CGDISK(8) GPT fdisk Manual CGDISK(8)
NAME
cgdisk - Curses-based GUID partition table (GPT) manipulator
SYNOPSIS
cgdisk [ -a ] device
DESCRIPTION
GPT fdisk is a text-mode family of programs for creation and manipula-
tion of partition tables. The cgdisk member of this family employs a
curses-based user interface for interaction using a text-mode menuing
system. It will automatically convert an old-style Master Boot Record
(MBR) partition table or BSD disklabel stored without an MBR carrier
partition to the newer Globally Unique Identifier (GUID) Partition Ta-
ble (GPT) format, or will load a GUID partition table. Other members of
this program family are gdisk (the most feature-rich program of the
group, with a non-curses-based interactive user interface) and sgdisk
(which is driven via command-line options for use by experts or in
scripts). FixParts is a related program for fixing a limited set of
problems with MBR disks.
For information on MBR vs. GPT, as well as GPT terminology and struc-
ture, see the extended GPT fdisk documentation at http://www.rods-
books.com/gdisk/ or consult Wikipedia.
The cgdisk program employs a user interface similar to that of Linux's
cfdisk, but cgdisk modifies GPT partitions. It also has the capability
of transforming MBR partitions or BSD disklabels into GPT partitions.
Like the original cfdisk program, cgdisk does not modify disk struc-
tures until you explicitly write them to disk, so if you make a mis-
take, you can exit from the program with the Quit option to leave your
partitions unmodified.
Ordinarily, cgdisk operates on disk device files, such as /dev/sda or
/dev/hda under Linux, /dev/disk0 under Mac OS X, or /dev/ad0 or
/dev/da0 under FreeBSD. The program can also operate on disk image
files, which can be either copies of whole disks (made with dd, for in-
stance) or raw disk images used by emulators such as QEMU or VMWare.
Note that only raw disk images are supported; cgdisk cannot work on
compressed or other advanced disk image formats.
Upon start, cgdisk attempts to identify the partition type in use on
the disk. If it finds valid GPT data, cgdisk will use it. If cgdisk
finds a valid MBR or BSD disklabel but no GPT data, it will attempt to
convert the MBR or disklabel into GPT form. (BSD disklabels are likely
to have unusable first and/or final partitions because they overlap
with the GPT data structures, though.) Upon exiting with the 'w' op-
tion, cgdisk replaces the MBR or disklabel with a GPT. This action is
potentially dangerous! Your system may become unbootable, and partition
type codes may become corrupted if the disk uses unrecognized type
codes. Boot problems are particularly likely if you're multi-booting
with any GPT-unaware OS. If you mistakenly launch cgdisk on an MBR
disk, you can safely exit the program without making any changes by us-
ing the Quit option.
When creating a fresh partition table, certain considerations may be in
order:
* For data (non-boot) disks, and for boot disks used on BIOS-based
computers with GRUB as the boot loader, partitions may be cre-
ated in whatever order and in whatever sizes are desired.
* Boot disks for EFI-based systems require an EFI System Partition
(GPT fdisk internal code 0xEF00) formatted as FAT-32. The rec-
ommended size of this partition is between 100 and 300 MiB.
Boot-related files are stored here. (Note that GNU Parted iden-
tifies such partitions as having the "boot flag" set.)
* The GRUB 2 boot loader for BIOS-based systems makes use of a
BIOS Boot Partition (GPT fdisk internal code 0xEF02), in which
the secondary boot loader is stored, without the benefit of a
filesystem. This partition can typically be quite small (roughly
32 KiB to 1 MiB), but you should consult your boot loader docu-
mentation for details.
* If Windows is to boot from a GPT disk, a partition of type Mi-
crosoft Reserved (GPT fdisk internal code 0x0C01) is recom-
mended. This partition should be about 128 MiB in size. It ordi-
narily follows the EFI System Partition and immediately precedes
the Windows data partitions. (Note that old versions of GNU
Parted create all FAT partitions as this type, which actually
makes the partition unusable for normal file storage in both
Windows and Mac OS X.)
* Some OSes' GPT utilities create some blank space (typically 128
MiB) after each partition. The intent is to enable future disk
utilities to use this space. Such free space is not required of
GPT disks, but creating it may help in future disk maintenance.
You can use GPT fdisk's relative partition positioning option
(specifying the starting sector as '+128M', for instance) to
simplify creating such gaps.
OPTIONS
Only one command-line option is accepted, aside from the device file-
name: -a. This option alters the highlighting of partitions and blocks
of free space: Instead of using ncurses, when -a is used cgdisk uses a
">" symbol to the left of the selected partition or free space. This
option is intended for use on limited display devices such as teletypes
and screen readers.
Interactions with cgdisk occur with its interactive text-mode menus.
The display is broken into two interactive parts:
* The partition display area, in which partitions and gaps between
them (marked as "free space") are summarized.
* The option selection area, in which buttons for the main options
appear.
In addition, the top of the display shows the program's name and ver-
sion number, the device filename associated with the disk, and the
disk's size in both sectors and IEEE-1541 units (GiB, TiB, and so on).
You can use the following keys to move among the various options and to
select among them:
up arrow
This key moves the partition selection up by one partition.
down arrow
This key moves the partition selection down by one partition.
Page Up
This key moves the partition selection up by one screen.
Page Down
This key moves the partition selection down by one screen.
right arrow
This key moves the option selection to the right by one item.
left arrow
This key moves the option selection to the left by one item.
Enter This key activates the currently selected option. You can also
activate an option by typing the capitalized letter in the op-
tion's name on the keyboard, such as a to activate the Align op-
tion.
If more partitions exist than can be displayed in one screen, you can
scroll between screens using the partition selection keys, much as in a
text editor.
Available options are as described below. (Note that cgdisk provides a
much more limited set of options than its sibling gdisk. If you need to
perform partition table recovery, hybrid MBR modification, or other ad-
vanced operations, you should consult the gdisk documentation.)
Align Change the sector alignment value. Disks with more logical sec-
tors than physical sectors (such as modern Advanced Format
drives), some RAID configurations, and many SSD devices, can
suffer performance problems if partitions are not aligned prop-
erly for their internal data structures. On new disks, GPT fdisk
attempts to align partitions on 1 MiB boundaries (2048-sectors
on disks with 512-byte sectors) by default, which optimizes per-
formance for all of these disk types. On pre-partitioned disks,
GPT fdisk attempts to identify the alignment value used on that
disk, but will set 8-sector alignment on disks larger than 300
GB even if lesser alignment values are detected. In either case,
it can be changed by using this option. The alignment value
also affects the default end sector value when creating a new
partition; it will be aligned to one less than a multiple of the
alignment value, when possible. This should keep partitions a
multiple of the alignment value in size. Some disk encryption
tools require partitions to be sized to some value, typically
4096 bytes, so the default alignment of 1 MiB works well for
them.
Backup Save partition data to a backup file. You can back up your cur-
rent in-memory partition table to a disk file using this option.
The resulting file is a binary file consisting of the protective
MBR, the main GPT header, the backup GPT header, and one copy of
the partition table, in that order. Note that the backup is of
the current in-memory data structures, so if you launch the pro-
gram, make changes, and then use this option, the backup will
reflect your changes.
Delete Delete a partition. This action deletes the entry from the par-
tition table but does not disturb the data within the sectors
originally allocated to the partition on the disk. If a corre-
sponding hybrid MBR partition exists, gdisk deletes it, as well,
and expands any adjacent 0xEE (EFI GPT) MBR protective partition
to fill the new free space.
Help Print brief descriptions of all the options.
Info Show detailed partition information. The summary information
shown in the partition display area necessarily omits many de-
tails, such as the partitions' unique GUIDs and the partitions'
sector-exact start and end points. The Info option displays this
information for a single partition.
Load Load partition data from a backup file. This option is the re-
verse of the Backup option. Note that restoring partition data
from anything but the original disk is not recommended.
naMe Change the GPT name of a partition. This name is encoded as a
UTF-16 string, but proper entry and display of anything beyond
basic ASCII values requires suitable locale and font support.
For the most part, Linux ignores the partition name, but it may
be important in some OSes. GPT fdisk sets a default name based
on the partition type code. Note that the GPT partition name is
different from the filesystem name, which is encoded in the
filesystem's data structures. Note also that to activate this
item by typing its alphabetic equivalent, you must use M, not
the more obvious N, because the latter is used by the next op-
tion....
New Create a new partition. You enter a starting sector, a size, a
type code, and a name. The start sector can be specified in ab-
solute terms as a sector number or as a position measured in
kibibytes (K), mebibytes (M), gibibytes (G), tebibytes (T), or
pebibytes (P); for instance, 40M specifies a position 40MiB from
the start of the disk. You can specify locations relative to the
start or end of the specified default range by preceding the
number by a '+' symbol, as in +2G to specify a point 2GiB after
the default start sector. The size value can use the K, M, G, T,
and P suffixes, too. Pressing the Enter key with no input speci-
fies the default value, which is the start of the largest avail-
able block for the start sector and the full available size for
the size.
Quit Quit from the program without saving your changes. Use this op-
tion if you just wanted to view information or if you make a
mistake and want to back out of all your changes.
Type Change a single partition's type code. You enter the type code
using a two-byte hexadecimal number. You may also enter a GUID
directly, if you have one and cgdisk doesn't know it. If you
don't know the type code for your partition, you can type L to
see a list of known type codes. The type code list may option-
ally be filtered by a search string; for instance, entering
linux shows only partition type codes with descriptions that in-
clude the string Linux. This search is performed case-insensi-
tively.
Verify Verify disk. This option checks for a variety of problems, such
as incorrect CRCs and mismatched main and backup data. This op-
tion does not automatically correct most problems, though; for
that, you must use gdisk. If no problems are found, this command
displays a summary of unallocated disk space.
Write Write data. Use this command to save your changes.
BUGS
Known bugs and limitations include:
* The program compiles correctly only on Linux, FreeBSD, and Mac
OS X. In theory, it should compile under Windows if the Ncurses
library for Windows is installed, but I have not tested this ca-
pability. Linux versions for x86-64 (64-bit), x86 (32-bit), and
PowerPC (32-bit) have been tested, with the x86-64 version hav-
ing seen the most testing. Under FreeBSD, 32-bit (x86) and
64-bit (x86-64) versions have been tested. Only 32-bit versions
for Mac OS X has been tested by the author.
* The FreeBSD version of the program can't write changes to the
partition table to a disk when existing partitions on that disk
are mounted. (The same problem exists with many other FreeBSD
utilities, such as gpt, fdisk, and dd.) This limitation can be
overcome by typing sysctl kern.geom.debugflags=16 at a shell
prompt.
* The program can load only up to 128 partitions (4 primary parti-
tions and 124 logical partitions) when converting from MBR for-
mat. This limit can be raised by changing the #define
MAX_MBR_PARTS line in the basicmbr.h source code file and recom-
piling; however, such a change will require using a
larger-than-normal partition table. (The limit of 128 partitions
was chosen because that number equals the 128 partitions sup-
ported by the most common partition table size.)
* Converting from MBR format sometimes fails because of insuffi-
cient space at the start or (more commonly) the end of the disk.
Resizing the partition table (using the 's' option in the ex-
perts' menu in gdisk) can sometimes overcome this problem; how-
ever, in extreme cases it may be necessary to resize a partition
using GNU Parted or a similar tool prior to conversion with GPT
fdisk.
* MBR conversions work only if the disk has correct LBA partition
descriptors. These descriptors should be present on any disk
over 8 GiB in size or on smaller disks partitioned with any but
very ancient software.
* BSD disklabel support can create first and/or last partitions
that overlap with the GPT data structures. This can sometimes be
compensated by adjusting the partition table size, but in ex-
treme cases the affected partition(s) may need to be deleted.
* Because of the highly variable nature of BSD disklabel struc-
tures, conversions from this form may be unreliable -- parti-
tions may be dropped, converted in a way that creates overlaps
with other partitions, or converted with incorrect start or end
values. Use this feature with caution!
* Booting after converting an MBR or BSD disklabel disk is likely
to be disrupted. Sometimes re-installing a boot loader will fix
the problem, but other times you may need to switch boot load-
ers. Except on EFI-based platforms, Windows through at least
Windows 7 doesn't support booting from GPT disks. Creating a hy-
brid MBR (using the 'h' option on the recovery & transformation
menu in gdisk) or abandoning GPT in favor of MBR may be your
only options in this case.
* The cgdisk Verify function and the partition type listing ob-
tainable by typing L in the Type function (or when specifying a
partition type while creating a new partition) both currently
exit ncurses mode. This limitation is a minor cosmetic blemish
that does not affect functionality.
AUTHORS
Primary author: Roderick W. Smith (rodsmith@rodsbooks.com)
Contributors:
* Yves Blusseau (1otnwmz02@sneakemail.com)
* David Hubbard (david.c.hubbard@gmail.com)
* Justin Maggard (justin.maggard@netgear.com)
* Dwight Schauer (das@teegra.net)
* Florian Zumbiehl (florz@florz.de)
SEE ALSO
cfdisk(8), fdisk(8), gdisk(8), mkfs(8), parted(8), sfdisk(8),
sgdisk(8), fixparts(8).
http://en.wikipedia.org/wiki/GUID_Partition_Table
http://developer.apple.com/technotes/tn2006/tn2166.html
http://www.rodsbooks.com/gdisk/
AVAILABILITY
The cgdisk command is part of the GPT fdisk package and is available
from Rod Smith.
Roderick W. Smith 1.0.9 CGDISK(8)
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