GDISK(8) GPT fdisk Manual GDISK(8)
NAME
gdisk - Interactive GUID partition table (GPT) manipulator
SYNOPSIS
gdisk [ -l ] device
DESCRIPTION
GPT fdisk (aka gdisk) is a text-mode menu-driven program for creation
and manipulation of partition tables. 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. When used with the -l command-line option, the program
displays the current partition table and then exits.
GPT fdisk operates mainly on the GPT headers and partition tables; how-
ever, it can and will generate a fresh protective MBR, when required.
(Any boot loader code in the protective MBR will not be disturbed.) If
you've created an unusual protective MBR, such as a hybrid MBR created
by gptsync or gdisk's own hybrid MBR creation feature, this should not
be disturbed by most ordinary actions. Some advanced data recovery op-
tions require you to understand the distinctions between the main and
backup data, as well as between the GPT headers and the partition ta-
bles. For information on MBR vs. GPT, as well as GPT terminology and
structure, see the extended gdisk documentation at http://www.rods-
books.com/gdisk/ or consult Wikipedia.
The gdisk program employs a user interface similar to that of Linux's
fdisk, but gdisk modifies GPT partitions. It also has the capability of
transforming MBR partitions or BSD disklabels into GPT partitions. Like
the original fdisk program, gdisk 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 'q' option to leave your partitions un-
modified.
Ordinarily, gdisk 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; gdisk cannot work on com-
pressed or other advanced disk image formats.
The MBR partitioning system uses a combination of cylinder/head/sector
(CHS) addressing and logical block addressing (LBA). The former is
klunky and limiting. GPT drops CHS addressing and uses 64-bit LBA mode
exclusively. Thus, GPT data structures, and therefore gdisk, do not
need to deal with CHS geometries and all the problems they create.
Users of fdisk will note that gdisk lacks the options and limitations
associated with CHS geometries.
For best results, you should use an OS-specific partition table program
whenever possible. For example, you should make Mac OS X partitions
with the Mac OS X Disk Utility program and Linux partitions with the
Linux gdisk or GNU Parted program.
Upon start, gdisk attempts to identify the partition type in use on the
disk. If it finds valid GPT data, gdisk will use it. If gdisk 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.) GPT fdisk can identify, but not use data
in, Apple Partition Map (APM) disks, which are used on 680x0- and Pow-
erPC-based Macintoshes. Upon exiting with the 'w' option, gdisk re-
places the MBR or disklabel with a GPT. This action is potentially dan-
gerous! Your system may become unbootable, and partition type codes may
become corrupted if the disk uses unrecognized type codes. Boot prob-
lems are particularly likely if you're multi-booting with any GPT-un-
aware OS. If you mistakenly launch gdisk on an MBR disk, you can safely
exit the program without making any changes by using the 'q' option.
The MBR-to-GPT conversion will leave at least one gap in the partition
numbering if the original MBR used logical partitions. These gaps are
harmless, but you can eliminate them by using the 's' option, if you
like. (Doing this may require you to update your /etc/fstab file.)
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
(gdisk internal code 0xEF00) formatted as FAT-32. I recommended
making this partition 550 MiB. (Smaller ESPs are common, but
some EFIs have flaky FAT drivers that necessitate a larger par-
tition for reliable operation.) Boot-related files are stored
here. (Note that GNU Parted identifies such partitions as having
the "boot flag" set.)
* Some boot loaders for BIOS-based systems make use of a BIOS Boot
Partition (gdisk internal code 0xEF02), in which the secondary
boot loader is stored, possibly without the benefit of a
filesystem. (GRUB2 may optionally use such a partition.) This
partition can typically be quite small (roughly 32 to 200 KiB,
although 1 MiB is more common in practice), but you should con-
sult your boot loader documentation for details.
* If Windows is to boot from a GPT disk, a partition of type Mi-
crosoft Reserved (gdisk 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
(specifying the starting sector as '+128M', for instance) to
simplify creating such gaps.
OPTIONS
-l List the partition table for the specified device and then ex-
its.
Most interactions with gdisk occur with its interactive text-mode
menus. Three menus exist: the main menu, the recovery & transformation
menu, and the experts' menu. The main menu provides the functions that
are most likely to be useful for typical partitioning tasks, such as
creating and deleting partitions, changing partition type codes, and so
on. Specific functions are:
b 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. Note also that the restore option is on
the recovery & transformation menu; the backup option is on the
main menu to encourage its use.
c 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.
d 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.
i Show detailed partition information. The summary information
produced by the 'p' command necessarily omits many details, such
as the partition's unique GUID and the translation of gdisk's
internal partition type code to a plain type name. The 'i' op-
tion displays this information for a single partition.
l Display a summary of partition types. GPT uses a GUID to iden-
tify partition types for particular OSes and purposes. For ease
of data entry, gdisk compresses these into two-byte (four-digit
hexadecimal) values that are related to their equivalent MBR
codes. Specifically, the MBR code is multiplied by hexadecimal
0x0100. For instance, the code for Linux swap space in MBR is
0x82, and it's 0x8200 in gdisk. A one-to-one correspondence is
impossible, though. Most notably, the codes for all varieties of
FAT and NTFS partition correspond to a single GPT code (entered
as 0x0700 in gdisk). Some OSes use a single MBR code but employ
many more codes in GPT. For these, gdisk adds code numbers se-
quentially, such as 0xa500 for a FreeBSD disklabel, 0xa501 for
FreeBSD boot, 0xa502 for FreeBSD swap, and so on. Note that
these two-byte codes are unique to gdisk. The type code list may
optionally be filtered by a search string; for instance, enter-
ing linux shows only partition type codes with descriptions that
include the string Linux. This search is performed case-insensi-
tively.
n Create a new partition. This command is modeled after the equiv-
alent fdisk option, although some differences exist. You enter a
partition number, starting sector, and an ending sector. Both
start and end sectors can be specified in absolute terms as sec-
tor numbers or as positions measured in kibibytes (K), mebibytes
(M), gibibytes (G), tebibytes (T), or pebibytes (P); for in-
stance, 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 '+' or
'-' symbol, as in +2G to specify a point 2GiB after the default
start sector, or -200M to specify a point 200MiB before the last
available sector. 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 end of the same block for the
end sector. Default start and end points may be adjusted to op-
timize partition alignment.
o Clear out all partition data. This includes GPT header data, all
partition definitions, and the protective MBR. The sector align-
ment is reset to the default (1 MiB, or 2048 sectors on a disk
with 512-byte sectors).
p Display basic partition summary data. This includes partition
numbers, starting and ending sector numbers, partition sizes,
gdisk's partition types codes, and partition names. For addi-
tional information, use the 'i' command.
q 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.
r Enter the recovery & transformation menu. This menu includes
emergency recovery options (to fix damaged GPT data structures)
and options to transform to or from other partitioning systems,
including creating hybrid MBRs.
s Sort partition entries. GPT partition numbers need not match the
order of partitions on the disk. If you want them to match, you
can use this option. Note that some partitioning utilities sort
partitions whenever they make changes. Such changes will be re-
flected in your device filenames, so you may need to edit
/etc/fstab if you use this option.
t Change a single partition's type code. You enter the type code
using a two-byte hexadecimal number, as described earlier. You
may also enter a GUID directly, if you have one and gdisk
doesn't know it.
v 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 options on the recovery & transformation
menu. If no problems are found, this command displays a summary
of unallocated disk space.
w Write data. Use this command to save your changes.
x Enter the experts' menu. Using this option provides access to
features you can use to get into even more trouble than the main
menu allows.
? Print the menu. Type this command (or any other unrecognized
command) to see a summary of available options.
The second gdisk menu is the recovery & transformation menu, which pro-
vides access to data recovery options and features related to the
transformation of partitions between partitioning schemes (converting
BSD disklabels into GPT partitions or creating hybrid MBRs, for in-
stance). A few options on this menu duplicate functionality on the
main menu, for the sake of convenience. The options on this menu are:
b Rebuild GPT header from backup. You can use the backup GPT
header to rebuild the main GPT header with this option. It's
likely to be useful if your main GPT header was damaged or de-
stroyed (say, by sloppy use of dd).
c Load backup partition table. Ordinarily, gdisk uses only the
main partition table (although the backup's integrity is checked
when you launch the program). If the main partition table has
been damaged, you can use this option to load the backup from
disk and use it instead. Note that this will almost certainly
produce no or strange partition entries if you've just converted
an MBR disk to GPT format, since there will be no backup parti-
tion table on disk.
d Use main GPT header and rebuild the backup. This option is
likely to be useful if the backup GPT header has been damaged or
destroyed.
e Load main partition table. This option reloads the main parti-
tion table from disk. It's only likely to be useful if you've
tried to use the backup partition table (via 'c') but it's in
worse shape then the main partition table.
f Load MBR and build fresh GPT from it. Use this option if your
GPT is corrupt or conflicts with the MBR and you want to use the
MBR as the basis for a new set of GPT partitions.
g Convert GPT into MBR and exit. This option converts as many par-
titions as possible into MBR form, destroys the GPT data struc-
tures, saves the new MBR, and exits. Use this option if you've
tried GPT and find that MBR works better for you. Note that
this function generates up to four primary MBR partitions or
three primary partitions and as many logical partitions as can
be generated. Each logical partition requires at least one unal-
located block immediately before its first block. Therefore, it
may be possible to convert a maximum of four partitions on disks
with tightly-packed partitions; however, if free space was in-
serted between partitions when they were created, and if the
disk is under 2 TiB in size, it should be possible to convert
all the partitions to MBR form. See also the 'h' option.
h Create a hybrid MBR. This is an ugly workaround that enables
GPT-unaware OSes, or those that can't boot from a GPT disk, to
access up to three of the partitions on the disk by creating MBR
entries for them. Note that these hybrid MBR entries can easily
go out of sync with the GPT entries, particularly when hy-
brid-unaware GPT utilities are used to edit the disk. Thus, you
may need to re-create the hybrid MBR if you use such tools. Un-
like the 'g' option, this option does not support converting any
partitions into MBR logical partitions.
i Show detailed partition information. This option is identical to
the 'i' option on the main menu.
l Load partition data from a backup file. This option is the re-
verse of the 'b' option on the main menu. Note that restoring
partition data from anything but the original disk is not recom-
mended.
m Return to the main menu. This option enables you to enter
main-menu commands.
o Print protective MBR data. You can see a summary of the protec-
tive MBR's partitions with this option. This may enable you to
spot glaring problems or help identify the partitions in a hy-
brid MBR.
p Print the partition table. This option is identical to the 'p'
option in the main menu.
q Quit without saving changes. This option is identical to the 'q'
option in the main menu.
t Transform BSD partitions into GPT partitions. This option works
on BSD disklabels held within GPT (or converted MBR) partitions.
Converted partitions' type codes are likely to need manual ad-
justment. gdisk will attempt to convert BSD disklabels stored on
the main disk when launched, but this conversion is likely to
produce first and/or last partitions that are unusable. The many
BSD variants means that the probability of gdisk being unable to
convert a BSD disklabel is high compared to the likelihood of
problems with an MBR conversion.
v Verify disk. This option is identical to the 'v' option in the
main menu.
w Write table to disk and exit. This option is identical to the
'w' option in the main menu.
x Enter the experts' menu. This option is identical to the 'x' op-
tion in the main menu.
? Print the menu. This option (or any unrecognized entry) displays
a summary of the menu options.
The third gdisk menu is the experts' menu. This menu provides advanced
options that aren't closely related to recovery or transformation be-
tween partitioning systems. Its options are:
a Set attributes. GPT provides a 64-bit attributes field that can
be used to set features for each partition. gdisk supports four
attributes: system partition, read-only, hidden, and do not au-
tomount. You can set other attributes, but their numbers aren't
translated into anything useful. In practice, most OSes seem to
ignore these attributes.
b Swap the byte order for the name of the specified partition.
Some partitioning tools, including GPT fdisk 1.0.7 and earlier,
can write the partition name in the wrong byte order on big-en-
dian computers, such as the IBM s390 mainframes and PowerPC-
based Macs. This feature corrects this problem.
c Change partition GUID. You can enter a custom unique GUID for a
partition using this option. (Note this refers to the GUID that
uniquely identifies a partition, not to its type code, which you
can change with the 't' main-menu option.) Ordinarily, gdisk as-
signs this number randomly; however, you might want to adjust
the number manually if you've wound up with the same GUID on two
partitions because of buggy GUID assignments (hopefully not in
gdisk) or sheer incredible coincidence.
d Display the sector alignment value. See the description of the
'l' option for more details.
e Move backup GPT data structures to the end of the disk. Use this
command if you've added disks to a RAID array, thus creating a
virtual disk with space that follows the backup GPT data struc-
tures. This command moves the backup GPT data structures to the
end of the disk, where they belong.
f Randomize the disk's GUID and all partitions' unique GUIDs (but
not their partition type code GUIDs). This function may be used
after cloning a disk with another utility in order to render all
GUIDs once again unique.
g Change disk GUID. Each disk has a unique GUID code, which gdisk
assigns randomly upon creation of the GPT data structures. You
can generate a fresh random GUID or enter one manually with this
option.
h Recompute CHS values in protective or hybrid MBR. This option
can sometimes help if a disk utility, OS, or BIOS doesn't like
the CHS values used by the partitions in the protective or hy-
brid MBR. In particular, the GPT specification requires a CHS
value of 0xFFFFFF for over-8GiB partitions, but this value is
technically illegal by the usual standards. Some BIOSes hang if
they encounter this value. This option will recompute a more
normal CHS value -- 0xFEFFFF for over-8GiB partitions, enabling
these BIOSes to boot.
i Show detailed partition information. This option is identical to
the 'i' option on the main menu.
j Adjust the location of the main partition table. This value is
normally 2, but it may need to be increased in some cases, such
as when a system-on-chip (SoC) is hard-coded to read boot code
from sector 2. I recommend against adjusting this value unless
doing so is absolutely necessary.
l Change the sector alignment value. Disks with more logical sec-
tors per 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, if possible. This should keep partitions a mul-
tiple 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.
m Return to the main menu. This option enables you to enter
main-menu commands.
n Create a new protective MBR. Use this option if the current pro-
tective MBR is damaged in a way that gdisk doesn't automatically
detect and correct, or if you want to convert a hybrid MBR into
a "pure" GPT with a conventional protective MBR.
o Print protective MBR data. You can see a summary of the protec-
tive MBR's partitions with this option. This may enable you to
spot glaring problems or help identify the partitions in a hy-
brid MBR.
p Print the partition table. This option is identical to the 'p'
option in the main menu.
q Quit without saving changes. This option is identical to the 'q'
option in the main menu.
r Enter the recovery & transformations menu. This option is iden-
tical to the 'r' option on the main menu.
s Resize partition table. The default partition table size is 128
entries. Officially, sizes of less than 16KB (128 entries,
given the normal entry size) are unsupported by the GPT specifi-
cation; however, in practice they seem to work, and can some-
times be useful in converting MBR disks. Larger sizes also work
fine. OSes may impose their own limits on the number of parti-
tions, though.
t Swap two partitions' entries in the partition table. One parti-
tion may be empty. For instance, if partitions 1-4 are defined,
transposing 1 and 5 results in a table with partitions numbered
from 2-5. Transposing partitions in this way has no effect on
their disk space allocation; it only alters their order in the
partition table.
u Replicate the current device's partition table on another de-
vice. You will be prompted to type the new device's filename.
After the write operation completes, you can continue editing
the original device's partition table. Note that the replicated
partition table is an exact copy, including all GUIDs; if the
device should have its own unique GUIDs, you should use the f
option on the new disk.
v Verify disk. This option is identical to the 'v' option in the
main menu.
z Zap (destroy) the GPT data structures and exit. Use this option
if you want to repartition a GPT disk using fdisk or some other
GPT-unaware program. You'll be given the choice of preserving
the existing MBR, in case it's a hybrid MBR with salvageable
partitions or if you've already created new MBR partitions and
want to erase the remnants of your GPT partitions. If you've al-
ready created new MBR partitions, it's conceivable that this op-
tion will damage the first and/or last MBR partitions! Such an
event is unlikely, but could occur if your new MBR partitions
overlap the old GPT data structures.
? Print the menu. This option (or any unrecognized entry) displays
a summary of the menu options.
In many cases, you can press the Enter key to select a default option
when entering data. When only one option is possible, gdisk usually by-
passes the prompt entirely.
BUGS
Known bugs and limitations include:
* The program compiles correctly only on Linux, FreeBSD, Mac OS X,
and Windows. 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 and Windows have been tested by the author, al-
though I've heard of 64-bit versions being successfully com-
piled.
* 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 fields used to display the start and end sector numbers for
partitions in the 'p' command are 14 characters wide. This
translates to a limitation of about 45 PiB. On larger disks, the
displayed columns will go out of alignment.
* In the Windows version, only ASCII characters are supported in
the partition name field. If an existing partition uses
non-ASCII UTF-16 characters, they're likely to be corrupted in
the 'i' and 'p' menu options' displays; however, they should be
preserved when loading and saving partitions. Binaries for
Linux, FreeBSD, and OS X support full UTF-16 partition names.
* 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) 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 gdisk.
* 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) or abandoning GPT in favor of MBR may be your only options
in this case.
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), cgdisk(8), fdisk(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 gdisk command is part of the GPT fdisk package and is available
from Rod Smith.
Roderick W. Smith 1.0.9 GDISK(8)
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