HFS Plus or HFS+ is a file system developed by Apple Inc. to replace their Hierarchical File System (HFS) as the primary file system used in Macintosh computers (or other systems running Mac OS). It is also one of the formats used by the iPod digital music player. HFS Plus is also referred to as Mac OS Extended (or, erroneously, “HFS Extended”), where its predecessor, HFS, is also referred to as Mac OS Standard (or, erroneously, as “HFS Standard”). During development, Apple referred to this filesystem with the codename Sequoia.
HFS Plus is an improved version of HFS, supporting much larger files (block addresses are 32-bit length instead of 16-bit) and using Unicode (instead of Mac OS Roman or any of several other character sets) for naming the items (files, folders) – names which are also character encoded in UTF-16 and normalized to a form very nearly the same as Unicode Normalization Form D (NFD) (which means that precomposed characters like å are decomposed in the HFS+ filename and therefore count as two characters and UTF-16 implies that characters from outside the Basic Multilingual Plane – often seldom used and characters from ancient writing systems – also count as two characters in an HFS+ filename). HFS Plus permits filenames up to 255 UTF-16 characters in length, and n-forked files similar to NTFS, though until recently, almost no systems software takes advantage of forks other than the data fork and resource fork. HFS Plus also uses a full 32-bit allocation mapping table, rather than HFS’s 16 bits. This was a serious limitation of HFS, meaning that no disk could support more than 65,536 allocation blocks under HFS. When disks were small, this was of little consequence, but as larger-capacity drives became available, it meant that the smallest amount of space that any file could occupy (a single allocation block) became excessively large, wasting significant amounts of 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. Unlike most other file systems HFS Plus supports hard links to directories.
Like HFS, HFS Plus uses B-trees to store most volume metadata.