UNIX inode

Data structure on disk that describes and stores a file's attributes, including its physical location on disk. When a filesystem is initially created, a specific number of inodes are created. In most cases, this becomes the maximum number of files of all types, including directories, special files, and links that can exist in the filesystem.
A typical formula is one inode for every 8 KB of actual file storage. This is more than sufficient in most situations. Inodes are given unique numbers, and each distinct file has its own inode. When a new file is created, an unused inode is assigned to it.

Information stored in inodes includes the following:
- User owner and group owner IDs.
- File type (regular, directory, etc., or 0 if the inode is unused).
- Access modes (permissions).
- Most recent inode modification, data access, and data modification times. If the file'smetadata does not change, the first item will correspond to the file creation time.
- Number of hard links to the file (links are discussed later in this chapter). This is 0 if the inode is unused, and one for most regular files.
- Size of the file.
- Disk addresses of:
a. Disk locations for the data blocks that make up the file, and/or
b. Disk locations of disk blocks that hold the disk locations of the file's data blocks (indirect blocks), and/or
c. Disk locations of disk blocks that hold the disk locations of indirect blocks (double indirect blocks: two disk addresses removed from the actual data blocks)

* From O'Reilly Essentail System Administration - 3rd Edition