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INODE Details
When a UNIX file system is created, data structures that contain information
about files are created. Each file has an inode and is identified by an inode
number (often "i-number" or even shorter, "ino") in the file system where it
resides.
Inodes store information on files such as user and group ownership, access
mode (read, write, execute permissions) and type of file. There is a fixed number
of inodes, which indicates the maximum number of files each file system can
hold. Typically when a file system is created about 1% of it is devoted to inodes.
The term inode usually refers to inodes on block devices that manage regular
files, directories, and possibly symbolic links. The concept is particularly
important to the recovery of damaged file systems.

The inode number indexes a table of inodes in a known location on the
device; from the inode number, the kernel can access the contents of the
inode, including the data pointers, and so the contents of the file.

A file's inode number can be found using the ls -i command, while the ls -l
command will retrieve inode information.

Non-traditional Unix-style filesystems such as ReiserFS may avoid having
a table of inodes, but must store equivalent data in order to provide
equivalent function. The data may be called stat data, in reference to the
stat
system call that provides the data to programs.
File names and directory implications

Inodes do not contain filenames, only file contents.

Unix directories are lists of "link" structures, each of which contains one
filename and one inode number.

The kernel must search a directory looking for a particular filename and
then convert the filename to the correct corresponding inode number if the
name is found.
The kernel's in-memory representation of this data is called struct inode in Linux.
Systems derived from BSD use the term vnode, with the v of vnode referring to
the kernel's virtual file system layer.
inode description
Regular files are required to have the following attributes:

The length of the file in bytes.

Device ID (this identifies the device containing the file).

The User ID of the file's owner.

The Group ID of the file.

The file mode, which determines what users can read, write, and execute
the file.

Timestamps telling when the inode itself was last modified (ctime, change
time), the file content last modified (mtime, modification time), and last
accessed (atime, access time).

A reference count telling how many hard links point to the inode.

Pointers to the disk blocks that store the file's contents.
Example of structure:
Implications
The properties of a file system that makes use of inodes surprise many users
who are not used to the concept:

If multiple names link to the same inode (they are all hard links to it) then
all of the names are equivalent. The first one to have been created has no
special status. This is unlike the sometimes more familiar symbolic links,
where all of the links depend on the original name.

An inode can even have no links at all. Normally such a file would be
removed from the disk and its resources freed for reallocation (the normal
process of deleting a file) but if any processes are holding the file open,
they may continue to access it, and the file will only be finally deleted
when the last reference to it is closed. This includes executable images
which are implicitly held open by the processes executing them. For this
reason, when programs are updated, it is recommended to delete the old
executable first and create a new inode for the updated version, so that
any instances of the old version currently executing may continue to do so
unbothered.

Typically, it is not possible to map from an open file to the filename that
was used to open it. The operating system would convert the filename to
an inode number at the first possible chance, then forget the filename.
This means that the getcwd() and getwd() library functions would need to
search the parent directory to find a file with an inode matching the "."
directory, then search the grandparent directory for that directory, and so
on until reaching the "/" directory. SVR4 and Linux systems retain extra
information to avoid this awkwardness.

Historically, it was possible to hard link directories. This made the directory
structure be an arbitrary directed graph instead of a DAG. It was possible
for a directory to be its own parent. Modern systems generally prohibit this
confusing state, except that the root directory is still its own parent.

A file's inode number will stay the same when it is moved to another
directory on the same device, or when the disk is defragmented.
Therefore, moving either a file's directory entry or its data (or both) is not
enough to prevent a running process from accessing it, if the process ever
had a chance of finding out the inode number. This also implies that
completely conforming behavior of inodes is impossible to implement with
many non-Unix file systems, such as FAT 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.
Variations in inode file systems
There are some important variations in inode file systems in current use that
must be noted.

Swap file systems in Linux typically don't support double indirection, or in
some cases single indirection.

The swap file and sleep mode swap file don't need to be any bigger than
10 GiB, as a matter of practice.
Defragmentation

An inode file system would have to be offline to be fully defragmented on
most systems -- but some online defragmenation tools exist.

inode systems when defragmented can have data extraction rates higher
than FAT32 or NTFS under optimal conditions
Practical considerations
Many computer programs used by system administrators in UNIX operating
systems often give inode numbers to designate a file. Popular disk integrity
checking utility fsck or pfiles command may serve here as examples.
Thus need naturally arises to translate inode numbers to file pathnames and vice
versa.
This can be accomplished using file-finding utility find with option -inum or ls
command with proper option which on many platforms is -i.
Y2038 problem
Some Inode file systems are Y2038 (aka Unix time) safe with respect to date
overflow prevention -- but not all Inode file systems in use are protected from this
problem. When setting up a server it will become more important over time to
avoid the use of these non-POSIX compliant file systems. POSIX in its latest
revsion supports system time and date