File System

Introduction to File Systems

All file systems consist of structures necessary for storing and managing data. These structures typically include an operating system boot record, directories, and files. Functions of a File System:Tracking allocated and free spaceMaintaining directories and file namesTracking where each file is physically stored

on the disk

Definition: Computers use particular kinds of file systems to store and organize data on media, such as a hard drive, the CDs, DVDs, and BDs in an optical drive or on a flash drive. Any place that a PC stores data is employing the use of some type of file system. A file system can be thought of as an index or database containing the physical location of every piece of data on a hard drive.

A file system is setup on a drive during a format. See How To Format a Hard Drive for more information.

Types of File Systems

FAT File SystemNTFS File SystemDisk file systemsFlash file systemsTape file systemsDatabase file systemsTransactional file systemsNetwork file systemswork file systemsShared disk file systemsSpecial file systemsDevice file systems

FAT File System

FAT12 - The initial version of the FAT file system, FAT12 was introduced in 1977, even before MS-DOS, and was the primary file system for Microsoft operating systems up to MS-DOS 4.0. FAT12 supports drive sizes up to 32MB.

FAT16 - The second implementation of FAT was FAT16, introduced in 1988. FAT16 was the primary file system for MS-DOS 4.0 up to Windows 95. FAT16 supports drive sizes up to 2GB.

FAT32 - FAT32 is the latest version of the FAT file system. It was introduced in 1996 for Windows 95 OSR2 users and was the primary file system for consumer Windows versions through Windows ME. FAT32 supports drive sizes up to 8TB.

NTFS File System

Definition: New Technology File System (NTFS) is a file system that was introduced by Microsoft in 1993 with Windows NT 3.1. NTFS supports hard drive sizes up to 256TB. NTFS is the primary file system used in Microsoft's Windows 7, Windows Vista, Windows XP, Windows 2000 and Windows NT operating systems. The Windows Server line of operating systems also primarily use NTFS.

The File Allocation Table (FAT) file system was the primary file system in Microsoft's older operating systems but it is still supported today along with NTFS.

Disk file systems

A disk file system takes advantages of the ability of disk storage media to randomly address data in a short amount of time. Additional considerations include the speed of accessing data following that initially requested and the anticipation that the following data may also be requested. This permits multiple users (or processes) access to various data on the disk without regard to the sequential location of the data. Examples include FAT (FAT12, FAT16, FAT32), exFAT, NTFS, HFS and HFS+, HPFS, UFS, ext2, ext3, ext4, XFS, btrfs, ISO 9660, Files-11, Veritas File System, VMFS, ZFS, ReiserFS and UDF. Some disk file systems are journaling file systems or versioning file systems.

Optical discs ISO 9660 and Universal Disk Format (UDF) are two common formats

that target Compact Discs, DVDs and Blu-ray discs. Mount Rainier is an extension to UDF supported by Linux 2.6 series and Windows Vista that facilitates rewriting to DVDs.

Flash file system

A flash file system considers the special abilities, performance and restrictions of flash memory devices. Frequently a disk file system can use a flash memory device as the underlying storage media but it is much better to use a file system specifically designed for a flash device.

Tape file systems

A tape file system is a file system and tape format designed to store files on tape in a self-describing form. Magnetic tapes are sequential storage media with significantly longer random data access times than disks, posing challenges to the creation and efficient management of a general-purpose file system.

In a disk file system there is typically a master file directory, and a map of used and free data regions. Any file additions, changes, or removals require updating the directory and the used/free maps. Random access to data regions is measured in milliseconds so this system works well for disks.

Tape requires linear motion to wind and unwind potentially very long reels of media. This tape motion may take several seconds to several minutes to move the read/write head from one end of the tape to the other.

Database file systems

Another concept for file management is the idea of a database-based file system. Instead of, or in addition to, hierarchical structured management, files are identified by their characteristics, like type of file, topic, author, or similar rich metadata.

Network file systems

A network file system is a file system that acts as a client for a remote file access protocol, providing access to files on a server. Examples of network file systems include clients for the NFS, AFS, SMB protocols, and file-system-like clients for FTP and WebDAV.

Shared disk file systems

A shared disk file system is one in which a number of machines (usually servers) all have access to the same external disk subsystem (usually a SAN). The file system arbitrates access to that subsystem, preventing write collisions. Examples include GFS2 from Red Hat, GPFS from IBM, SFS from DataPlow, CXFS from SGI and StorNext from Quantum Corporation.

Special file systems

A special file system presents non-file elements of an operating system as files so they can be acted on using file system APIs. This is most commonly done in Unix-like operating systems, but devices are given file names in some non-Unix-like operating systems as well.

Device file systems

A device file system represents I/O devices and pseudo-devices as files, called device files. Examples in Unix-like systems include devfs and, in Linux 2.6 systems, udev. In non-Unix-like systems, such as TOPS-10 and other operating systems influenced by it, where the full filename or pathname of a file can include a device prefix, devices other than those containing file systems are referred to by a device prefix specifying the device, without anything following it.

Flat file systems

In a flat file system, there are no subdirectories.

When floppy disk media was first available this type of file system was adequate due to the relatively small amount of data space available. CP/M machines featured a flat file system, where files could be assigned to one of 16 user areas and generic file operations narrowed to work on one instead of defaulting to work on all of them. These user areas were no more than special attributes associated with the files, that is, it was not necessary to define specific quota for each of these areas and files could be added to groups for as long as there was still free storage space on the disk..

Aspects of file systems

Space management

File systems allocate space in a granular manner, usually multiple physical units on the device. The file system is responsible for organizing files and directories, and keeping track of which areas of the media belong to which file and which are not being used. For example, in Apple DOS of the early 1980s, 256-byte sectors on 140 kilobyte floppy disk used a track/sector map.

Filenames

A filename (or file name) is used to identify a storage location in the file system. Most file systems have restrictions on the length of filenames. In some file systems, filenames are not case sensitive (i.e., filenames such as FOO and foo refer to the same file); in others, filenames are case sensitive (i.e., the names FOO and foo refer to two separate files).

Directories

File systems typically have directories (also called folders) which allow the user to group files into separate collections. This may be implemented by associating the file name with an index in a table of contents or an inode in a Unix-like file system. Directory structures may be flat (i.e. linear), or allow hierarchies where directories may contain subdirectories. The first file system to support arbitrary hierarchies of directories was used in the Multics operating system. The native file systems of Unix-like systems also support arbitrary directory hierarchies, as do, for example, Apple's Hierarchical File System, and its successor HFS+ in classic Mac OS (HFS+ is still used in Mac OS X), the FAT file system in MS-DOS 2.0 and later and Microsoft Windows, the NTFS file system in the Windows NT family of operating systems, and the ODS-2 (On-Disk Structure-2) and higher levels of the Files-11 file system in OpenVMS.

Metadata

Other bookkeeping information is typically associated with each file within a file system. The length of the data contained in a file may be stored as the number of blocks allocated for the file or as a byte count. The time that the file was last modified may be stored as the file's timestamp. File systems might store the file creation time, the time it was last accessed, the time the file's metadata was changed, or the time the file was last backed up. Other information can include the file's device type (e.g. block, character, socket, subdirectory, etc.), its owner user ID and group ID, its access permissions and other file attributes (e.g. whether the file is read-only, executable, etc.).

The Second Extended File system (EXT2)

The Second Extended File system was devised (by Rémy Card) as an extensible and powerful file system for Linux. It is also the most successful file system so far in the Linux community and is the basis for all of the currently shipping Linux distributions.

The EXT2 file system, like a lot of the file systems, is built on the premise that the data held in files is kept in data blocks. These data blocks are all of the same length and, although that length can vary between different EXT2 file systems the block size of a particular EXT2 file system is set when it is created (using mke2fs). Every file's size is rounded up to an integral number of blocks. If the block size is 1024 bytes, then a file of 1025 bytes will occupy two 1024 byte blocks. Unfortunately this means that on average you waste half a block per file. Usually in computing you trade off CPU usage for memory and disk space utilisation. In this case Linux, along with most operating systems, trades off a relatively inefficient disk usage in order to reduce the workload on the CPU. Not all of the blocks in the file system hold data, some must be used to contain the information that describes the structure of the file system. EXT2 defines the file system topology by describing each file in the system with an inode data structure. An inode describes which blocks the data within a file occupies as well as the access rights of the file, the file's modification times and the type of the file. Every file in the EXT2 file system is described by a single inode and each inode has a single unique number identifying it. The inodes for the file system are all kept together in inode tables. EXT2 directories are simply special files (themselves described by inodes) which contain pointers to the inodes of their directory entries.

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The EXT2 Inode

In the EXT2 file system, the inode is the basic building block; every file and directory in the file system is described by one and only one inode. The EXT2 inodes for each Block Group are kept in the inode table together with a bitmap that allows the system to keep track of allocated and unallocated inodes. Figure  9.2 shows the format of an EXT2 inode, amongst other information, it contains the following fields:

mode This holds two pieces of information; what this inode describes and the permissions that users have to it. For EXT2, an inode can describe one of file, directory, symbolic link, block device, character device or FIFO. Owner Information The user and group identifiers of the owners of this file or directory. This allows the file system to correctly allow the right sort of accesses, Size The size of the file in bytes, Timestamps The time that the inode was created and the last time that it was modified, Datablocks Pointers to the blocks that contain the data that this inode is describing. The first twelve are pointers to the physical blocks containing the data described by this inode and the last three pointers contain more and more levels of indirection.

The EXT2 Superblock

The Superblock contains a description of the basic size and shape of this file system. The information within it allows the file system manager to use and maintain the file system. Usually only the Superblock in Block Group 0 is read when the file system is mounted but each Block Group contains a duplicate copy in case of file system corruption. Amongst other information it holds the:

Magic Number This allows the mounting software to check that this is indeed the Superblock for an EXT2 file system. For the current version of EXT2 this is 0xEF53. Revision Level The major and minor revision levels allow the mounting code to determine whether or not this file system supports features that are only available in particular revisions of the file system. There are also feature compatibility fields which help the mounting code to determine which new features can safely be used on this file system, Mount Count and Maximum Mount Count Together these allow the system to determine if the file system should be fully checked.

Conclusion This paper discusses how the Modify-on-Access file system

efficiently extends the capabilities of conventional file systems. It demonstrates how an active file system can simplify both applications and system usage by performing computations on behalf of processes. Furthermore, the paper describes the implementation of the MonA file system and the export transformation. Section 5 shows that the overhead of a kernel-resident transformation is very small and that transforming data inline provides performance benefits. Furthermore, it shows that the export transformation provides user extensibility.

The MonA file system is the first component of a suite of system software designed for a collaborative memory system in which intelligent peripheral devices collaborate with a host processor to accomplish tasks. Current projects include a MonA virtual memory system and a MonA peripheral device. These implementations are similar to the Active Page and Active Disk simulations described in related work.