Explainer: Inodes and Inode Numbers

Summary

Every self-respecting file system identifies files and directories using numbered data structures. In most modern file systems, those data structures are known as inodes, and their numbers are inode numbers, sometimes shortened to inodes. The term is thought to be a contraction of index node, which certainly makes sense, but is lost in the mists of time. In any file system, for example an individual APFS volume, the inode numbers uniquely identify each inode, and each object within that file system has its own inode. Whatever else the file system might do, the inode number identifies one and only one object within it. Thus one invariant way of identifying any file is by referencing the file system containing it, and its inode number. HFS+ The Mac’s original native file systems, ending most recently in Mac OS Extended File System, or HFS+ from its origins in the Hierarchical File System, don’t use inodes as such, and don’t strictly speaking have inode numbers. Instead, the data structures for their files and folders are kept in Catalogue Nodes, and their numbers are Catalogue Node IDs, CNIDs. With Mac OS X came Unix APIs, and their requirement to use inodes and inode numbers. As CNIDs are unique to each file and folder within an HFS+ volume, for HFS+ they are used as inode numbers, although in some ways they differ. CNIDs are unsigned 32-bit integers, with the numbers 0-15 reserved for system use. For example, CNID 7 is normally used as the Startup file’s ID. Although not necessarily related to CNIDs, it’s worth noting that the Mac’s original file system MFS allowed a maximum of 4,094 files, its successor HFS was limited to 65,535, and HFS+ to 4,294,967,295. Those are in effect the maximum number of inode numbers or their equivalents allowed in that file system. APFS Unlike HFS+, APFS was designed from the outset to support standard Posix features, so has inodes numbered using unsigned 64-bit integers. Strictly, the APFS inode number is the object identifier in the h...