Hierarchical File System
Hierarchical File System (HFS) is a proprietary 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 HFS Standard), while its successor, HFS Plus, is also called Mac OS Extended (or HFS Extended). With the introduction of Mac OS X 10.6, Apple dropped support for formatting or writing HFS disks and images, which remain supported as read-only volumes. Starting with  macOS 10.15, HFS disks can no longer be read. History. Apple introduced HFS 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 operating system (SOS) for the failed Apple III, which also served as the basis for hierarchical file 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 the main data of the file 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 redesigned 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; 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, where it was loaded into RAM from a MFS floppy disk on boot using a patch file ("Hard Disk 20"). However, HFS was not widely introduced until it was included in the 128K ROM that debuted with the Macintosh Plus in January 1986 along with the larger 800 KB floppy disk drive for  the Macintosh that also used HFS. 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 Mac OS X, an HFS volume cannot be used for booting, and beginning with Mac OS X 10.6 (Snow Leopard), HFS volumes are read-only  and cannot be created or updated. In macOS Sierra (10.12), Apple's release notes state that "The HFS Standard filesystem is no longer supported." However, read-only HFS Standard support is still present in Sierra and works as it did in previous versions. Design. A storage volume is inherently divided into "logical blocks" of 512 bytes. The Hierarchical File System groups these logical blocks 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 (2-1). Five structures make up an HFS volume: Limitations. 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. 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 saves the case of a file that is created or renamed but is case-insensitive in operation. According to bombich.com, HFS is no  longer supported on Catalina and future macOS releases.