NAME
fdisk —
MS-DOS partition maintenance
program
SYNOPSIS
fdisk |
[-aBFfIiSuv]
[-0 | -1 |
-2 | -3 | -E
number [-s
[id][/[start][/[size][/[bootmenu]]]]]]
[-r file |
-w file]
[-A
ptn_alignment[/ptn_0_offset]]
[-b
cylinders/heads/sectors]
[-c
bootcode]
[-T
disktype]
[-t
disktab]
[-z
sectorsize]
[device] |
DESCRIPTION
The
fdisk program is used to display or update the
master boot record or
MBR in the first
sector (sector 0) of a disk that uses the MBR style of partitioning. The
following
NetBSD ports use this style of disk
partitioning: amd64, arc, bebox, cobalt, hpcarm, hpcmips, hpcsh, i386, macppc,
mvmeppc, netwinder, ofppc, playstation2, and prep.
The MBR contains bootable code, a partition table, an indication of which
partition is ‘active’, and (optionally, depending on the boot
code) a menu for selecting a partition to be booted. There can be at most 4
partitions defined in sector 0, one of which can be an extended partition
which can be split into any number of sub-partitions (then called logical
partitions).
The boot code in the MBR is usually invoked by the BIOS or firmware, and the MBR
passes control to the next stage boot code stored in the first sector of the
partition to be booted (the
partition boot record or
PBR).
After booting,
NetBSD does not use the partitioning done
by
fdisk, instead it uses a
NetBSD
disklabel saved in sector 1 of the
NetBSD partition.
See
mbrlabel(8) for a way of
using information from the MBR to construct a
NetBSD
disklabel.
The standard MBR boot code will only boot the ‘active’ partition.
However,
NetBSD contains additional boot programs
which allow the user to interactively select which of the partitions to boot.
The ‘mbr_ext’ code will also boot
NetBSD
from an extended partition but will not work on old systems that do not
support LBA reads, the ‘mbr_com0’ and ‘mbr_com0_9600’
will read and write from a serial port. At the start the
fdisk program will determine whether the disk sector 0 is
valid as a boot sector. (This is determined by checking the magic number.) If
not,
fdisk will initialise the boot code as well as the
partition table. During this, all four partitions will be marked empty.
The flags
-a,
-i or
-u are
used to indicate that the partition data is to be updated. The
fdisk program will enter an interactive conversational mode.
This mode is designed not to change any data unless you explicitly tell it to;
fdisk selects defaults for its questions to guarantee that
behaviour.
If partition data is going to be updated and the disk carries GUID Partition
Tables,
fdisk will remove both primary and backup GPT
headers from the disk. See
gpt(8)
for information on how to manipulate GUID Partition Tables.
fdisk will calculate the correct
cylinder,
head, and
sector values for any partition
you edit. If you specify
-v you will be asked whether you
want to specify them yourself.
Finally, when all the data for the first sector has been accumulated,
fdisk will ask if you really want to write the new partition
table. Only if you reply affirmatively to this question will
fdisk write anything to the disk.
Available options:
-
-
- -0
- Specify partition slot 0 to be printed or updated.
-
-
- -1
- Specify partition slot 1 to be printed or updated.
-
-
- -2
- Specify partition slot 2 to be printed or updated.
-
-
- -3
- Specify partition slot 3 to be printed or updated.
-
-
- -A
ptn_alignment[/ptn_0_offset]
- Specify the alignment for all partitions and optionally the
offset for the first partition of the disk and of logical partitions. If
ptn_alignment is specified and
ptn_0_offset is not specified, then the offset is
set to the alignment. If -A is not specified, then the
alignment of the first partition is inspected. If it ends on a 2048 sector
boundary, then the alignment is set to 2048, if the start is a power of 2
less than, or equal to 2048 then the offset is set to the start sector. If
the first partition is not defined then the alignment and offset for disks
larger than 128GB is set to 2048 (1MB). In all other cases the alignment
default to a cylinder and the offset to a track (both using the BIOS
geometry). The 1MB alignment is the same as that used by recent windows
versions.
-
-
- -a
- Change the active partition. In interactive mode this
question will be asked after the partitions have been processed.
-
-
- -B
- On an i386 or amd64 system, interactively update the boot
selector settings. (The boot selector permits the user to interactively
select the boot partition, and thus which operating system is run, at
system boot time; see mbr(8)
for more information.)
-
-
- -b
cylinders/heads/sectors
- Specify the BIOS geometry parameters for
cylinders, heads, and
sectors. It is used only in conjunction with the
-u flag. If not specified the BIOS geometry will be
obtained using sysctl (i386 and amd64) or by solving the simultaneous
equations from the existing partition information. If that fails then
either the geometry from the disklabel or 63 sectors and 16 heads is used.
For modern disks larger than about 8GB, and where the BIOS is configured
to use LBA-Assisted translation, a setting of -b
1023/255/63 is likely to work.
-
-
- -c
bootcode
- Specify the filename that fdisk should
read the bootcode from. If the name of a directory is specified, then
fdisk will look for files with the default names in that
directory. The default is to read from /usr/mdec/mbr,
/usr/mdec/mbr_bootsel or
/usr/mdec/mbr_ext depending on whether
bootmenu was specified for any partitions on an i386
machine, and leave the bootcode empty for other machines.
-
-
- -E
number
- Specify logical partition number to
be printed or updated. If the specified logical partition does not exist
on updating partition data an additional logical partition will be
created.
-
-
- -F
- Indicate that device is a regular
file. Unless the geometry of device is told to
fdisk by -T
disktype, fdisk will count the
512-byte sectors in device and produce a fake
geometry. If device is a regular file,
-F will be used implicitly.
-
-
- -f
- Run fdisk in a non-interactive mode. In
this mode, you can only change the disk parameters by using the
-b flag. This is provided only so scripts or other
programs may use fdisk as part of an automatic
installation process.
Using the -f flag with -u makes it
impossible to specify the starting and ending
cylinder, head, and
sector fields (only start
and size
can be specified by -s
option). They will be automatically computed using
the BIOS geometry.
-
-
- -I
- Ignore errors from overlapping partitions. Some devices
(cameras CHDK) require overlapping partitions to support bigger than 4GB
cards. The -I flag ignores overlapping error checks and
does not fix them, allowing these incorrect configurations to be
used.
-
-
- -i
- Explicitly request initialisation of the master boot code
(similar to what fdisk /mbr does under MS-DOS), even if
the magic number in the first sector is ok. The partition table is left
alone by this (but see above).
-
-
- -l
- Lists known sysid values and exit.
-
-
- -r
file
- Read the boot record from file file
instead of the specified disk. The geometry information used is still that
of the disk volume. Any changes are written back to the file.
-
-
- -S
- When used with no other flags print a series of
/bin/sh commands for setting variables to the partition
information. This could be used by installation scripts.
-
-
- -s
[id][/[start][/[size][/[bootmenu]]]]
- Specify the partition id,
start, size, and
bootmenu. If the optional arguments are not
provided, they stay as before or use the same defaults as the interactive
mode, if new. This flag requires the use of a partition selection flag
(-0, -1, -2,
-3, or -E
number).
-
-
- -T
disktype
- Use the disklabel disktype instead of
the disklabel on device.
-
-
- -t
disktab
- Read disktype from the named
disktab(5) file instead of
from /etc/disktab.
-
-
- -u
- Update partition data, including id,
start, and
size. Unless -f option
(non-interactive mode) is specified, fdisk will display
the partitions and interactively ask which one you want to edit.
fdisk will step through each field showing the old value
and asking for a new one. The start and
size can be specified in blocks (NN), cylinders (NNc or
NNcyl), megabytes (NNm or NNMB), or gigabytes (NNg or NNGB), values in
megabytes and gigabytes will be rounded to the nearest cylinder boundary.
The size may be specified as $ in
which case the partition will extend to the end of the available free
space.
In a non-interactive mode (specified by -f
option), partition data should be specified by
-s option. A partition selection option
(-0, -1, -2,
-3, or -E
number) should also be specified to select a
partition slot to be updated.
fdisk will not allow you to create partitions which
overlap. If -u and -s are specified in
a non-interactive mode then the details of the specified partition will be
changed. Any other partitions which overlap the requested part of the disk
will be silently deleted.
If bootmenu is specified for any partition
fdisk will determine whether the installed boot code
supports the bootselect code, if it does not you will be asked whether you
want to install the required boot code. To remove a
bootmenu label, simply press ⟨space⟩
followed by ⟨return⟩.
-
-
- -v
- Be more verbose, specifying -v more than
once may increase the amount of output.
Using -v with -u allows the user to
change more parameters than normally permitted.
-
-
- -w
file
- Write the modified partition table to file
file instead of the disk.
-
-
- -z
sectorsize
- Specify a sector size other than 512, for devices that only
support larger sector sizes. The sector size needs to be a power of two
greater than 512.
When called with no arguments, it prints the partition table. An example
follows:
Disk: /dev/rwd0d
NetBSD disklabel disk geometry:
cylinders: 16383, heads: 16, sectors/track: 63 (1008 sectors/cylinder)
total sectors: 40032696
BIOS disk geometry:
cylinders: 1023, heads: 255, sectors/track: 63 (16065 sectors/cylinder)
total sectors: 40032696
Partition table:
0: NetBSD (sysid 169)
bootmenu: net 1.5.
start 4209030, size 8289540 (4048 MB, Cyls 262-778), Active
1: Primary DOS with 32 bit FAT (sysid 11)
bootmenu: win98
start 63, size 4208967 (2055 MB, Cyls 0-262)
2: NetBSD (sysid 169)
bootmenu: current
start 32515560, size 7517136 (3670 MB, Cyls 2024-2491/234/40)
3: Ext. partition - LBA (sysid 15)
start 12498570, size 20016990 (9774 MB, Cyls 778-2024)
Extended partition table:
E0: NetBSD (sysid 169)
bootmenu: test
start 12498633, size 12305727 (6009 MB, Cyls 778-1544)
E1: Primary DOS with 32 bit FAT (sysid 11)
start 24804423, size 4096512 (2000 MB, Cyls 1544-1799)
E2: Primary DOS with 32 bit FAT (sysid 11)
start 28900998, size 3614562 (1765 MB, Cyls 1799-2024)
Bootselector enabled, infinite timeout.
First active partition: 0
This example disk is divided into four partitions, the last of which is an
extended partition. The logical partitions of the extended partition are also
shown. In this case there is no free space in either the disk or in the
extended partition.
The various fields in each partition entry are:
ptn_number:
id_name (sysid id_number)
bootmenu:
bootmenu
start
start, size
size (
MB
MB, Cyls
first-next)
[
, Active]
-
-
- ptn_number
- is the number of the partition.
-
-
- id_name
- is the name of the filesystem type or operating system that
uses this partition.
-
-
- id_number
- is the number that identifies the partition type. 169
decimal is used for NetBSD partitions, 15 decimal
to create an extended partition and 0 to mark a partition as unused. Use
fdisk -l to list the known partition
types.
-
-
- bootmenu
- is the menu prompt output by the interactive boot code for
this partition. This line is omitted if the prompt is not defined.
-
-
- start,
size
- are the start address and size of the partition in
sectors.
-
-
- MB
- is the size of the partition in megabytes.
-
-
- first,
next
- are the bounds of this partition displayed as
cylinder/head/sector. If the partition starts (or ends) on a cylinder
boundary the head and sector values are omitted. If -v
is not specified the start of logical partitions and the first partition
on the disk are rounded down to include the mandatory red tape in the
preceding track.
-
-
- Active
- is output if this is the active partition.
If the
-v flag is specified, the beginning and end of each
partition are also displayed as follows:
beg: cylinder
cylinder, head
head, sector
sector
end: cylinder
cylinder, head
head, sector
sector
-
-
- cylinder,
head, sector
- are the beginning or ending address of a partition.
Note: these numbers are read from the bootblock, so are
the values calculated by a previous run of fdisk.
fdisk attempts to check whether each partition is bootable, by
checking the magic number and some other characteristics of the first sector
of each partition (the PBR). If the partition does not appear to be bootable,
fdisk will print a line containing “PBR is not
bootable” followed by an error message. If the partition is bootable,
and if the
-v flag is specified,
fdisk
will print “PBR appears to be bootable”. If the
-v flag is specified more than once,
fdisk
will print the heading “Information from PBR:” followed by one or
more lines of information gleaned from the PBR; this additional information
may be incorrect or misleading, because different operating systems use
different PBR formats. Note that, even if no errors are reported, an attempt
to boot from the partition might fail.
NetBSD
partitions may be made bootable using
installboot(8).
NOTES
This program is only available (and useful) on systems with PC-platform-style
MBR partitioning.
Traditionally the partition boundaries should be on cylinder boundaries using
the BIOS geometry, with the exception of the first partition, which
traditionally begins in the second track of the first cylinder (cylinder 0,
head 1, sector 1). Although the BIOS geometry is typically different from the
geometry reported by the drive, neither will match the actual physical
geometry for modern disks (the actual geometry will vary across the disk).
Keeping the partition boundaries on cylinder boundaries makes partitioning a
driver easier as only relatively small numbers need be entered.
The automatic calculation of the starting cylinder and other parameters uses a
set of figures that represent what the BIOS thinks is the geometry of the
drive. The default values should be correct for the system on which
fdisk is run; however, if you move the disk to a different
system, the BIOS of that system might use a different geometry translation.
If you run the equivalent of
fdisk on a different operating
system then the
bootmenu strings associated with
extended partitions may be lost.
Editing an existing partition is risky, and may cause you to lose all the data
in that partition.
You should run this program interactively once or twice to see how it works.
This is completely safe as long as you answer the last question in the
negative. You can also specify
-w file
to write the output to a file and later specify
-r
file to read back the updated information. This can be
done without having write access to the disk volume.
FILES
- /usr/mdec/mbr
- Default location of i386 bootcode
- /usr/mdec/mbr_bootsel
- Default location of i386 bootselect code
- /usr/mdec/mbr_ext
- Default location of i386 bootselect for extended partitions
(i.e., NetBSD on logical partitions)
EXAMPLES
Update MBR partition data of
/dev/rwd0d in interactive mode:
fdisk -u
/dev/rwd0d
Change active MBR partition of
/dev/rwd0d in interactive mode:
fdisk -a
/dev/rwd0d
Install MBR bootcode
/usr/mdec/mbr_bootsel into
/dev/rwd0d:
fdisk -c /usr/mdec/mbr_bootsel
/dev/rwd0d
Set MBR partition data for slot 0 of
/dev/rwd0d specifying
values without prompt:
fdisk -f -u -0 -s 169/63/2097089
/dev/rwd0d
Make partition slot 0 of
/dev/rwd0d active without prompt:
fdisk -f -a -0
/dev/rwd0d
Initialize and create MBR partition data using bootcode
destdir/usr/mdec/mbr without prompt against 1GB disk image
file
diskimg:
fdisk -f -i -b 130/255/63 -c
destdir/usr/mdec/mbr -F diskimg
Create MBR partition data for slot 0 which has an active
NetBSD partition using whole disk without prompt
against 1GB disk image file
diskimg:
fdisk -f -a -u -0 -s
169/63/2097089 -F diskimg
SEE ALSO
disktab(5),
boot(8),
disklabel(8),
gpt(8),
installboot(8),
mbr(8),
mbrlabel(8)
HISTORY
A version of
fdisk first appeared in the Mach Operating
System. It was subsequently ported to
386BSD.
AUTHORS
fdisk for Mach Operating System was written by
Robert Baron
<
rvb@cs.cmu.edu>. It was
ported to
386BSD by
Julian
Elischer
<
julian@tfs.com>.
BUGS
The word ‘partition’ is used to mean both an MBR partition and a
NetBSD partition, sometimes in the same sentence.
There are subtleties that the program detects that are not explained in this
manual page.