gdisk.8

.\" Copyright 2009 Roderick W. Smith (rodsmith@rodsbooks.com)
.\" May be distributed under the GNU General Public License
.TH "GDISK" "8" "0.5.1" "Roderick W. Smith" "GPT fdisk Manual"
.SH "NAME"
gdisk \- GUID partition table (GPT) manipulator for Linux and Unix
.SH "SYNOPSIS"
.BI "gdisk "
[ \-l ]
.I device

.SH "DESCRIPTION"

GPT fdisk (aka \fBgdisk\fR) 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 \fI\-l\fR 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
\fBgptsync\fR or \fBgdisk\fR'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 \fBgdisk\fR documentation at
\fIhttp://www.rodsbooks.com/gdisk/\fR or consult Wikipedia.

The \fBgdisk\fR program employs a user interface similar to that of Linux's
\fBfdisk\fR, but \fBgdisk\fR modifies GPT partitions. It also has the
capability of transforming MBR partitions or BSD disklabels into GPT
partitions. Like the original \fBfdisk\fR program, \fBgdisk\fR 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 save
your partitions.

Ordinarily, \fBgdisk\fR operates on disk device files, such as
\fI/dev/sda\fR or \fI/dev/hda\fR under Linux, \fI/dev/disk0\fR under
Mac OS X, or \fI/dev/ad0\fR or \fI/dev/da0\fR under FreeBSD. The program
can also operate on disk image files, which can be either copies of whole
disks (made with \fBdd\fR, for instance) or raw disk images used by
emulators such as QEMU or VMWare. Note that only \fIraw\fR disk images
are supported; \fBgdisk\fR 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
\fBgdisk\fR, do not need to deal with CHS geometries and all the problems
they create. Users of \fBfdisk\fR will note that \fBgdisk\fR
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 \fBgdisk\fR or GNU Parted program.

Upon start, \fBgdisk\fR attempts to identify the partition type in use on
the disk. If it finds valid GPT data, \fBgdisk\fR will use it. If
\fBgdisk\fR 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, \fBgdisk\fR
replaces the MBR or disklabel with a GPT. \fIThis action is potentially
dangerous!\fR 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 \fBgdisk\fR 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 \fI/etc/fstab\fR file.)

When creating a fresh partition table, certain considerations may be in
order:

.TP 
.B *
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.

.TP 
.B *
Boot disks for EFI\-based systems require an \fIEFI System
Partition\fR (\fBgdisk\fR internal code 0xEF00) formatted as FAT\-32.
The recommended size of this partition is between 100 and 200 MiB.
Boot\-related files are stored here. (Note that GNU Parted identifies
such partitions as having the "boot flag" set.)

.TP 
.B *
Some boot loaders for BIOS\-based systems make use of a \fIBIOS Boot
Partition\fR (\fBgdisk\fR internal code 0xEF02), in which the secondary
boot loader is stored, possibly without the benefit of a filesystem. This
partition can typically be quite small (roughly 32 to 200 KiB), but you
should consult your boot loader documentation for details.

.TP 
.B *
If Windows is to boot from a GPT disk, a partition of type \fIMicrosoft
Reserved\fR (\fBgdisk\fR
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 GNU Parted creates all
FAT partitions as this type, which actually makes the partition unusable
for normal file storage in both Windows and Mac OS X.)

.TP 
.B *
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.

.SH "OPTIONS"
.TP 
.B \-l
List the partition tables for the specified devices and then exit.
.PP 

Most interactions with \fBgdisk\fR
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:

.TP 
.B 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.


.TP 
.B c
Change the GPT name of a partition. This name is encoded as a UTF\-16
string, but \fBgdisk\fR
supports only ASCII characters as names. 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.

.TP 
.B 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,
\fBgdisk\fR deletes it, as well, and expands any adjacent 0xEE (EFI GPT)
MBR protective partition to fill the new free space.

.TP 
.B 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 \fBgdisk\fR's
internal partition type code to a plain type name. The 'i' option
displays this information for a single partition.

.TP 
.B 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,
\fBgdisk\fR 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 \fBgdisk\fR.
A one\-to\-one correspondence is impossible, though. Most notably, many DOS,
Windows, and Linux data partition codes correspond to a single GPT code
(entered as 0x0700 in \fBgdisk\fR). Some OSes use a single MBR code but
employ many more codes in GPT. For these, \fBgdisk\fR
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 \fBgdisk\fR.

.TP 
.B n
Create a new partition. This command is modelled after the equivalent
\fBfdisk\fR 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 kilobytes (K), megabytes (M), gigabytes (G), or terabytes (T);
for instance, \fI\fB40M\fR\fR specifies a position 40MiB from the start of
the disk. You can specify locations relative to the start or end of the
specified range by preceding the number by a '+' or '\-' symbol, as in
\fI\fB+2G\fR\fR to specify a point 2GiB after the first available sector,
or \fI\fB\-200M\fR\fR 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 last available block for the end sector.

.TP 
.B o
Clear out all partition data. This includes GPT header data,
all partition definitions, and the protective MBR.

.TP 
.B p
Display basic partition summary data. This includes partition
numbers, starting and ending sector numbers, partition sizes,
\fBgdisk\fR's partition types codes, and partition names. For
additional information, use the 'i' command.

.TP 
.B q
Quit from the program \fIwithout saving your changes\fR.
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.

.TP 
.B 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.

.TP 
.B 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 \fI/etc/fstab\fR if you use this option.

.TP 
.B 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 \fBgdisk\fR doesn't know it.

.TP 
.B 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.

.TP 
.B w
Write data. Use this command to save your changes.

.TP 
.B 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.
.PP 

.TP 
.B ?
Print the menu. Type this command (or any other unrecognized command) to
see a summary of available options.

.PP 
The second \fBgdisk\fR 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:

.TP 
.B 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
\fBdd\fR).

.TP 
.B c
Load backup partition table. Ordinarily, \fBgdisk\fR
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.

.TP 
.B 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.

.TP 
.B 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.

.TP 
.B 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.

.TP 
.B g
Convert GPT into MBR and exit. This option converts up to four GPT partitions
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 \fIprimary\fR MBR partitions;
it cannot generate logical partitions, and so it cannot transform more than
four partitions. If four or fewer partitions exist, and if they can be represented
in the 32\-bit MBR LBA scheme, this function converts
them all. If more than four partitions exist, you'll be asked to select which
ones to convert. See also the 'h' option.

.TP 
.B 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 recreate the hybrid MBR if you use such tools.

.TP 
.B i
Show detailed partition information. This option is identical to the 'i'
option on the main menu.

.TP 
.B 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.

.TP 
.B m
Return to the main menu. This option enables you to enter main\-menu commands.

.TP 
.B 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.

.TP 
.B p
Print the partition table. This option is identical to the 'p' option in
the main menu.

.TP 
.B q
Quit without saving changes. This option is identical to the 'q' option in
the main menu.

.TP 
.B 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. \fBgdisk\fR
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 \fBgdisk\fR being unable to convert a BSD disklabel is
high compared to the likelihood of problems with an MBR conversion.

.TP 
.B v
Verify disk. This option is identical to the 'v' option in the main menu.

.TP 
.B w
Write table to disk and exit. This option is identical to the 'w' option in
the main menu.

.TP 
.B x
Enter the experts' menu. This option is identical to the 'x' option in the
main menu.

.TP 
.B ?
Print the menu. This option (or any unrecognized entry) displays a summary
of the menu options.

.PP 
The third \fBgdisk\fR 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:

.TP 
.B a
Set attributes. GPT provides a 64\-bit attributes field that can be used to
set features for each partition. \fBgdisk\fR supports four attributes:
\fIsystem partition\fR, \fIread\-only\fR, \fIhidden\fR, and
\fIdo not automount\fR. You can set other attributes, but their numbers
aren't translated into anything useful. In practice, most OSes seem to
ignore these attributes.

.TP 
.B 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, \fBgdisk\fR 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 \fBgdisk\fR) or sheer incredible coincidence.

.TP
.B d
Display the number of logical sectors per physical sector. This value
determines the sector alignment that GPT fdisk enforces. See the
description of the 'l' option for more details. Note that this value
is not actually detected on a disk-by-disk basis; it's set to 8 as a
blanket default.

.TP
.B 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.

.TP 
.B g
Change disk GUID. Each disk has a unique GUID code, which \fBgdisk\fR
assigns randomly upon creation of the GPT data structures. You can generate
a fresh random GUID or enter one manually with this option.

.TP 
.B i
Show detailed partition information. This option is identical to the 'i'
option on the main menu.

.TP
.B l
Change the number of logical sectors per physical sector. Prior to December
of 2009, most hard disks used 512-byte physical sectors. Starting in
December of 2009, disk manufacturers began transitioning to disks with
larger physical sectors, but their firmware translated to 512-byte logical
sectors to maintain compatibility with older OSes. If partitions begin
mid-physical-sector, though, performance can suffer on such drives, since
important filesystem data structures can span physical sectors on the disk.
To minimize such problems, GPT fdisk aligns the start of partitions on the
boundary of presumed physical sectors. You can set the number of logical
sectors per physical sector with this option. The default is 8, which is
set blindly; GPT fdisk does not currently read this information from the
disk. The default value will result in a tiny amount of wasted disk space
on older disks with true 512-byte sectors but will otherwise be harmless.

.TP 
.B m
Return to the main menu. This option enables you to enter main\-menu commands.

.TP 
.B n
Create a new protective MBR. Use this option if the current protective MBR
is damaged in a way that \fBgdisk\fR doesn't automatically detect and
correct, or if you want to convert a hybrid MBR into a "pure" GPT with a
conventional protective MBR.

.TP 
.B 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.

.TP 
.B p
Print the partition table. This option is identical to the 'p' option in
the main menu.

.TP 
.B q
Quit without saving changes. This option is identical to the 'q' option in
the main menu.

.TP 
.B r
Enter the recovery & transformations menu. This option is identical to
the 'r' option on the main menu.

.TP 
.B 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.

.TP 
.B v
Verify disk. This option is identical to the 'v' option in the main menu.

.TP 
.B z
Zap (destroy) the GPT data structures and exit. Use this option if you want to
repartition a GPT disk using \fBfdisk\fR 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. \fIIf you've
already created new MBR partitions, it's conceivable that this option will
damage the first and/or last MBR partitions!\fR Such an event is unlikely, but
could occur if your new MBR partitions overlap the old GPT data structures.

.TP 
.B ?
Print the menu. This option (or any unrecognized entry) displays a summary
of the menu options.

.PP 
In many cases, you can press the Enter key to select a default option when
entering data. When only one option is possible, \fBgdisk\fR
usually bypasses the prompt entirely.

.SH "BUGS"
As of November 2009 (version 0.5.1), \fBgdisk\fR
should be considered beta software. Known bugs and limitations include:

.TP 
.B *
The program compiles correctly only on Linux, FreeBSD, and Mac OS X. 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. The Mac OS X
support was added with version 0.3.1 and has not been as thoroughly tested.
FreeBSD support was added with version 0.4.0 and has seen even less
testing.

.TP 
.B *
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
\fBgpt\fR, \fBfdisk\fR, and \fBdd\fR.)

.TP 
.B *
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.

.TP 
.B *
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.

.TP 
.B *
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 \fI#define MAX_MBR_PARTS\fR line in the
\fImbr.h\fR 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.)

.TP 
.B *
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
\fBgdisk\fR.

.TP 
.B *
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.

.TP 
.B *
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.

.TP 
.B *
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!

.TP 
.B *
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 RC 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.

.PP 

The support for big\-endian CPUs (PowerPC, for example) is new, as of version
0.3.5. I advise using caution on that platform, particularly with the more
obscure features of the program.

.SH "AUTHORS"
Primary author: Roderick W. Smith (rodsmith@rodsbooks.com)

Contributors:

* Yves Blusseau (1otnwmz02@sneakemail.com)

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

.SH "SEE ALSO"
\fBcfdisk (8)\fR,
\fBfdisk (8)\fR,
\fBmkfs (8)\fR,
\fBparted (8)\fR,
\fBsfdisk (8)\fR

\fIhttp://en.wikipedia.org/wiki/GUID_Partition_Table\fR

\fIhttp://developer.apple.com/technotes/tn2006/tn2166.html\fR

\fIhttp://www.rodsbooks.com/gdisk/\fR

.SH "AVAILABILITY"
The \fBgdisk\fR command is part of the \fIGPT fdisk\fR package and is
available from Rod Smith.