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HFS Plus or HFS+ is a file system developed by Apple Inc. It serves as the primary file system of OS X. HFS+ was developed to replace the 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 file system 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〔There are some minor differences derived from the fact that the HFS Plus format was finalized before Unicode had standardized the NFD format (see ("Unicode Subtleties" ) for more information)〕 and UTF-16 implies that characters from outside the Basic Multilingual Plane – typically seldom-used characters or those 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 2005,〔http://en.wikipedia.org/w/index.php?title=HFS_Plus&diff=prev&oldid=258069029〕 almost no systems software took 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. == History == HFS+ was introduced with the January 19, 1998 release of Mac OS 8.1. With the release of the Mac OS X 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 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. Mac OS X 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 optionally support case sensitivity for file and folder names. HFSX volumes can be recognized by two entries in the Volume Header, a value of HX in the signature field and 5 in the version field.〔 Mac OS X 10.3 also marked Apple's adoption of Unicode 3.2 decomposition, superseding the Unicode 2.1 decomposition used previously. This change has caused problems for developers writing software for OS X.〔 〕 With Mac OS X 10.4, Apple added support for Inline Attribute Data records, something that had been a part of the OS X implementation of HFS Plus since at least 10.0, but always marked as "reserved for future use". Until the release of OS X Server 10.4, HFS Plus supported only the standard UNIX file system permissions; however, 10.4 introduced support for access control list–based file security, which provides a richer mechanism to define file permissions and is also designed to be fully compatible with the file permission models on other platforms such as Microsoft Windows XP and Windows Server 2003. In Mac OS X Leopard 10.5, directory hard-linking was added as a fundamental part of Time Machine. In Mac OS X Snow Leopard 10.6, HFS+ compression was added. In open source and some other areas this is referred to as AppleFSCompression. Compressed data may be stored in either an extended attribute or the resource fork. When using non-Apple APIs, AppleFSCompression is not always completely transparent. In OS X Lion 10.7, logical volume encryption (known as FileVault 2) was added to the operating system. This addition to the operating system in no way changed the logical structure of the file system. Apple's logical volume manager is known as Core Storage and its encryption at the volume level can apply to file systems other than HFS Plus. With appropriate hardware, both encryption and decryption should be transparent. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「HFS Plus」の詳細全文を読む スポンサード リンク
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