Information Technology Reference
In-Depth Information
The HFS Plus volume
The HFS Plus volume contains a number of internal structures to manage the organization
of data. These structures include a header, alternate header, and five special files: an alloca-
tion file, an Extents Overflow file, a Catalog file, an Attributes file, and a Startup file.
Among the five files, three files, the Extents Overflow file, the Catalog file, and the Attrib-
ute file, use a B-tree structure, a data structure that allows data to be efficiently searched,
viewed, modified, or removed. The HFS Plus volume structure is shown in the following
figure:
The volume structure is described as follows:
• The first 1,024 bytes are reserved for boot load information.
•
Volume Header
: This stores volume information, such as the size of allocation
blocks, a timestamp of when the volume was created, and metadata about each of
the five special files.
•
Allocation File
: This file is used to track which allocation blocks are in use by the
system. The file format consists of one bit for every allocation block. If the bit is
set, the block is in use. If it is not set, the block is free.
•
Extents Overflow File
: This file records the allocation blocks that are allocated
when the file size exceeds eight blocks, which helps in locating the actual data
when referred. Bad blocks are also recorded in the file.
•
Catalog File
: This file contains information about the hierarchy of files and
folders, which is used to locate any file and folder within the volume.
•
Attribute File
: This file contains inline data attribute records, fork data attribute
records, and extension attribute records.
•
Startup File
: This file holds the information needed to assist in booting a system
that does not have HFS Plus support.
