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SFDISK
!!!SFDISK
NAME
SYNOPSIS
DESCRIPTION
OPTIONS
THEORY
INPUT FORMAT
EXAMPLE
DOS 6.x WARNING
DRDOS WARNINGS
BUGS
AUTHOR
SEE ALSO
----
!!NAME


sfdisk - Partition table manipulator for Linux
!!SYNOPSIS


__sfdisk__ [[options] device__
sfdisk -s__ [[partition]
!!DESCRIPTION


__sfdisk__ has four (main) uses: list the size of a
partition, list the partitions on a device, check the
partitions on a device, and - very dangerous - repartition a
device.


__List Sizes__


__sfdisk -s__ ''partition'' gives the size of
''partition'' in blocks. This may be useful in connection
with programs like __mkswap__ or so. Here
''partition'' is usually something like ''/dev/hda1''
or ''/dev/sdb12'', but may also be an entire disk, like
''/dev/xda''.


 % sfdisk -s /dev/hda9
 81599
 %


If the partition argument is omitted, __sfdisk__ will
list the sizes of all disks, and the total:


 % sfdisk -s
 /dev/hda: 208896
 /dev/hdb: 1025136
 /dev/hdc: 1031063
 /dev/sda: 8877895
 /dev/sdb: 1758927
 total: 12901917 blocks
 %


__List Partitions__


The second type of invocation: __sfdisk -l__ ''[[options]
device'' will list the partitions on this device. If the
device argument is omitted, the partitions on all hard disks
are listed.


 % sfdisk -l /dev/hdc

 Disk /dev/hdc: 16 heads, 63 sectors, 2045 cylinders
 Units = cylinders of 516096 bytes, blocks of 1024 bytes, counting from 0
 Device Boot Start     End   #cyls   #blocks   Id  System

 /dev/hdc1          0+    406     407-   205096+  83  Linux native
 /dev/hdc2        407     813     407    205128   83  Linux native
 /dev/hdc3        814    2044    1231    620424   83  Linux native
 /dev/hdc4          0       -       0         0    0  Empty
 %

The trailing - and + signs indicate that rounding has taken place, and that the actual value is slightly less (more). To see the exact values, ask for a listing with sectors as unit.


__Check partitions__


The third type of invocation: __sfdisk -V__ ''device''
will apply various consistency checks to the partition
tables on ''device''. It prints `OK' or complains. The -V
option can be used together with -l. In a shell script one
might use __sfdisk -V -q__ ''device'' which only
returns a status.


__Create partitions__


The fourth type of invocation: __sfdisk__ ''device''
will cause __sfdisk__ to read the specification for the
desired partitioning of ''device'' from its standard
input, and then to change the partition tables on that disk.
Thus, it is possible to use __sfdisk__ from a shell
script. When __sfdisk__ determines that its standard
input is a terminal, it will be conversational; otherwise it
will abort on any error.


BE EXTREMELY CAREFUL - ONE TYPING MISTAKE AND ALL YOUR DATA
IS LOST


As a precaution, one can save the sectors changed by
__sfdisk__:


 % sfdisk /dev/hdd -O hdd-partition-sectors.save
 ...
 %


Then, if you discover that you did something stupid before
anything else has been written to disk, it may be possible
to recover the old situation with


 % sfdisk /dev/hdd -I hdd-partition-sectors.save
 %


(This is not the same as saving the old partition table: a
readable version of the old partition table can be saved
using the -d option. However, if you create logical
partitions, the sectors describing them are located
somewhere on disk, possibly on sectors that were not part of
the partition table before. Thus, the information the -O
option saves is not a binary version of the output of
-d.)


There are many options.
!!OPTIONS


__-v__ or __--version__


Print version number of __sfdisk__ and exit
immediately.


__-?__ or __--help__


Print a usage message and exit immediately.


__-T__ or __--list-types__


Print the recognized types (system Id's).


__-s__ or __--show-size__


List the size of a partition.


__-g__ or __--show-geometry__


List the kernel's idea of the geometry of the indicated
disk(s).


__-l__ or __--list__


List the partitions of a device.


__-d__


Dump the partitions of a device in a format useful as input
to sfdisk. For example,


% sfdisk -d /dev/hda
will correct the bad last extended partition that the OS/2 fdisk creates.


__-V__ or __--verify__


Test whether partitions seem correct. (See
above.)


__-i__ or __--increment__


Number cylinders etc. starting from 1 instead of
0.


__-N__ ''number''


Change only the single partition indicated. For
example:


 % sfdisk /dev/hdb -N5
 ,,,*
 %
will make the fifth partition on /dev/hdb bootable (`active') and change nothing else. (Probably this fifth partition is called /dev/hdb5, but you are free to call it something else, like `/my_equipment/disks/2/5' or so).


__-A__''number''


Make the indicated partition(s) active, and all others
inactive.


__-c__ ''or'' __--id__ ''number
[[Id]''


If no Id argument given: print the partition Id of the
indicated partition. If an Id argument is present: change
the type (Id) of the indicated partition to the given value.
This option has the two very long forms --print-id and
--change-id. For example:


 % sfdisk --print-id /dev/hdb 5
 6
 % sfdisk --change-id /dev/hdb 5 83
 OK
first reports that /dev/hdb5 has Id 6, and then changes that into 83.


__-uS__ or __-uB__ or __-uC__ or
__-uM__


Accept or report in units of sectors (blocks, cylinders,
megabytes, respectively). The default is cylinders, at least
when the geometry is known.


__-x__ or __--show-extended__


Also list non-primary extended partitions on output, and
expect descriptors for them on input.


__-C__ ''cylinders''


Specify the number of cylinders, possibly overriding what
the kernel thinks.


__-H__ ''heads''


Specify the number of heads, possibly overriding what the
kernel thinks.


__-S__ ''sectors''


Specify the number of sectors, possibly overriding what the
kernel thinks.


__-f__ or __--force__


Do what I say, even if it is stupid.


__-q__ or __--quiet__


Suppress warning messages.


__-L__ or __--Linux__


Do not complain about things irrelevant for
Linux.


__-D__ or __--DOS__


For DOS-compatibility: waste a little space. (More
precisely: if a partition cannot contain sector 0, e.g.
because that is the MBR of the device, or contains the
partition table of an extended partition, then __sfdisk__
would make it start the next sector. However, when this
option is given it skips to the start of the next track,
wasting for example 33 sectors (in case of 34
sectors/track), just like certain versions of DOS do.)
Certain Disk Managers and boot loaders (such as OSBS, but
not LILO or the OS/2 Boot Manager) also live in this empty
space, so maybe you want this option if you use
one.


__-E__ or __--DOS-extended__


Take the starting sector numbers of


__--IBM__ or __--leave-last__


Certain IBM diagnostic programs assume that they can use the
last cylinder on a disk for disk-testing purposes. If you
think you might ever run such programs, use this option to
tell __sfdisk__ that it should not allocate the last
cylinder. Sometimes the last cylinder contains a bad sector
table.


__-n__


Go through all the motions, but do not actually write to
disk.


__-R__


Only execute the BLKRRPART ioctl (to make the kernel re-read
the partition table). This can be useful for checking in
advance that the final BLKRRPART will be successful, and
also when you changed the partition table `by hand' (e.g.,
using dd from a backup). If the kernel complains (`device
busy for revalidation (usage = 2)') then something still
uses the device, and you still have to unmount some file
system, or say swapoff to some swap partition.


__--no-reread__


When starting a repartitioning of a disk, sfdisk checks that
this disk is not mounted, or in use as a swap device, and
refuses to continue if it is. This option suppresses the
test. (On the other hand, the -f option would force sfdisk
to continue even when this test fails.)


__-O__ ''file''


Just before writing the new partition, output the sectors
that are going to be overwritten to ''file'' (where
hopefully ''file'' resides on another disk, or on a
floppy).


__-I__ ''file''


After destroying your filesystems with an unfortunate
__sfdisk__ command, you would have been able to restore
the old situation if only you had preserved it using the -O
flag.
!!THEORY


Block 0 of a disk (the Master Boot Record) contains among
other things four partition descriptors. The partitions
described here are called ''primary''
partitions.


A partition descriptor has 6 fields:


struct partition {
unsigned char bootable;        /* 0 or 0x80 */
hsc begin_hsc;
unsigned char id;
hsc end_hsc;
unsigned int starting_sector;
unsigned int nr_of_sectors;
}
The two hsc fields indicate head, sector and cylinder of the begin and the end of the partition. Since each hsc field only takes 3 bytes, only 24 bits are available, which does not suffice for big disks (say lilo__ documentation.


Each partition has a type, its `Id', and if this type is 5
or f (`''extended partition''') the starting sector of
the partition again contains 4 partition descriptors. MSDOS
only uses the first two of these: the first one an actual
data partition, and the second one again an extended
partition (or empty). In this way one gets a chain of
extended partitions. Other operating systems have slightly
different conventions. Linux also accepts type 85 as
equivalent to 5 and f - this can be useful if one wants to
have extended partitions under Linux past the 1024 cylinder
boundary, without DOS FDISK hanging. (If there is no good
reason, you should just use 5, which is understood by other
systems.)


Partitions that are not primary or extended are called
''logical''. Often, one cannot boot from logical
partitions (because the process of finding them is more
involved than just looking at the MBR). Note that of an
extended partition only the Id and the start are used. There
are various conventions about what to write in the other
fields. One should not try to use extended partitions for
data storage or swap.
!!INPUT FORMAT


__sfdisk__ reads lines of the form




where each line fills one partition descriptor.


Fields are separated by whitespace, or comma or semicolon
possibly followed by whitespace; initial and trailing
whitespace is ignored. Numbers can be octal, decimal or
hexadecimal, decimal is default. When a field is absent or
empty, a default value is used.


The
sfdisk__ computes them from
__


Bootable is specified as [[*|-], with as default
not-bootable. (The value of this field is irrelevant for
Linux - when Linux runs it has been booted already - but
might play a role for certain boot loaders and for other
operating systems. For example, when there are several
primary DOS partitions, DOS assigns C: to the first among
these that is bootable.)


Id is given in hex, without the 0x prefix, or is [[E|S|L|X],
where L (LINUX_NATIVE (83)) is the default, S is LINUX_SWAP
(82), E is EXTENDED_PARTITION (5), and X is LINUX_EXTENDED
(85).


The default value of start is the first nonassigned
sector/cylinder/...


The default value of size is as much as possible (until next
partition or end-of-disk).


However, for the four partitions inside an extended
partition, the defaults are: Linux partition, Extended
partition, Empty, Empty.


But when the -N option (change a single partition only) is
given, the default for each field is its previous
value.
!!EXAMPLE


The command


 sfdisk /dev/hdc


will partition /dev/hdc just as indicated
above.


With the -x option, the number of input lines must be a
multiple of 4: you have to list the two empty partitions
that you never want using two blank lines. Without the -x
option, you give one line for the partitions inside a
extended partition, instead of four, and terminate with
end-of-file (^D). (And __sfdisk__ will assume that your
input line represents the first of four, that the second one
is extended, and the 3rd and 4th are empty.)
!!DOS 6.x WARNING


The DOS 6.x FORMAT command looks for some information in the
first sector of the data area of the partition, and treats
this information as more reliable than the information in
the partition table. DOS FORMAT expects DOS FDISK to clear
the first 512 bytes of the data area of a partition whenever
a size change occurs. DOS FORMAT will look at this extra
information even if the /U flag is given -- we consider this
a bug in DOS FORMAT and DOS FDISK.


The bottom line is that if you use sfdisk to change the size
of a DOS partition table entry, then you must also use
__dd__ to zero the first 512 bytes of that partition
before using DOS FORMAT to format the partition. For
example, if you were using sfdisk to make a DOS partition
table entry for /dev/hda1, then (after exiting sfdisk and
rebooting Linux so that the partition table information is
valid) you would use the command
__BE EXTREMELY CAREFUL__ if you
use the __dd__ command, since a small typo can make all
of the data on your disk useless.


For best results, you should always use an OS-specific
partition table program. For example, you should make DOS
partitions with the DOS FDISK program and Linux partitions
with the Linux sfdisk program.
!!DRDOS WARNINGS


Stephen Tweedie reported (930515): `Most reports of
superblock corruption turn out to be due to bad
partitioning, with one filesystem overrunning the start of
the next and corrupting its superblock. I have even had this
problem with the supposedly-reliable DRDOS. This was quite
possibly due to DRDOS-6.0's FDISK command. Unless I created
a blank track or cylinder between the DRDOS partition and
the immediately following one, DRDOS would happily stamp all
over the start of the next partition. Mind you, as long as I
keep a little free disk space after any DRDOS partition, I
don't have any other problems with the two coexisting on the
one drive.'


A. V. Le Blanc writes in README.esfdisk: `Dr. DOS 5.0 and
6.0 has been reported to have problems cooperating with
Linux, and with this version of efdisk in particular. This
efdisk sets the system type to hexadecimal 81. Dr. DOS seems
to confuse this with hexadecimal 1, a DOS code. If you use
Dr. DOS, use the efdisk command 't' to change the system
code of any Linux partitions to some number less than
hexadecimal 80; I suggest 41 and 42 for the
moment.'


A. V. Le Blanc writes in his README.fdisk: `DR-DOS 5.0 and
6.0 are reported to have difficulties with partition ID
codes of 80 or more. The Linux `fdisk' used to set the
system type of new partitions to hexadecimal 81. DR-DOS
seems to confuse this with hexadecimal 1, a DOS code. The
values 82 for swap and 83 for file systems should not cause
problems with DR-DOS. If they do, you may use the `fdisk'
command `t' to change the system code of any Linux
partitions to some number less than hexadecimal 80; I
suggest 42 and 43 for the moment.'


In fact, it seems that only 4 bits are significant for the
DRDOS FDISK, so that for example 11 and 21 are listed as DOS
2.0. However, DRDOS itself seems to use the full byte. I
have not been able to reproduce any corruption with DRDOS or
its fdisk.
!!BUGS


A corresponding interactive __cfdisk__ (with curses
interface) is still lacking.


There are too many options.


There is no support for non-DOS partition
types.
!!AUTHOR


A. E. Brouwer (aeb@cwi.nl)
!!SEE ALSO


cfdisk(8), fdisk(8),
parted(8)
----
3 pages link to sfdisk(8):
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Last edited on Saturday, November 6, 2004 12:55:44 pm by "YazzAtlas"
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