Hierarchical File System

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Hierarchical File System (HFS), is a file system developed by Apple Inc. for use in computer systems running Mac OS. Originally designed for use on floppy and hard disks, it can also be found on read-only media such as CD-ROMs. HFS is also referred to as Mac OS Standard (or, erroneously, "HFS Standard"), where its successor, HFS Plus, is also called Mac OS Extended (or, erroneously, “HFS Extended”). With the introduction of OS X 10.6, Apple has dropped support to format or write HFS disks and images, which are only supported as read-only volumes.[1]



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. Drawing 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 Lisa, HFS was developed by Patrick Dirks and Bill Bruffey and 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 localised. 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 was optimised 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 64k files.

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