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HFS Plus

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HFS+
Developer(s)Apple Inc.
Full nameHierarchical File System Plus
IntroducedJanuary 19, 1998 with Mac OS 8.1
Partition IDsApple_HFS (Apple Partition Map)
0xAF (MBR)
Apple_HFSX (Apple Partition Map) when HFSX
48465300-0000-11AA-
AA11-00306543ECAC (GPT)
Structures
Directory contentsB-tree
File allocationBitmap
Bad blocksB-tree
Limits
Max volume size8 EiB [1]
Max file size8 EiB[2]
Max no. of files4,294,967,295 (232-1)
Max filename length255 characters (255 UTF-16 encoding units, normalized to Apple-modified variant of Unicode Normalization Format D)
Allowed filename
characters		Unicode, any character, including NUL. OS APIs may limit some characters for legacy reasons
Features
Dates recordedaccess, attributes modified, backed up, contents modified, created
Date rangeJanuary 1, 1904 – February 6, 2040
Date resolution1 s
ForksYes
AttributesColor (3 bits, all other flags 1 bit), locked, custom icon, bundle, invisible, alias, system, stationery, inited, no INIT resources, shared, desktop
File system
permissions		Unix permissions, NFSv4 ACLs (Mac OS X v10.4 onward)
Transparent
compression		Yes (on Mac OS X 10.6 and higher)
Transparent
encryption		Yes (on Mac OS X 10.7 and up). Per-home directory encryption is available with AES using HFS+ formatted .dmg volumes on Mac OS X versions prior to 10.7 but later than Mac OS X 10.3
Other
Supported
operating systems		Mac OS 8.1, Mac OS 9, Mac OS X, & Darwin, Linux, Microsoft Windows (through Boot Camp[3] IFS drivers)

HFS Plus or HFS+ 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 filesystem with the codename Sequoia.[4]

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[verification needed] and normalized to a form very nearly the same as Unicode Normalization Form D (NFD)[5] (which means that precomposed characters like å are decomposed in the HFS+ filename and therefore count as two characters[6] and UTF-16 implies that characters from outside the Basic Multilingual Plane – often seldom used and characters 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, the one which was changed was taken into the systems 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. 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.[2] However, its first appearance, as a beta filesystem, was in the never-released Copland OS betas.

With the release of the 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.[7] With Mac 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.

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.[2]

Additionally, Mac OS X 10.3 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 Mac OS X.[8]

With 10.4, Apple added support for Inline Attribute Data records, something that had been a part of the Mac OS X implementation of HFS Plus since at least 10.0, but always marked as "reserved for future use".[9] Until the release of Mac 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.[10]

In Mac OS X Snow Leopard 10.6 file system compression was added.

Design

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 (= 232) allocation blocks rather than the 65,536 (= 216) allocation blocks available to HFS.[2]

Formerly, HFS Plus volumes were embedded inside an HFS standard filesystem. This was phased out by the Tiger transition to Intel Macs, where the HFS Plus filesystem was not embedded inside a wrapper. The wrapper was designed for two purposes; it allowed Macintosh computers without HFS Plus support in their ROM to boot HFS Plus volumes and it also was designed to help users transition to HFS Plus by including a minimal HFS volume with a read-only file called Where_have_all_my_files_gone?, explaining to users with versions of Mac OS 8.0 and earlier without HFS Plus, that the volume requires a system with HFS Plus support. The original HFS volume contains a signature and an offset to the embedded HFS Plus volume within its volume header. All allocation blocks in the HFS volume which contain the embedded volume are mapped out of the HFS allocation file as bad blocks.[2]

There are nine structures that make up a typical HFS Plus volume:[2]

  1. Sectors 0 and 1 of the volume are HFS boot blocks. These are identical to the boot blocks in an HFS volume. They are part of the HFS wrapper.
  2. Sector 2 contains the Volume Header equivalent to the Master Directory Block in an HFS volume. The Volume Header stores a wide variety of data about the volume itself, for example the size of allocation blocks, a timestamp that indicates when the volume was created or the location of other volume structures such as the Catalog File or Extent Overflow File. The Volume Header is always located in the same place.
  3. The Allocation File which keeps track of which allocation blocks are free and which are in use. It is similar to the Volume Bitmap in HFS, in which each allocation block is represented by one bit. A zero means the block is free and a one means the block is in use. The main difference with the HFS Volume Bitmap, is that the Allocation File is stored as a regular file, it does not occupy a special reserved space near the beginning of the volume. The Allocation File can also change size and does not have to be stored contiguously within a volume.
  4. The Catalog File is a B-tree that contains records for all the files and directories stored in the volume. The HFS Plus Catalog File is very similar to the HFS Catalog File, the main differences being records are larger to allow more fields and to allow for those fields to be larger (for example to allow the longer 255-character unicode file nameas of which the things was having a S Catalog File is 512 bytes in size, a record in the HFS Plus Catalog File is 4 KB in Mac OS and 8 KB in Mac OS X. Fields in HFS are of fixed size, in HFS Plus the size can vary depending on the actual size of the data they store.
  5. The Extents Overflow File is another B-tree that records the allocation blocks that are allocated to each file as extents. Each file record in the Catalog File is capable of recording eight extents for each fork of a file; once those are used extents are recorded in the Extents Overflow File. Bad blocks are also recorded as extents in the Extents Overflow File. The default size of an extent record in Mac OS is 1 KB and 4 KB in Mac OS X.
  6. The Attributes File is a new B-tree in HFS Plus that does not have a corresponding structure in HFS. The Attributes File can store three different types of 4 KB records: Inline Data Attribute records, Fork Data Attribute records and Extension Attribute records. Inline Data Attribute records store small attributes that can fit within the record itself. Fork Data Attribute records contain references to a maximum of eight extents that can hold larger attributes. Extension Attributes are used to extend a Fork Data Attribute record when its eight extent records are already used.
  7. The Startup File is designed for non-Mac OS systems that don't have HFS or HFS Plus support. It is similar to the Boot Blocks of an HFS volume.
  8. The second to last sector contains the Alternate Volume Header equivalent to the Alternate Master Directory Block of HFS.
  9. The last sector in the volume is reserved for use by Apple. It is used during the computer manufacturing process.

Other operating systems

Linux

The Linux kernel includes the hfsplus module[11] for mounting HFS+ filesystems. HFS+ fsck and mkfs have been ported to Linux and are part of the hfsprogs package.[12]

In 2009, these drivers were diagnosed to be corrupting HFS+ drives with a capacity greater than 2 TB.[13] Consequently, Linux distributions such as Debian and Ubuntu also stopped allowing mounting of HFS+ drives or partitions greater than 2 TB.[14] As of February 2011, work is in progress to lift this restriction.[15]

The Linux HFS+ kernel driver has support to read and write to HFS+ non-journaled drives/partitions but only has read support of journaled HFS+. Journaling is a redundant behavior of a filesystem that helps protect against data loss. If planning to write to an HFS+ partition then drive journaling must be turned off in OSX.[16] As of 2011, the Linux Foundation is working on hfsplus journal support.[citation needed]

Windows

There are a few commercial products that allow Windows users to read and write to HFS Plus formatted media, but to date Apple has only released read-only HFS+ drivers for Windows XP, Windows Vista, and Windows 7 as part of the Boot Camp software in Mac OS X 10.6. This means users on these systems can read data on the HFS+ drive, but not write to them. Microsoft has created an HFS+ driver for the Xbox 360 mainly for the purpose of reading HFS+ formatted iPods.

A free (GPL) alternative is HFSExplorer written by Erik Larsson.[17] HFSExplorer is an application for viewing and extracting files from an HFS+ volume (Mac OS Extended) or an HFSX volume (Mac OS Extended, Case-sensitive) located either on a physical disk, on a .dmg disk image, or in a raw file system dump. However HFSExplorer can only read, but not write to HFS formatted volumes.

A free and opensource software – jHFSplus, based on HFSExplorer and jpfm – can be used to mounting hfs/hfs+ partitions as read-only virtual folders.[18]

See also

References

  1. ^ "Mac OS X: Mac OS Extended format (HFS Plus) volume and file limits". Support.apple.com. 2008-07-29. Retrieved 2010-07-05.
  2. ^ a b c d e f "Technical Note TN1150: HFS Plus Volume Format". Apple Developer Connection. March 5, 2004. Retrieved 2007-03-28.
  3. ^ http://support.apple.com/kb/HT3777
  4. ^ "New Mac OS Extended Format (HFS+) Available". Apple Developer News. 1997. Retrieved 2007-03-28.
  5. ^ "Technical Q&A QA1235: Converting to Precomposed Unicode". Apple Developer Connection. February 7, 2003. Retrieved 2007-03-28.
  6. ^ 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)
  7. ^ John Gruber (November 11, 2002). "Crow". DaringFireball.net. Retrieved 2007-03-28.
  8. ^ "Re: git on MacOSX and files with decomposed utf-8 file names". KernelTrap. 2010-05-07. Retrieved 2010-07-05.
  9. ^ John Siracusa (April 28, 2005). "Mac OS X 10.4 Tiger: Metadata revisited". Ars Technica. Retrieved 2007-03-28.
  10. ^ "Apple - Mac OS X Server - Windows Services". Apple.com. Archived from the original on 2007-12-31. Retrieved 2007-11-12.
  11. ^ Git Repository Kernel.org
  12. ^ User-space supporting tools for HFS+ filesystems under Posix systems.
  13. ^ http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5c36fe3d87b3f0c85894a49193c66096a3d6b26f
  14. ^ http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=550010
  15. ^ http://www.spinics.net/lists/linux-fsdevel/msg42242.html
  16. ^ http://ubuntuforums.org/showthread.php?p=2346494
  17. ^ "Catacombae - HFSExplorer". Erik Larsson. 2008-12-23. Retrieved 2010-12-19.
  18. ^ "jHFSplus". Shashank Tulsyan. 2011-06-29.
  • hfsexplorer - Open Source Explorer for MS-Windows® written in Java, with support for HFS, HFS+, and HFSX
  • Apple "legacy" Technote 1150 - HFS Plus Volume Format
  • Apple "legacy" Technote 1189 - The Monster Disk Driver Technote
  • hfsdebug - A debugger for HFS Plus volumes by Amit Singh
  • hfsprogs - Userspace support tools for HFS+ filesystems under Linux systems (adapted from Apple's native tools).
  • iBored - A disk editor and viewer supporting HFS Plus


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