Rocstor Arcticroc 2T Manuel d'utilisateur télécharger pdf (Page 58)

ARCTICROC 2T – 2Bay RAID System - User Manual Page | 58

HFS Plus or HFS+

This 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 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 were normalized to a form very

nearly the same as NFD (there are some minor differences derived from the fact that the HFS

Plus format was finalized before Unicode had standardized the NFD format). HFS Plus permits

filenames up to 255 UTF-16 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 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.

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 (=2

) allocation blocks rather than the 65,536 (=2

)

allocation blocks available to HFS.

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Rocstor Arcticroc 2T Manuel d'utilisateur Page 58