Information Technology Reference
In-Depth Information
The HFS Plus file system
In 1996, Apple developed a new file system,
Hierarchical File System
(
HFS
), to accom-
modate the storage of large datasets. In an HFS file system the storage medium is represen-
ted as volumes. HFS volumes are divided into logical blocks of 512 bytes. The logical
blocks are numbered from first to last on a given volume and will remain static with the
same size as physical blocks, that is, 512 bytes. These logical blocks are grouped together
into allocation blocks, which are used by the HFS file system to track data in a more effi-
cient way. HFS uses a 16-bit value to address allocation blocks, which limits the number of
allocation blocks to 65,535. To overcome the inefficient allocations of disk space and some
of the limitations of HFS, Apple introduced the
HFS Plus
file system (
http://dubeiko.com/
The HFS Plus file system was designed to support larger file sizes. HFS volumes are di-
vided into sectors that are usually 512 bytes in size. These sectors are grouped together into
allocation blocks. The number of allocation blocks depends on the total size of the volume.
HFS Plus uses block addresses of 32 bits to address allocation blocks. HFS Plus uses journ-
aling by default. Journaling is the process of logging every transaction to the disk, which
helps in preventing file system corruption. The key characteristics of the HFS Plus file sys-
tem are: efficient use of disk space, unicode support for filenames, support for name forks,
file compression, journaling, dynamic resizing, dynamic defragmentation, and an ability to
boot on operating systems other than Mac OS.
