Hierarchical File System - History

History

HFS was introduced by Apple in September 1985, specifically to support Apple's first hard disk drive for the Macintosh, replacing the Macintosh File System (MFS), the original file system which had been introduced over a year and a half earlier with the first Macintosh computer. HFS drew heavily upon Apple's first hierarchical SOS operating system for the failed Apple III, which also served as the basis for hierarchical filing systems on the Apple IIe and Apple Lisa. HFS was developed by Patrick Dirks and Bill Bruffey. It shared a number of design features with MFS that were not available in other file systems of the time (such as DOS's FAT). Files could have multiple forks (normally a data and a resource fork), which allowed program code to be stored separately from resources such as icons that might need to be localized. Files were referenced with unique file IDs rather than file names, and file names could be 255 characters long (although the Finder only supported a maximum of 31 characters).

However, MFS had been optimized to be used on very small and slow media, namely floppy disks, so HFS was introduced to overcome some of the performance problems that arrived with the introduction of larger media, notably hard drives. The main concern was the time needed to display the contents of a folder. Under MFS all of the file and directory listing information was stored in a single file, which the system had to search to build a list of the files stored in a particular folder. This worked well with a system with a few hundred kilobytes of storage and perhaps a hundred files, but as the systems grew into megabytes and thousands of files, the performance degraded rapidly.

The solution was to replace MFS's directory structure with one more suitable to larger file systems. HFS replaced the flat table structure with the Catalog File which uses a B-tree structure that could be searched very quickly regardless of size. HFS also re-designed various structures to be able to hold larger numbers, 16-bit integers being replaced by 32-bit almost universally. Oddly, one of the few places this "upsizing" did not take place was the file directory itself, which limits HFS to a total of 65,535 files on each logical disk.

While HFS is a proprietary file system-format, it is well-documented, so there are usually solutions available to access HFS formatted disks from most modern operating systems.

Apple introduced HFS out of necessity with its first 20 MB hard disk offering for the Macintosh in September 1985. However, HFS was not widely introduced until System 3.0, which debuted with the Macintosh Plus in January 1986, along with the larger 800 KB floppy disk drive for the Macintosh, which also required HFS support. More importantly, HFS was hard-coded into new Plus' 128K ROM, freeing not only space from the system software disk, but also RAM. However, RAM-based HFS support was also implemented for use with the earlier Macintosh 512K's 64K ROM through the addition of an INIT file on the system disk. The introduction of HFS was the first advancement by Apple to leave a Macintosh computer model behind: the original 128K Macintosh, which lacked sufficient memory to load the HFS code and was promptly discontinued.

In 1998, Apple introduced HFS Plus to address inefficient allocation of disk space in HFS and to add other improvements. HFS is still supported by current versions of Mac OS, but starting with OS X, an HFS volume cannot be used for booting, and beginning with OS X 10.6 (Snow Leopard), HFS volumes are read-only and cannot be created or updated.