path: root/README

GPT fdisk (aka gdisk and sgdisk) and FixParts
by Roderick W. Smith, rodsmith@rodsbooks.com

Introduction
------------

This binary archive includes the source code for three related disk
partitioning programs:

- gdisk -- This program is modeled after Linux fdisk, but it operates on
  GUID Partition Table (GPT) disks rather than the Master Boot Record (MBR)
  disks that fdisk modifies. As such, gdisk is an interactive text-mode
  tool for manipulating partitions, but it does nothing to the contents of
  those partitions (usually filesystems, but sometimes swap space or other
  data).

- sgdisk -- This program is conceptually similar to the Linux sfdisk and
  FreeBSD gpt programs, but its operational details differ. It enables
  manipulation of GPT disks using command-line options, so it's suitable
  for use in scripts or by experts to perform specific tasks that might
  take several commands in gdisk to accomplish.

- fixparts -- This program, unlike the preceding two, operates on MBR
  disks. It's intended to fix certain problems that can be created by
  various utilities. Specifically, it can fix mis-sized extended partitions
  and primary partitions located in the middle of extended partitions. It
  also enables changing primary vs. logical partition status (within limits
  of what's legal in the MBR scheme) and making a few other minor changes.
  It does NOT support creating new partitions; for that, you should use
  fdisk, parted, or some other tool.

More details about the abilities of these tools follows.

All three programs rely on the same set of underlying code base; they
differ only in their control interfaces (defined in gdisk.cc, sgdisk.cc,
and fixparts.cc, respectively) and in which support code they use.

GPT fdisk (gdisk and sgdisk) Details
------------------------------------

The gdisk program is intended as a (somewhat) fdisk-workalike program for
GPT-partitioned disks, and sgdisk provides most of gdisk's functionality in
a more script-friendly program. Although libparted and programs that use it
(GNU Parted, gparted, etc.) provide the ability to handle GPT disks, they
have certain limitations that gdisk overcomes. Specific advantages of gdisk
and sgdisk include:

* The ability to convert MBR-partitioned disks in-place to GPT format,
  without losing data

* The ability to convert BSD disklabels in-place to create GPT
  partitions, without losing data

* The ability to convert from GPT format to MBR format without data loss

* More flexible specification of filesystem type code GUIDs, which
  GNU Parted tends to corrupt

* Clear identification of the number of unallocated sectors on a
  disk

* A user interface that's familiar to long-time users of Linux
  fdisk (gdisk only)

* The MBR boot loader code is left alone (GNU Parted tends to
  wipe it out with every change)

* The ability to create a hybrid MBR, which permits GPT-unaware
  OSes to access up to three GPT partitions on the disk

Of course, GPT fdisk isn't without its limitations. Most notably, it lacks
the filesystem awareness and filesystem-related features of GNU Parted. You
can't resize a partition's filesystem or create a partition with a
filesystem already in place with gdisk, for instance. There's no GUI
version of gdisk.

The GPT fdisk package provides two program files: the interactive text-mode
gdisk and the command-line-driven sgdisk. The former is intended for use in
manually partitioning disks or changing partitioning details; the latter is
intended for use in scripts to help automate tasks such as disk cloning or
preparing multiple disks for Linux installation.

FixParts Details
----------------

This program's creation was motivated by cries for help I've seen in online
forums from users who have found their partition tables to be corrupted by
various buggy partitioning tools. Although most OSes can handle the
afflicted disks fine, libparted-based tools (GParted, parted, most Linux
installers, etc.) tend to flake out when presented with these disks.
Typically, the symptom is a disk that appears to hold no partitions;
however, sometimes the libparted tool presents partitions other than those
that the OS sees.

I've observed four causes of these symptoms, three of which FixParts can
correct:

- Old GPT data -- If a disk is used as a GPT disk and then re-used as an
  MBR disk, the GPT data may be incompletely erased. This happens if the
  disk is repartitioned with fdisk or the Microsoft Windows installer, for
  instance. (Tools based on libparted correctly remove the old GPT data
  when converting from GPT to MBR format.) FixParts checks for this problem
  when it starts and offers to correct it. If you opt to erase the GPT
  data, this erasure occurs immediately, unlike other changes the program
  makes.

- Mis-sized extended partitions -- Some tools create an extended partition
  that's too large, typically ending after the last sector of the disk.
  FixParts automatically corrects this problem (if you use the 'w' option
  to save the partition table).

- Primary partitions inside an extended partition -- Some utilities create
  or move primary partitions to within the range covered by the extended
  partition. FixParts can usually correct this problem by turning the
  primary partition into a logical partition or by changing one or more
  other logical partitions into primaries. Such corrections aren't always
  possible, though, at least not without deleting or resizing other
  partitions.

- Leftover RAID data -- If a disk is used in a RAID array and then re-used
  as a non-RAID disk, some utilities can become confused and fail to see
  the disk. FixParts can NOT correct this problem. You must destroy the old
  RAID data, or possibly remove the dmraid package from the system, to fix
  this problem.

When run, FixParts presents an fdisk-like interface, enabling you to adjust
partition types (primary, logical, or omitted), change type codes, change
the bootable flag, and so on. Although you can delete a partition (by
omitting it), you can't create new partitions with the program. If you're
used to partitioning disks, particularly with Linux fdisk, two unusual
features of FixParts require elaboration:

- No extended partitions -- Internally, FixParts reads the partition table
  and discards data on any extended partition(s) it finds. When you save
  the partition table, the program generates a new extended partition. This
  design means that the program automatically corrects many problems
  related to the extended partition. It also means that you'll see no
  evidence of extended partitions in the FixParts user interface, although
  it keeps track of the requirements and prevents you from creating illegal
  layouts, such as a primary between two logicals.

- Partition numbering -- In most Linux tools, partitions 1-4 are primaries
  and partitions 5 and up are logicals. Although a legal partition table
  loaded into FixParts will initially conform to this convention, some
  types of damaged table might not, and various changes you make can also
  cause deviations. When FixParts writes the partition table, its numbering
  will be altered to conform to the standard MBR conventions, but you
  should use the explicit labeling of partitions as primary or logical
  rather than the partition numbers to determine a partition's status.

Installing
----------

To compile GPT fdisk, you must have appropriate development tools
installed, most notably the GNU Compiler Collection (GCC) and its g++
compiler for C++. (Under Windows, Microsoft Visual C++ 2008 can also be
used.) In addition, note these requirements:

* On Linux, FreeBSD, and OS X, libuuid must be installed. This is the
  standard for Linux and OS X, although you may need to install a package
  called uuid-dev or something similar to get the headers. On FreeBSD, the
  e2fsprogs-libuuid port must be installed.

* The ICU library (http://site.icu-project.org) is required on all
  platforms. This library is normally installed in Linux, but you may need
  to install the development headers (libicu-dev or something similar).

* The sgdisk program also requires the popt library and its development
  files (headers). Most Linux distributions install popt by default, but
  you may need to install a package called popt-dev, popt-devel, or
  something similar to obtain the header files. Mac OS users can find a
  version of popt for Mac OS from http://popt.darwinports.com; however,
  you'll first need to install DarwinPorts (instructions exist on the
  preceding page). Alternatively, you can compile gdisk alone, without
  sgdisk; gdisk doesn't require popt.

When all the necessary development tools and libraries are installed, you
can uncompress the package and type "make" at the command prompt in the
resulting directory. (You may need to type "make -f Makefile.mac" on Mac OS
X, "make -f Makefile.freebsd" on FreeBSD, or "make -f Makefile.mingw" to
compile using MinGW for Windows.) You may also need to add header (include)
directories or library directories by setting the CXXFLAGS environment
variable or by editing the Makefile. The result should be program files
called gdisk, sgdisk, and fixparts. Typing "make gdisk", "make sgdisk", or
"make fixparts" will compile only the requested programs. You can use these
programs in place or copy the files to a suitable directory, such as
/usr/local/sbin. You can copy the man pages (gdisk.8, sgdisk.8, and
fixparts.8) to /usr/local/man/man8 to make them available.

Caveats
-------

THIS SOFTWARE IS BETA SOFTWARE! IF IT WIPES OUT YOUR HARD DISK OR EATS YOUR
CAT, DON'T BLAME ME! To date, I've tested the software on several USB flash
drives, a handful of PATA and SATA hard disks, and several virtual disks in
the QEMU and VirtualBox environments. Many others have now used the
software on their computers, as well. I believe all data-corruption bugs to
be squashed, but I know full well that the odds of my missing something are
high. This is particularly true for large drives; my only direct testing
with such disks is with virtual QEMU disks. I've received user reports of
success with RAID arrays over 2TiB in size, though.

My main development platform is a system running the 64-bit version of
Gentoo Linux (previously Ubuntu 8.04). I've also tested on several other
32- and 64-bit Linux distributions Intel-based Mac OS X 10.5 and 10.6,
64-bit FreeBSD 7.1, and Windows 7.

Redistribution
--------------

This program is licensed under terms of the GNU GPL (see the file COPYING).

Acknowledgements
----------------

This code is mostly my own; however, I've used three functions from two
other GPLed programs:

- The code used to generate CRCs is taken from the efone program by
  Krzysztof Dabrowski and ElysiuM deeZine. (See the crc32.h and
  crc32.cc source code files.)

- A function to find the disk size is taken from Linux fdisk by A. V. Le
  Blanc. This code has subsequently been heavily modified.

Additional code contributors include:

- Yves Blusseau (1otnwmz02@sneakemail.com)

- David Hubbard (david.c.hubbard@gmail.com)

- Justin Maggard (justin.maggard@netgear.com)

- Dwight Schauer (dschauer@ti.com)

- Florian Zumbiehl (florz@florz.de)