Inode

An inode is a data structure describing an object in an inode-based file system. For an ordinary file, it records file metadata such as type, owner, access permissions, size and hard link count, together with information locating the contents. In the direct-reference toy layout, inode 42 points to file data blocks 7 and 9; its two names a and b live in directory entries. Other file systems can locate contents differently, so the diagram is a model of the separation rather than a universal storage layout. ^[ref]

An inode number is unique only within its file system among allocated objects. Two different file systems can each have inode 42, referring to unrelated files. POSIX exposes the file system's device identity as st_dev and the file's serial number as st_ino; the pair identifies a file within a running system. Neither the bare inode number nor that pair is a permanent historical identifier: after reclamation, a later file can reuse the number. In particular, comparing an old saved number with a new one does not establish continuity of the file. ^[ref]

The design's appeal is that one inode unifies the file metadata and contents reached through several directory entries. Ritchie and Thompson's original 1974 account explicitly describes a directory entry as a name and an i-number, with the corresponding inode holding ownership, protection, size, content addresses and the hard link count. It gives a concrete early explanation of how names and file identity can evolve separately. ^[ref]

// Target: computer-science.bigb