Inode
Encyclopedia
In computing
, an inode is a data structure
on a traditional Unix
-style file system
such as UFS
. An inode stores all the information about a regular file
, directory
, or other file system
object, except its data and name.
replied:
—data which describes data. Each file is associated with an inode, which is identified by an integer number, often referred to as an i-number or inode number.
Inodes store information about files and folders, such as file ownership, access mode (read, write, execute permissions), and file type. On many types of file system implementations, the maximum number of inodes is fixed at file system creation, limiting the maximum number of files the file system can hold. A typical allocation heuristic for inodes in a file system is one percent of total size.
The inode number indexes a table of inodes in a known location on the device; from the inode number, the file system driver portion of the kernel can access the contents of the inode, including the location of the file allowing access to the file.
A file's inode number can be found using the ls -i command. The ls -l command displays some of the inode contents for each file.
Some Unix-style file systems such as ReiserFS
omit an inode table, but must store equivalent data in order to provide equivalent capabilities. The data may be called stat data, in reference to the
that provides the data to programs.
File names and directory implications:
The operating system kernel's in-memory representation of this data is called
. Systems derived from BSD use the term
layer.
standard mandates filesystem behavior that is strongly influenced by traditional UNIX
filesystems. Regular files must have the following attributes:
The
pathnames and vice versa. This can be accomplished using the file finding utility
It is possible to use up a device's set of inodes. When this happens, new files cannot be created on the device, even though there may be free space available. For example, a mail server may have many small files that don't fill up the disk, but use many inodes to point to the numerous files.
Filesystems (such as JFS, or XFS
) escape this limitation with extents and/or dynamic inode allocation, which can 'grow' the filesystem and/or increase the number of inodes.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, an inode is a data structure
Data structure
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
on a traditional Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
-style file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
such as UFS
Unix File System
The Unix file system is a file system used by many Unix and Unix-like operating systems. It is also called the Berkeley Fast File System, the BSD Fast File System or FFS...
. An inode stores all the information about a regular file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
, directory
Directory (file systems)
In computing, a folder, directory, catalog, or drawer, is a virtual container originally derived from an earlier Object-oriented programming concept by the same name within a digital file system, in which groups of computer files and other folders can be kept and organized.A typical file system may...
, or other file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
object, except its data and name.
Origin of term according to Dennis Ritchie
The reason for designating these as "i" nodes is unknown. When asked, Unix pioneer Dennis RitchieDennis Ritchie
Dennis MacAlistair Ritchie , was an American computer scientist who "helped shape the digital era." He created the C programming language and, with long-time colleague Ken Thompson, the UNIX operating system...
replied:
Details
A file system relies on data structures that contain information about the files, beside the file content. The former is called metadataMetadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
—data which describes data. Each file is associated with an inode, which is identified by an integer number, often referred to as an i-number or inode number.
Inodes store information about files and folders, such as file ownership, access mode (read, write, execute permissions), and file type. On many types of file system implementations, the maximum number of inodes is fixed at file system creation, limiting the maximum number of files the file system can hold. A typical allocation heuristic for inodes in a file system is one percent of total size.
The inode number indexes a table of inodes in a known location on the device; from the inode number, the file system driver portion of the kernel can access the contents of the inode, including the location of the file allowing access to the file.
A file's inode number can be found using the ls -i command. The ls -l command displays some of the inode contents for each file.
Some Unix-style file systems such as ReiserFS
ReiserFS
ReiserFS is a general-purpose, journaled computer file system designed and implemented by a team at Namesys led by Hans Reiser. ReiserFS is currently supported on Linux . Introduced in version 2.4.1 of the Linux kernel, it was the first journaling file system to be included in the standard kernel...
omit an inode table, but must store equivalent data in order to provide equivalent capabilities. The data may be called stat data, in reference to the
statStat (Unix)stat is a Unix system call that returns useful data about a file inode. The semantics of stat vary between operating systems. As an example, the Unix command ls uses it to retrieve information on : time of last modification , time of last status change and time of last access .-stat functions and...
system callSystem call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
that provides the data to programs.
File names and directory implications:
- inodes do not contain file names, only file metadata.
- Unix directories are lists of association structures, each of which contains one filename and one inode number.
- The file system driver must search a directory looking for a particular filename and then convert the filename to the correct corresponding inode number.
The operating system kernel's in-memory representation of this data is called
struct inode in LinuxLinux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
. Systems derived from BSD use the term
vnode, with the v of vnode referring to the kernel's virtual file systemVirtual file system
A virtual file system or virtual filesystem switch is an abstraction layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way...
layer.
POSIX inode description
The POSIXPOSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
standard mandates filesystem behavior that is strongly influenced by traditional UNIX
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
filesystems. Regular files must have the following attributes:
- The size of the file in byteByteThe byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s. - Device ID (this identifies the device containing the file).
- The User IDUser identifier (Unix)Unix-like operating systems identify users within the kernel by an unsigned integer value called a user identifier, often abbreviated to UID or User ID...
of the file's owner. - The Group IDGroup identifier (Unix)In Unix-like systems, multiple users can be categorized into groups. POSIX and conventional Unix file system permissions are organized into three classes, user, group, and others. The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks,...
of the file. - The file modeFile system permissionsMost current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users to view or make changes to the contents of the filesystem....
which determines the file type and how the file's owner, its group, and others can access the file. - Additional system and user flags to further protect the file (limit its use and modification).
- TimestampTimestampA timestamp is a sequence of characters, denoting the date or time at which a certain event occurred. A timestamp is the time at which an event is recorded by a computer, not the time of the event itself...
s telling when the inode itself was last modified (ctime, inode change time), the file content last modified (mtime, modification time), and last accessed (atime, access time). - A link countReference countingIn computer science, reference counting is a technique of storing the number of references, pointers, or handles to a resource such as an object, block of memory, disk space or other resource...
telling how many hard linkHard linkIn computing, a hard link is a directory entry that associates a name with a file on a file system. . The term is used in file systems which allow multiple hard links to be created for the same file. This has the effect of creating multiple names for the same file, causing an aliasing effect: e.g...
s point to the inode. - Pointers to the disk blocks that store the file's contents (see inode pointer structureInode pointer structureThe inode pointer structure is a structure adopted by the inode of a file in the Unix File System or other related file systems to list the addresses of a file's data blocks. In the past, the structure may have consisted of eleven or thirteen pointers, but most modern file systems use fifteen...
).
The
statStat (Unix)stat is a Unix system call that returns useful data about a file inode. The semantics of stat vary between operating systems. As an example, the Unix command ls uses it to retrieve information on : time of last modification , time of last status change and time of last access .-stat functions and...
system call retrieves a file's inode number and some of the information in the inode.Implications
- Files can have multiple names. If multiple names hard link to the same inode then the names are equivalent. I.e., the first to be created has no special status. This is unlike symbolic linkSymbolic linkIn computing, a symbolic link is a special type of file that contains a reference to another file or directory in the form of an absolute or relative path and that affects pathname resolution. Symbolic links were already present by 1978 in mini-computer operating systems from DEC and Data...
s, which depend on the original name, not the inode (number). - An inode may have no links. An unlinked file is removed from disk, and its resources are freed for reallocation but deletion must wait until all processes that have opened it finish accessing it. This includes executable files which are implicitly held open by the processes executing them.
- It is typically not possible to map from an open file to the filename that was used to open it. The operating system immediately converts the filename to an inode number then discards the filename. This means that the getcwdDirect.hDirect.h is a C++ header file that is used to manipulate directories.- Member functions :-External references:* *...
and getwd library functions search the parent directoryParent directoryIn computing, the parent directory of a given directory A is the directory B in which A is located. In As absolute path, B is the predecessor of A....
to find a file with an inode matching the working directoryWorking directoryIn computing, the working directory of a process is a directory of a hierarchical file system, if any, dynamically associated with each process. When the process refers to a file using a simple file name or relative path , the reference is interpreted relative to the current working directory of...
, then search that directory's parent, and so on until reaching the root directoryRoot directoryIn computer file systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the root of a tree — the starting point where all branches originate.-Metaphor:...
. SVR4 and LinuxLinuxLinux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
systems maintain extra information to make this possible. - Historically, it was possible to hard link directories. This made the directory structure into an arbitrary directed graphDirected graphA directed graph or digraph is a pair G= of:* a set V, whose elements are called vertices or nodes,...
as opposed to a directed acyclic graph (DAG)Directed acyclic graphIn mathematics and computer science, a directed acyclic graph , is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of...
. It was even possible for a directory to be its own parent. Modern systems generally prohibit this confusing state, except that the parent of root is still defined as root. - A file's inode number stays the same when it is moved to another directory on the same device, or when the disk is defragmentedDefragmentationIn the maintenance of file systems, defragmentation is a process that reduces the amount of fragmentation. It does this by physically organizing the contents of the mass storage device used to store files into the smallest number of contiguous regions . It also attempts to create larger regions of...
which may change its physical location. This also implies that completely conforming inode behavior is impossible to implement with many non-Unix file systems, such as FATFile Allocation TableFile Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
and its descendants, which don't have a way of storing this lasting "sameness" when both a file's directory entry and its data are moved around. - Installation of new libraries is simple with inode filesystems. A running process can access a library file while another process replaces that file, creating a new inode, and an all new mapping will exist for the new file so that subsequent attempts to access the library get the new version. This facility eliminates the need to reboot to replace currently mapped libraries. For this reason, when updating programs, best practice is to delete the old executable first and create a new inode for the updated version, so that any processes executing the old version may proceed undisturbed.
Practical considerations
Many computer programs used by system administrators in UNIX operating systems often designate files with inode numbers. Examples include popular disk integrity checking utilities such as thefsckFsckThe system utility fsck is a tool for checking the consistency of a file system in Unix and Unix-like operating systems such as Linux.-Use:...
or pfiles. Thus, the need naturally arises to translate inode numbers to fileComputer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
pathnames and vice versa. This can be accomplished using the file finding utility
findFindIn Unix-like and some other operating systems, find is a command-line utility that searches through one or more directory trees of a file system, locates files based on some user-specified criteria and applies a user-specified action on each matched file...
with the -inum option, or the lsLsIn computing, ls is a command to list files in Unix and Unix-like operating systems. ls is specified by POSIX and the Single UNIX Specification.- History :An ls utility appeared in the original version of AT&T UNIX...
command with the proper option (-i on POSIX compliant platforms).It is possible to use up a device's set of inodes. When this happens, new files cannot be created on the device, even though there may be free space available. For example, a mail server may have many small files that don't fill up the disk, but use many inodes to point to the numerous files.
Filesystems (such as JFS, or XFS
XFS
XFS is a high-performance journaling file system created by Silicon Graphics, Inc. It is the default file system in IRIX releases 5.3 and onwards and later ported to the Linux kernel. XFS is particularly proficient at parallel IO due to its allocation group based design...
) escape this limitation with extents and/or dynamic inode allocation, which can 'grow' the filesystem and/or increase the number of inodes.