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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.
History HFS Plus was introduced with the January 19 1998 release of Mac OS 8.1. However its first appearance, as a beta filesystem, was in the never-released Copland OS betas. With the release of the 10.2.2 update on November 11 2002, Apple added optional journaling features to HFS Plus for improved data reliability. These features were easily accessible in Mac OS X Server, but only accessible through the command line in the standard desktop client. With Mac OS X v10.3, all HFS Plus volumes on all Macs are set to be journaled by default. Within the system, an HFS Plus volume with a journal is identified as HFSJ. 10.3 also introduced another version of HFS Plus called HFSX. HFSX volumes are almost identical to HFS Plus volumes, except that they are never surrounded by the HFS Wrapper that is typical of HFS Plus volumes and they support case sensitivity for file and folder names. HFSX volumes can be recognised by two entries in the Volume Header, a value of HX in the signature field and 5 in the version field. With 10.4, Apple added support for Inline Attribute Data records, something that had been a part of the Mac OS X implementation of HFS Plus since at least 10.0, but always marked as "reserved for future use". Until the release of Mac OS X Server 10.4, HFS Plus only supported the standard UNIX file system permissions, however 10.4 introduced support for access control list-based file security, which is designed to be fully compatible with the file permission system used by Microsoft Windows XP and Windows Server 2003. While the HFS Plus format provides for almost limitless capacity, the various Mac OS versions support only a subset of it: The maximum number of files or folders within a folder is further limited by the maximum volume size divided by the block size. More information on limitations can be found in file system comparison. Design HFS Plus volumes are divided into sectors (called logical blocks in HFS), that are usually 512 bytes in size. These sectors are then grouped together into allocation blocks which can contain one or more sectors; the number of allocation blocks depends on the total size of the volume. HFS Plus uses a larger value to address allocation blocks than HFS, 32 bits rather than 16 bits; this means it can access 4,294,967,296 (=) allocation blocks rather than the 65,536 (=) allocation blocks available to HFS. Typically an HFS Plus volume is embedded inside an HFS Wrapper, although this is becoming less prevalent. The wrapper was designed for two purposes; it allowed Macintosh computers without HFS Plus support in their ROM to boot HFS Plus volumes and it also was designed to help users transition to HFS Plus by including a minimal, bootable HFS volume with a read-only file called Where_have_all_my_files_gone?, explaining to users with versions of the Mac OS without HFS Plus, that the volume requires a system with HFS Plus support. The original HFS volume contains a signature and an offset to the embedded HFS Plus volume within its volume header. All allocation blocks in the HFS volume which contain the embedded volume are mapped out of the HFS allocation file as bad blocks. There are nine structures that make up a typical HFS Plus volume: See also | |||||||||||||||||||||||||||||||||||||||||||||||||||||
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