gdisk - Interactive GUID partition table (GPT) manipulator
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;
however, 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 options require you to understand the distinctions between the main
and backup data, as well as between the GPT headers and the partition
tables. For information on MBR vs. GPT, as well as GPT terminology and
structure, see the extended gdisk documentation at
http://www.rodsbooks.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 unmodified.
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 instance) or raw disk images used by emulators
such as QEMU or VMWare. Note that only raw disk images are supported;
gdisk cannot work on compressed 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
PowerPC-based Macintoshes. Upon exiting with the 'w' option, gdisk
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 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 created 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 partition 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 consult your boot loader
documentation for details.
- *
- If Windows is to boot from a GPT disk, a partition of type
Microsoft 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.
- -l
- List the partition table for the specified device and then exits.
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 current
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 program, 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 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.
- 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' option displays this information for a
single partition.
- l
- Display a summary of partition types. GPT uses a GUID to identify
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 sequentially, 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, entering linux
shows only partition type codes with descriptions that include the string
Linux. This search is performed case-insensitively.
- n
- Create a new partition. This command is modeled after the equivalent
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 sector numbers or as
positions 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 '+' 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
optimize partition alignment.
- o
- Clear out all partition data. This includes GPT header data, all partition
definitions, and the protective MBR. The sector alignment 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 additional information,
use the 'i' command.
- q
- 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.
- 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 reflected 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 option 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 provides 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 instance). 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 destroyed (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 partition 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 partition 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 partitions as
possible into MBR form, destroys the GPT data structures, 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
unallocated 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 inserted 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 hybrid-unaware GPT utilities are used to edit the disk.
Thus, you may need to re-create the hybrid MBR if you use such tools.
Unlike 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 reverse of the
'b' option on the main menu. Note that restoring partition data from
anything but the original disk is not recommended.
- 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 protective MBR's
partitions with this option. This may enable you to spot glaring problems
or help identify the partitions in a hybrid 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 adjustment. 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' option 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 between 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
automount. 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-endian 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 assigns 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 structures. 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 hybrid 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 sectors 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 properly 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
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. 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 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.
- 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 protective 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 protective MBR's
partitions with this option. This may enable you to spot glaring problems
or help identify the partitions in a hybrid 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 identical 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 specification; however, in practice they
seem to work, and can sometimes be useful in converting MBR disks. Larger
sizes also work fine. OSes may impose their own limits on the number of
partitions, though.
- t
- Swap two partitions' entries in the partition table. One partition 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 device. 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 already created new MBR partitions,
it's conceivable that this option 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 bypasses the prompt entirely.
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, although I've heard of 64-bit versions being
successfully compiled.
- *
- 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 partitions and
124 logical partitions) when converting from MBR format. This limit can be
raised by changing the #define MAX_MBR_PARTS line in the
basicmbr.h source code file and recompiling; 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
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) 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 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 (using the 'h' option on the recovery
& transformation menu) or abandoning GPT in favor of MBR may be your
only options in this case.
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)
The gdisk command is part of the GPT fdisk package
and is available from Rod Smith.