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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 manipulation of partition ta-
bles. The cgdisk member of this family employs a curses-based user interface for interac-
tion 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 Table (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 structure, see the extended
GPT fdisk documentation at http://www.rodsbooks.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 structures until you explicitly write them to disk, so if you make a mistake, 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 instance) 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 im-
age 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' option, cgdisk replaces the MBR or
disklabel with a GPT. This action is potentially dangerous! Your system may become un-
bootable, 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 with-
out making any changes by using 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 created in whatever order and in what-
ever sizes are desired.
* Boot disks for EFI-based systems require an EFI System Partition (GPT fdisk inter-
nal code 0xEF00) formatted as FAT-32. The recommended size of this partition is
between 100 and 300 MiB. Boot-related files are stored here. (Note that GNU Parted
identifies 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 documentation
for details.
* If Windows is to boot from a GPT disk, a partition of type Microsoft Reserved (GPT
fdisk internal code 0x0C01) is recommended. This partition should be about 128 MiB
in size. It ordinarily 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 (speci-
fying the starting sector as '+128M', for instance) to simplify creating such gaps.
OPTIONS
Only one command-line option is accepted, aside from the device filename: -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 bro-
ken 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 version number, the de-
vice 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 option's name on the keyboard, such as a to
activate the Align option.
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 advanced operations, you should consult the gdisk docu-
mentation.)
Align Change the sector alignment value. Disks with more logical sectors 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 properly
for their internal data structures. On new disks, GPT fdisk attempts to align par-
titions on 1 MiB boundaries (2048-sectors on disks with 512-byte sectors) by de-
fault, which optimizes performance 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.
Backup Save partition data to a backup file. You can back up your current in-memory parti-
tion 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 cur-
rent in-memory data structures, so if you launch the program, make changes, and
then use this option, the backup will reflect your changes.
Delete Delete a partition. This action deletes the entry from the partition table but does
not disturb the data within the sectors originally allocated to the partition on
the disk. If a corresponding 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 details, 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 reverse 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 parti-
tion 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 acti-
vate this item by typing its alphabetic equivalent, you must use M, not the more
obvious N, because the latter is used by the next option....
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 absolute 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 specifies the default value,
which is the start of the largest available block for the start sector and the full
available size for the size.
Quit Quit from the program without saving your changes. Use this option 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 optionally be filtered
by a search string; for instance, entering linux shows only partition type codes
with descriptions that include the string Linux. This search is performed case-in-
sensitively.
Verify Verify disk. This option checks for a variety of problems, such as incorrect CRCs
and mismatched main and backup data. This option 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 capability. Linux versions for x86-64 (64-bit), x86 (32-bit),
and PowerPC (32-bit) have been tested, with the x86-64 version having 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 partitions and 124 logi-
cal partitions) when converting from MBR format. This limit can be raised by chang-
ing the #define MAX_MBR_PARTS line in the basicmbr.h source code file and recompil-
ing; however, such a change will require using a larger-than-normal partition ta-
ble. (The limit of 128 partitions was chosen because that number equals the 128
partitions supported by the most common partition table size.)
* Converting from MBR format sometimes fails because of insufficient space at the
start or (more commonly) the end of the disk. Resizing the partition table (using
the 's' option in the experts' menu in gdisk) can sometimes overcome this problem;
however, 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 extreme cases the affected partition(s) may need to be deleted.
* Because of the highly variable nature of BSD disklabel structures, conversions from
this form may be unreliable -- partitions 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 loaders. Except on EFI-based platforms, Windows through at
least Windows 7 doesn't support booting from GPT disks. Creating a hybrid MBR (us-
ing 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 obtainable by typing L in
the Type function (or when specifying a partition type while creating a new parti-
tion) both currently exit ncurses mode. This limitation is a minor cosmetic blemish
that does not affect functionality.
AUTHORS
Primary author: Roderick W. Smith (rodsmith AT rodsbooks.com)
Contributors:
* Yves Blusseau (1otnwmz02 AT sneakemail.com)
* David Hubbard (david.c.hubbard AT gmail.com)
* Justin Maggard (justin.maggard AT netgear.com)
* Dwight Schauer (das AT teegra.net)
* Florian Zumbiehl (florz AT 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 1.0.8 CGDISK(8)
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