HFS+

HFS+, or Hierarchical File System Plus, is the file system designed by Apple Computer[1] to supersede HFS. First introduced with Mac OS 8.1, one of the biggest differences was the lower allocation block size of 4kb, which increased performance and lowered fragmentation [2]. It also implemented Unicode (rather than Mac proprietary formats) for naming files.

There are structurally many differences between HFS and HFS+, which are listed below[3]:

Feature	HFS	HFS Plus	Benefit/Comment
User visible name	Mac OS Standard	Mac OS Extended
Number of allocation blocks	16 bits worth	32 bits worth	Radical decrease in disk space used on large volumes, and a larger number of files per volume.
Long file names	31 characters	255 characters	Obvious user benefit; also improves cross-platform compatibility
File name encoding	MacRoman	Unicode	Allows for international-friendly file names, including mixed script names
File/folder attributes	Support for fixed size attributes (FileInfo and ExtendedFileInfo)	Allows for future meta-data extensions	Future systems may use metadata for a richer Finder experience
OS startup support	System Folder ID	Also supports a dedicated startup file	May help non-Mac OS systems to boot from HFS Plus volumes
catalog node size	512 bytes	4 KB	Maintains efficiency in the face of the other changes. (This larger catalog node size is due to the much longer file names [512 bytes as opposed to 32 bytes], and larger catalog records (because of more/larger fields)).
Maximum file size	231 bytes	263 bytes	Obvious user benefit, especially for multimedia content creators.

An HFS+ volume contains five special files:

1. Catalog file - Describes the folder and file hierarchy of the volume. It is organized as a "balanced tree" for fast and efficient searches
2. Extents overflow file - Additional extents (contiguous allocation blocks allocated to forks) are stored in a b-tree in this file
3. Allocation file - Specifies whether an allocation block is free (similar to $Bitmap in NTFS). This is stored in a bitmap, specifying a free allocation block with a "clear bit"
4. Attributes file - Contains attribute information regarding files or folders
5. Startup file - Allows computers to boot that do have built in support for HFS+ file systems

HFS+ also implements journaling, which allows fast recovery in the case of a crash or power outage. According to Apple, "The purpose of the journal is to ensure that when a group of related changes are being made, that either all of those changes are actually made, or none of them are made."[4]

Apple technical notes are available for the HFS+ file system from their website.

External Links

• Internet Archive Copy of: Technical Note TN1121