sfdisk - Unix, Linux Command

sfdisk doesn’t understand GUID Partition Table (GPT) and
it is not designed for large partitions. In particular case use more advanced GNU
parted(8). 



% sfdisk -s /dev/hda9
81599
%


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




% 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
%


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
%






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.




    % sfdisk -d /dev/hda > hda.out
    % sfdisk /dev/hda < hda.out


    % sfdisk /dev/hdb -N5
    ,,,*
    %


    % sfdisk --print-id /dev/hdb 5
    6
    % sfdisk --change-id /dev/hdb 5 83
    OK




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 > 8GB). In fact, due to the wasteful
representation (that uses a byte for the number of heads, which
is typically 16), problems already start with 0.5GB.
However Linux does not use these fields, and problems can arise
only at boot time, before Linux has been started. For more
details, see the
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.




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 <c,h,s> parts can (and probably should) be omitted -
sfdisk computes them from <start> and <size> and the disk geometry
as given by the kernel or specified using the -H, -S, -C flags.


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.



sfdisk /dev/hdc << EOF
0,407
,407
;
;
EOF


The command

sfdisk /dev/hdb << EOF
,3,L
,60,L
,19,S
,,E
,130,L
,130,L
,130,L
,,L
EOF


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.)




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 "dd if=/dev/zero of=/dev/hda1 bs=512 count=1" to zero
the first 512 bytes of the partition.
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.




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.efdisk: ‘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.




There are too many options.


There is no support for non-DOS partition types.



cfdisk (8)
cfdisk (8)
fdisk (8)
fdisk (8)
mkfs (8)
mkfs (8)
parted (8)
parted (8)

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