File Organization: The Inode

One of the most important on-disk structures of a file system is the inode; virtually all file systems have a structure similar to this. The name inode is short for index node, the historical name given to it in UNIX“The UNIX Time-Sharing System” by M. Ritchie, K. Thompson. CACM Volume 17:7, 1974. The original paper about UNIX. Read it to see the underpinnings of much of modern operating systems., and possibly earlier systems, used because these nodes were originally arranged in an array, and the array indexed into when accessing a particular inode.

ASIDE: DATA STRUCTURE — THE INODE

The inode is the generic name that is used in many file systems to describe the structure that holds the metadata for a given file, such as its length, permissions, and the location of its constituent blocks. The name goes back at least as far as UNIX and probably further back to Multics if not earlier systems. It is short for index node, as the inode number is used to index into an array of on-disk inodes in order to find the inode of that number. As we’ll see, the design of the inode is one key part of file system design. Most modern systems have some kind of structure like this for every file they track, but perhaps call them different things (such as dnodes, fnodes, etc.).

Each inode is implicitly referred to by a number (called the i-number), which we’ve earlier called the low-level name of the file. In vsfs (and other simple file systems), given an i-number, you should directly be able to calculate where on the disk the corresponding inode is located. For example, take the inode table of vsfs as shown in the previous lesson: 20-KB in size (5 4-KB blocks) and thus consisting of 80 inodes (assuming each inode is 256 bytes). Further assume that the inode region starts at 12KB (i.e, the superblock starts at 0KB, the inode bitmap is at address 4KB, the data bitmap at 8KB, and thus the inode table comes right after). In vsfs, we thus have the following layout for the beginning of the file system partition (in closeup view):