer called the Virtual File System. That change has allowed Linux to easily incorporate new file systems. The VFS allows the operating system to use exactly the same code to mount, read, and write to file systems without worrying about the specific format of the file system. | | | | | | | | | Linux files possess the properties of both user and group ownership, in addition to permissions based on reading, writing, and execution permission. They also have long, case sensitive filenames. Not every file system supports these notions, in which case their files are assigned reasonable defaults. Files may be up to 2 GBytes long, a limitation imposed by the 31-bit signed integer passed to the lseek system | | | | |
