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

An address in a traditional inode-based file system that points to a block of further disk block numbers, rather than directly to user contents. If a block can store $q$ addresses, one singly indirect block can locate up to $q$ file data blocks, and a doubly indirect arrangement can locate up to $q^2$. The address blocks themselves consume allocated storage and must be reclaimed when their file no longer needs them. This lets small files use a short direct layout while supporting larger files.

An inode-based file system uses direct pointers for small files and successive levels of indirect block pointers for larger ones. If an address block contains $q$ pointers, a depth-$k$ region addresses $q^k$ file data blocks. With the inode resident, an uncached byte access in that region requires $k$ address-block reads and one file data block read.

A persistent collection of inodes indexed by inode numbers in an inode-based file system. The same term can also denote an operating system's in-memory collection of active inodes; these must be distinguished. A persistent table belongs to a particular volume, while the system-wide active collection keys entries by both volume identity and inode number.

An inode number identifies an allocated inode within one file system. A directory entry stores a name and this number; it does not normally store the file data block addresses. Different volumes can reuse the same number for unrelated files, and a reclaimed number can later be reused on the same volume. This is therefore a local, current identity, rather than a permanent identifier.