HFS Plus or HFS+ is a file system developed by Apple Computer to replace their Hierarchical File System (HFS) as the primary file system used on Macintosh computers. It is also one of the formats used by the iPod digital music player. HFS Plus is the name used by developers, but in user documentation the format is referred to as Mac OS Extended. 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) for naming the items (files, folders). HFS Plus permits filenames up to 255 characters in length, and n-forked files similar to NTFS, though almost no 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 they started to approach the 1GB mark, it meant that 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 1GB disk, the allocation block size under HFS is 16KB, so even a 1-byte file would take up 16K of disk space.
Like HFS, HFS Plus uses B*-trees to store most volume metadata.