FDISK(8) | System Manager's Manual | FDISK(8) |
fdisk
—
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] |
fdisk |
-l |
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
-1
-2
-3
-A
ptn_alignment[/ptn_0_offset]-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
-B
-b
cylinders/heads/sectors-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
bootcodefdisk
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-F
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
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
-I
flag ignores overlapping error checks and does
not fix them, allowing these incorrect configurations to be used.-i
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
-r
file-S
-s
[id][/[start][/[size][/[bootmenu]]]]-0
, -1
,
-2
, -3
,
or -E
number).-T
disktype-t
disktab-u
-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
-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-z
sectorsizeWhen 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:
fdisk
-l
to list the known
partition types.-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.If the -v
flag is specified, the beginning
and end of each partition are also displayed as follows:
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).
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.
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
fdisk
first appeared in the Mach Operating
System. It was subsequently ported to 386BSD.
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>.
There are subtleties that the program detects that are not explained in this manual page.
April 9, 2019 | NetBSD 9.4 |