| Developer | Apple Computer |
|---|---|
| Full name | Hierarchical File System |
| Introduced | September 17, 1985 (System 2.1) |
| Partition identifier | Apple_HFS (Apple Partition Map) 0xAF (MBR) |
| Structures | |
| Directory contents | B-tree |
| File allocation | Bitmap |
| Bad blocks | B-tree |
| Limits | |
| Max file size | 2 GB (2 × 10243 bytes) |
| Max number of files | 65535 |
| Max filename length | 31 characters |
| Max volume size | 2 TB (2 × 10244 bytes) |
| Allowed characters in filenames | All 8-bit values except colon ":". Discouraged null and nonprints. |
| Features | |
| Dates recorded | Creation, modification, backup |
| Date range | January 1, 1904 - February 6, 2040 |
| Date resolution | 1s |
| Forks | Only 2 (data and resource) |
| Attributes | Color (3 bits, all other flags 1 bit), locked, custom icon, bundle, invisible, alias, system, stationery, inited, no INIT resources, shared, desktop |
| File system permissions | AppleShare |
| Transparent compression | Yes (third-party), Stacker |
| Transparent encryption | No |
| Supported operating systems | Mac OS, Mac OS X, Linux, Microsoft Windows (through MacDrive or Boot Camp IFS drivers) |
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]
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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.
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.
Although Apple introduced HFS out of necessity with its first 20MB hard disk offering for the Macintosh in September 1985, HFS wasn't widely introduced until System 3.0 which debuted with the Macintosh Plus in January 1986 along with the larger 800K 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 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 Mac 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.
The Hierarchical File System divides a volume into logical blocks of 512 bytes. These logical blocks are then grouped together into allocation blocks which can contain one or more logical blocks depending on the total size of the volume. HFS uses a 16 bit value to address allocation blocks, limiting the number of allocation blocks to 65,535.
There are five structures that make up an HFS volume:
The Catalog File, which stores all the file and directory records in a single data structure, results in performance problems when the system allows multitasking, as only one program can write to this structure at a time, meaning that many programs may be waiting in queue due to one program "hogging" the system.[2] It is also a serious reliability concern, as damage to this file can destroy the entire file system. This contrasts with other file systems that store file and directory records in separate structures (such as DOS's FAT file system or the Unix File System), where having structure distributed across the disk means that damaging a single directory is generally non-fatal and the data may possibly be re-constructed with data held in the non-damaged portions.
Additionally, the limit of 65,535 allocation blocks resulted in files having a "minimum" size equivalent 1/65,535th the size of the disk. Thus, any given volume, no matter its size, could only store a maximum of 65,535 files. Moreover, any file would be allocated more space than it actually needed, up to the allocation block size. When disks were small, this was of little consequence, because the individual allocation block size was trivial, but as disks started to approach the 1 GB mark, the smallest amount of space that any file could occupy (a single allocation block) became excessively large, wasting significant amounts of disk space. For example, on a 1 GB disk, the allocation block size under HFS is 16 KB, so even a 1 byte file would take up 16 KB of disk space. This situation was less of a problem for users having large files (such as pictures, databases or audio) because these larger files wasted less space as a percentage of their file size. Users with many small files, on the other hand, could lose a copious amount of space due to large allocation block size. This made partitioning disks into smaller logical volumes very appealing for Mac users, because small documents stored on a smaller volume would take up much less space than if they resided on a large partition. The same problem existed in the FAT16 file system.
HFS supports case sensitive and case insensitive operation. Case sensitive mode causes errors with some applications.
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