freebsd/sbin/fdisk/fdisk.8

.\" $FreeBSD$
.\"
.Dd May 24, 2009
.Dt FDISK 8
.Os
.Sh NAME
.Nm fdisk
.Nd PC slice table maintenance utility
.Sh SYNOPSIS
.Nm
.Op Fl BIaipqstu
.Op Fl b Ar bootcode
.Op Fl 1234
.Op Ar disk
.Nm
.Fl f Ar configfile
.Op Fl itv
.Op Ar disk
.Sh PROLOGUE
In order for the BIOS to boot the kernel,
certain conventions must be adhered to.
Sector 0 of the disk must contain boot code,
a slice table,
and a magic number.
BIOS slices can be used to break the disk up into several pieces.
The BIOS brings in sector 0 and verifies the magic number.
The sector
0 boot code then searches the slice table to determine which
slice is marked
.Dq active .
This boot code then brings in the bootstrap from the
active slice and, if marked bootable, runs it.
Under
.Tn DOS ,
you can have one or more slices with one active.
The
.Tn DOS
.Nm
utility can be used to divide space on the disk into slices and set one
active.
.Sh DESCRIPTION
The
.Fx
utility,
.Nm ,
serves a similar purpose to the
.Tn DOS
utility.
The first form is used to
display slice information or to interactively edit the slice
table.
The second is used to write a slice table using a
.Ar configfile ,
and is designed to be used by other scripts/programs.
.Pp
Options are:
.Bl -tag -width indent
.It Fl a
Change the active slice only.
Ignored if
.Fl f
is given.
.It Fl b Ar bootcode
Get the boot code from the file
.Ar bootcode .
Default is
.Pa /boot/mbr .
.It Fl B
Reinitialize the boot code contained in sector 0 of the disk.
Ignored if
.Fl f
is given.
.It Fl f Ar configfile
Set slice values using the file
.Ar configfile .
The
.Ar configfile
only modifies explicitly specified slices, unless
.Fl i
is also given, in which case all existing slices are deleted (marked
as
.Dq unused )
before the
.Ar configfile
is read.
The
.Ar configfile
can be
.Sq Fl ,
in which case standard input is read.
See
.Sx CONFIGURATION FILE ,
below, for file syntax.
.Pp
.Em WARNING :
when
.Fl f
is used, you are not asked if you really want to write the slices
table (as you are in the interactive mode).
Use with caution!
.It Fl i
Initialize sector 0 of the disk.
Existing slice entries will be cleared
(marked as unused) before editing.
(Compare with
.Fl u . )
.It Fl I
Initialize sector 0 slice table
for one
.Fx
slice covering the entire disk.
.It Fl p
Print a slice table in
.Nm
configuration file format and exit; see
.Sx CONFIGURATION FILE ,
below.
.It Fl q
Be quiet.
Benign warnings (such as "GEOM not found") are suppressed.
.It Fl s
Print summary information and exit.
.It Fl t
Test mode; do not write slice values.
Generally used with the
.Fl f
option to see what would be written to the slice table.
Implies
.Fl v .
.It Fl u
Update (edit) the disk's sector 0 slice table.
Ignored if
.Fl f
is given.
.It Fl v
Be verbose.
When
.Fl f
is used,
.Nm
prints out the slice table that is written to the disk.
.It Fl 1234
Operate on a single slice table entry only.
Ignored if
.Fl f
is given.
.El
.Pp
The final disk name can be provided as a
.Dq bare
disk name only, e.g.\&
.Pa da0 ,
or as a full pathname.
If omitted,
.Nm
tries to figure out the default disk device name from the
mounted root device.
.Pp
When called with no arguments, it prints the sector 0 slice table.
An example follows:
.Bd -literal
	******* Working on device /dev/ad0 *******
	parameters extracted from in-core disklabel are:
	cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

	parameters to be used for BIOS calculations are:
	cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

	Warning: BIOS sector numbering starts with sector 1
	Information from DOS bootblock is:
	The data for partition 1 is:
	sysid 165,(FreeBSD/NetBSD/386BSD)
    	    start 495, size 380160 (185 Meg), flag 0
		beg: cyl 1/ sector 1/ head 0;
		end: cyl 768/ sector 33/ head 14
	The data for partition 2 is:
	sysid 164,(unknown)
    	    start 378180, size 2475 (1 Meg), flag 0
		beg: cyl 764/ sector 1/ head 0;
		end: cyl 768/ sector 33/ head 14
	The data for partition 3 is:
	<UNUSED>
	The data for partition 4 is:
	sysid 99,(ISC UNIX, other System V/386, GNU HURD or Mach)
    	    start 380656, size 224234 (109 Meg), flag 80
		beg: cyl 769/ sector 2/ head 0;
		end: cyl 197/ sector 33/ head 14
.Ed
.Pp
The disk is divided into three slices that happen to fill the disk.
The second slice overlaps the end of the first.
(Used for debugging purposes.)
.Bl -tag -width ".Em cyl , sector No and Em head"
.It Em sysid
is used to label the slice.
.Fx
reserves the
magic number 165 decimal (A5 in hex).
.It Xo
.Em start
and
.Em size
.Xc
fields provide the start address
and size of a slice in sectors.
.It Em "flag 80"
specifies that this is the active slice.
.It Xo
.Em cyl , sector
and
.Em head
.Xc
fields are used to specify the beginning and end addresses of the slice.
.El
.Pp
.Em Note :
these numbers are calculated using BIOS's understanding of the disk geometry
and saved in the bootblock.
.Pp
The
.Fl i
and
.Fl u
flags are used to indicate that the slice data is to be updated.
Unless the
.Fl f
option is also given,
.Nm
will enter a conversational mode.
In this mode, no changes will be written to disk unless you explicitly tell
.Nm
to.
.Pp
The
.Nm
utility will display each slice and ask whether you want to edit it.
If you say yes,
.Nm
will step through each field, show you the old value,
and ask you for a new one.
When you are done with the slice,
.Nm
will display it and ask you whether it is correct.
It will then proceed to the next entry.
.Pp
Getting the
.Em cyl , sector ,
and
.Em head
fields correct is tricky, so by default,
they will be calculated for you;
you can specify them if you choose to though.
.Pp
After all the slices are processed,
you are given the option to change the
.Dq active
slice.
Finally, when all the new data for sector 0 has been accumulated,
you are asked to confirm whether you really want to rewrite it.
.Pp
The difference between the
.Fl u
and
.Fl i
flags is that
the
.Fl u
flag edits (updates) the existing slice parameters
while the
.Fl i
flag is used to
.Dq initialize
them (old values will be ignored);
if you edit the first slice,
.Fl i
will also set it up to use the whole disk for
.Fx
and make it active.
.Sh NOTES
The automatic calculation of starting cylinder etc.\& uses
a set of figures that represent what the BIOS thinks the
geometry of the drive is.
These figures are taken from the in-core disklabel by default,
but
.Nm
initially gives you an opportunity to change them.
This allows you to create a bootblock that can work with drives
that use geometry translation under the BIOS.
.Pp
If you hand craft your disk layout,
please make sure that the
.Fx
slice starts on a cylinder boundary.
.Pp
Editing an existing slice will most likely result in the loss of
all data in that slice.
.Pp
You should run
.Nm
interactively once or twice to see how it works.
This is completely safe as long as you answer the last question
in the negative.
There are subtleties that
.Nm
detects that are not fully explained in this manual page.
.Sh CONFIGURATION FILE
When the
.Fl f
option is given, a disk's slice table can be written using values
from a
.Ar configfile .
The syntax of this file is very simple;
each line is either a comment or a specification, as follows:
.Bl -tag -width indent
.It Ic # Ar comment ...
Lines beginning with a
.Ic #
are comments and are ignored.
.It Ic g Ar spec1 spec2 spec3
Set the BIOS geometry used in slice calculations.
There must be
three values specified, with a letter preceding each number:
.Bl -tag -width indent
.It Cm c Ns Ar num
Set the number of cylinders to
.Ar num .
.It Cm h Ns Ar num
Set the number of heads to
.Ar num .
.It Cm s Ns Ar num
Set the number of sectors/track to
.Ar num .
.El
.Pp
These specs can occur in any order, as the leading letter determines
which value is which; however, all three must be specified.
.Pp
This line must occur before any lines that specify slice
information.
.Pp
It is an error if the following is not true:
.Bd -literal -offset indent
1 <= number of cylinders
1 <= number of heads <= 256
1 <= number of sectors/track < 64
.Ed
.Pp
The number of cylinders should be less than or equal to 1024, but this
is not enforced, although a warning will be printed.
Note that bootable
.Fx
slices (the
.Dq Pa /
file system) must lie completely within the
first 1024 cylinders; if this is not true, booting may fail.
Non-bootable slices do not have this restriction.
.Pp
Example (all of these are equivalent), for a disk with 1019 cylinders,
39 heads, and 63 sectors:
.Bd -literal -offset indent
g       c1019   h39     s63
g       h39     c1019   s63
g       s63     h39     c1019
.Ed
.It Ic p Ar slice type start length
Set the slice given by
.Ar slice
(1-4) to type
.Ar type ,
starting at sector
.Ar start
for
.Ar length
sectors.
If the
.Ar start
or
.Ar length
is suffixed with a
.Em K ,
.Em M
or
.Em G ,
it is taken as a
.Em Kilobyte ,
.Em Megabyte
or
.Em Gigabyte
measurement respectively.
If the
.Ar start
is given as
.Qq *
it is set to the value of the previous partition end.
If the
.Ar length
is given as
.Qq *
the partition end is set to the end of the disk.
.Pp
Only those slices explicitly mentioned by these lines are modified;
any slice not referenced by a
.Ic p
line will not be modified.
However, if an invalid slice table is present, or the
.Fl i
option is specified, all existing slice entries will be cleared
(marked as unused), and these
.Ic p
lines will have to be used to
explicitly set slice information.
If multiple slices need to be
set, multiple
.Ic p
lines must be specified; one for each slice.
.Pp
These slice lines must occur after any geometry specification lines,
if one is present.
.Pp
The
.Ar type
is 165 for
.Fx
slices.
Specifying a slice type of zero is
the same as clearing the slice and marking it as unused; however,
dummy values (such as
.Dq 0 )
must still be specified for
.Ar start
and
.Ar length .
.Pp
Note: the start offset will be rounded upwards to a head boundary if
necessary, and the end offset will be rounded downwards to a cylinder
boundary if necessary.
.Pp
Example: to clear slice 4 and mark it as unused:
.Pp
.Dl "p       4       0       0       0"
.Pp
Example: to set slice 1 to a
.Fx
slice, starting at sector 1
for 2503871 sectors (note: these numbers will be rounded upwards and
downwards to correspond to head and cylinder boundaries):
.Pp
.Dl "p       1       165     1       2503871"
.Pp
Example: to set slices 1, 2 and 4 to
.Fx
slices, the first being 2 Gigabytes, the second being 10 Gigabytes and the
forth being the remainder of the disk (again, numbers will be rounded
appropriately):
.Pp
.Dl "p       1       165     63      2G"
.Dl "p       2       165     *       10G"
.Dl "p       3       0       0       0"
.Dl "p       4       165     *       *"
.It Ic a Ar slice
Make
.Ar slice
the active slice.
Can occur anywhere in the config file, but only
one must be present.
.Pp
Example: to make slice 1 the active slice:
.Pp
.Dl "a       1"
.El
.Sh FILES
.Bl -tag -width ".Pa /boot/mbr" -compact
.It Pa /boot/mbr
The default boot code.
.El
.Sh SEE ALSO
.Xr boot0cfg 8 ,
.Xr bsdlabel 8 ,
.Xr gpart 8 ,
.Xr newfs 8
.Sh BUGS
The default boot code will not necessarily handle all slice types
correctly, in particular those introduced since
.Tn MS-DOS
6.x.
.Pp
The entire utility should be made more user-friendly.
.Pp
Most users new to
.Fx
do not understand the difference between
.Dq slice
and
.Dq partition ,
causing difficulty to adjust.
.Pp
You cannot use this command to completely dedicate a disk to
.Fx .
The
.Xr bsdlabel 8
command must be used for this.