OS/File/System/Inode

• Inode

  • An inode (index node) is the data structure in a Unix-like file system that stores a file's metadata and data-block pointers.
  • Each inode has a unique number within its file system. The OS/File/System/Directory stores the mapping from names to inode numbers; the inode itself does not store the filename.

  • What an inode stores

    • File type (regular file, directory, symlink, device, …)
    • Permissions and owner (UID, GID)
    • Timestamps: atime (last access), mtime (last modification), ctime (last status change)
    • File size
    • Link count — the number of OS/File/System/Link/Hards pointing to this inode
    • Pointers to the data blocks that hold the file's content

  • What an inode does not store

    • The filename — that lives in the OS/File/System/Directory entry
    • The full path — a file can be reached by multiple paths if multiple hard links exist

  • Lifecycle

    • The file's data is freed only when the link count reaches zero and no process holds the file open.
    • Hard links share one inode; each new hard link increments the link count.