File systems under Mac OS X
Mac OS X uses a file system that it inherited from classic
Mac OS called
HFS Plus. HFS Plus is a
metadata-rich and
case preserving file system. Due to the Unix roots of Mac OS X, Unix permissions were added to HFS Plus. Later versions of HFS Plus added
journaling to prevent corruption of the file system structure and introduced a number of optimizations to the allocation algorithms in an attempt to defragment files automatically without requiring an external defragmenter.
Filenames can be up to 255 characters. HFS Plus uses
Unicode to store filenames. On Mac OS X, the
filetype can come from the
type code, stored in file's metadata, or the filename.
HFS Plus has three kinds of links: Unix-style
hard links, Unix-style
symbolic links and
aliases. Aliases are designed to maintain a link to their original file even if they are moved or renamed; they are not interpreted by the file system itself, but by the File Manager code in
userland.
Mac OS X also supports the
UFS file system, derived from the
BSD Unix Fast File System via
NeXTSTEP. However, as of Mac OS X 10.5 (Leopard), Mac OS X can no longer be installed on a UFS volume, nor can a pre-Leopard system installed on a UFS volume be upgraded to Leopard.
[1]