1FDISK(8) Linux Programmer's Manual FDISK(8)
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6 fdisk - partition table manipulator for Linux
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9 fdisk [-uc] [-b sectorsize] [-C cyls] [-H heads] [-S sects] device
10 fdisk -l [-u] [device...]
12 fdisk -s partition...
14 fdisk -v
16 fdisk -h
18
20 fdisk (in the first form of invocation) is a menu-driven program for
21 creation and manipulation of partition tables. It understands DOS-type
22 partition tables and BSD- or SUN-type disklabels.
23 fdisk does not understand GUID partition tables (GPTs) and it is not
25 designed for large partitions. In these cases, use the more advanced
26 GNU parted(8).
27 fdisk does not use DOS-compatible mode and cylinders as display units
29 by default. The old deprecated DOS behavior can be enabled with the
30 '-c=dos -u=cylinders' command-line options.
31 Hard disks can be divided into one or more logical disks called parti‐
33 tions. This division is recorded in the partition table, found in sec‐
34 tor 0 of the disk. (In the BSD world one talks about `disk slices' and
35 a `disklabel'.)
36 Linux needs at least one partition, namely for its root file system.
38 It can use swap files and/or swap partitions, but the latter are more
39 efficient. So, usually one will want a second Linux partition dedi‐
40 cated as swap partition. On Intel-compatible hardware, the BIOS that
41 boots the system can often only access the first 1024 cylinders of the
42 disk. For this reason people with large disks often create a third
43 partition, just a few MB large, typically mounted on /boot, to store
44 the kernel image and a few auxiliary files needed at boot time, so as
45 to make sure that this stuff is accessible to the BIOS. There may be
46 reasons of security, ease of administration and backup, or testing, to
47 use more than the minimum number of partitions.
48
51 The device is usually /dev/sda, /dev/sdb or so. A device name refers
52 to the entire disk. Old systems without libata (a library used inside
53 the Linux kernel to support ATA host controllers and devices) make a
54 difference between IDE and SCSI disks. In such cases the device name
55 will be /dev/hd* (IDE) or /dev/sd* (SCSI).
56 The partition is a device name followed by a partition number. For
58 example, /dev/sda1 is the first partition on the first hard disk in the
59 system. See also Linux kernel documentation (the Documenta‐
60 tion/devices.txt file).
61
64 A BSD/SUN-type disklabel can describe 8 partitions, the third of which
65 should be a `whole disk' partition. Do not start a partition that
66 actually uses its first sector (like a swap partition) at cylinder 0,
67 since that will destroy the disklabel.
68 An IRIX/SGI-type disklabel can describe 16 partitions, the eleventh of
70 which should be an entire `volume' partition, while the ninth should be
71 labeled `volume header'. The volume header will also cover the parti‐
72 tion table, i.e., it starts at block zero and extends by default over
73 five cylinders. The remaining space in the volume header may be used
74 by header directory entries. No partitions may overlap with the volume
75 header. Also do not change its type or make some filesystem on it,
76 since you will lose the partition table. Use this type of label only
77 when working with Linux on IRIX/SGI machines or IRIX/SGI disks under
78 Linux.
79 A DOS-type partition table can describe an unlimited number of parti‐
81 tions. In sector 0 there is room for the description of 4 partitions
82 (called `primary'). One of these may be an extended partition; this is
83 a box holding logical partitions, with descriptors found in a linked
84 list of sectors, each preceding the corresponding logical partitions.
85 The four primary partitions, present or not, get numbers 1-4. Logical
86 partitions start numbering from 5.
87 In a DOS-type partition table the starting offset and the size of each
89 partition is stored in two ways: as an absolute number of sectors
90 (given in 32 bits), and as a Cylinders/Heads/Sectors triple (given in
91 10+8+6 bits). The former is OK -- with 512-byte sectors this will work
92 up to 2 TB. The latter has two problems. First, these C/H/S fields
93 can be filled only when the number of heads and the number of sectors
94 per track are known. And second, even if we know what these numbers
95 should be, the 24 bits that are available do not suffice. DOS uses
96 C/H/S only, Windows uses both, Linux never uses C/H/S.
97 If possible, fdisk will obtain the disk geometry automatically. This
99 is not necessarily the physical disk geometry (indeed, modern disks do
100 not really have anything like a physical geometry, certainly not some‐
101 thing that can be described in simplistic Cylinders/Heads/Sectors
102 form), but it is the disk geometry that MS-DOS uses for the partition
103 table.
104 Usually all goes well by default, and there are no problems if Linux is
106 the only system on the disk. However, if the disk has to be shared
107 with other operating systems, it is often a good idea to let an fdisk
108 from another operating system make at least one partition. When Linux
109 boots it looks at the partition table, and tries to deduce what (fake)
110 geometry is required for good cooperation with other systems.
111 Whenever a partition table is printed out, a consistency check is per‐
113 formed on the partition table entries. This check verifies that the
114 physical and logical start and end points are identical, and that each
115 partition starts and ends on a cylinder boundary (except for the first
116 partition).
117 Some versions of MS-DOS create a first partition which does not begin
119 on a cylinder boundary, but on sector 2 of the first cylinder. Parti‐
120 tions beginning in cylinder 1 cannot begin on a cylinder boundary, but
121 this is unlikely to cause difficulty unless you have OS/2 on your
122 machine.
123 A sync() and an ioctl(BLKRRPART) (reread partition table from disk) are
125 performed before exiting when the partition table has been updated.
126 Long ago it used to be necessary to reboot after the use of fdisk. I
127 do not think this is the case anymore -- indeed, rebooting too quickly
128 might cause loss of not-yet-written data. Note that both the kernel
129 and the disk hardware may buffer data.
130
133 The DOS 6.x FORMAT command looks for some information in the first sec‐
134 tor of the data area of the partition, and treats this information as
135 more reliable than the information in the partition table. DOS FORMAT
136 expects DOS FDISK to clear the first 512 bytes of the data area of a
137 partition whenever a size change occurs. DOS FORMAT will look at this
138 extra information even if the /U flag is given -- we consider this a
139 bug in DOS FORMAT and DOS FDISK.
140 The bottom line is that if you use cfdisk or fdisk to change the size
142 of a DOS partition table entry, then you must also use dd to zero the
143 first 512 bytes of that partition before using DOS FORMAT to format the
144 partition. For example, if you were using cfdisk to make a DOS parti‐
145 tion table entry for /dev/sda1, then (after exiting fdisk or cfdisk and
146 rebooting Linux so that the partition table information is valid) you
147 would use the command "dd if=/dev/zero of=/dev/sda1 bs=512 count=1" to
148 zero the first 512 bytes of the partition.
149 BE EXTREMELY CAREFUL if you use the dd command, since a small typo can
151 make all of the data on your disk useless.
152 For best results, you should always use an OS-specific partition table
154 program. For example, you should make DOS partitions with the DOS
155 FDISK program and Linux partitions with the Linux fdisk or Linux cfdisk
156 program.
157
160 -b sectorsize
161 Specify the sector size of the disk. Valid values are 512,
162 1024, 2048 or 4096. (Recent kernels know the sector size. Use
163 this only on old kernels or to override the kernel's ideas.)
164 Since util-linux-ng-2.17, fdisk differentiates between logical
165 and physical sector size. This option changes both sector sizes
166 to sectorsize.
167 -c[=mode]
169 Specify the compatiblity mode, 'dos' or 'nondos'. The default
170 is non-DOS mode. For backward compatibility, it is possible to
171 use the option without the <mode> argument -- then the default
172 is used. Note that the optional <mode> argument cannot be sepa‐
173 rated from the -c option by a space, the correct form is for
174 example '-c=dos'.
175 -C cyls
177 Specify the number of cylinders of the disk. I have no idea why
178 anybody would want to do so.
179 -H heads
181 Specify the number of heads of the disk. (Not the physical num‐
182 ber, of course, but the number used for partition tables.) Rea‐
183 sonable values are 255 and 16.
184 -S sects
186 Specify the number of sectors per track of the disk. (Not the
187 physical number, of course, but the number used for partition
188 tables.) A reasonable value is 63.
189 -h Print help and then exit.
191 -l List the partition tables for the specified devices and then
193 exit. If no devices are given, those mentioned in /proc/parti‐
194 tions (if that exists) are used.
195 -s partition...
197 Print the size (in blocks) of each given partition.
198 -u[=unit]
200 When listing partition tables, show sizes in 'sectors' or in
201 'cylinders'. The default is to show sizes in sectors. For
202 backward compatibility, it is possible to use the option without
203 the <units> argument -- then the default is used. Note that the
204 optional <unit> argument cannot be separated from the -u option
205 by a space, the correct form is for example '-u=cylinders'.
206 -v Print version number of fdisk program and exit.
208
210 There are several *fdisk programs around. Each has its problems and
211 strengths. Try them in the order cfdisk, fdisk, sfdisk. (Indeed,
212 cfdisk is a beautiful program that has strict requirements on the par‐
213 tition tables it accepts, and produces high quality partition tables.
214 Use it if you can. fdisk is a buggy program that does fuzzy things -
215 usually it happens to produce reasonable results. Its single advantage
216 is that it has some support for BSD disk labels and other non-DOS par‐
217 tition tables. Avoid it if you can. sfdisk is for hackers only -- the
218 user interface is terrible, but it is more correct than fdisk and more
219 powerful than both fdisk and cfdisk. Moreover, it can be used nonin‐
220 teractively.)
221 These days there also is parted. The cfdisk interface is nicer, but
223 parted does much more: it not only resizes partitions, but also the
224 filesystems that live in them.
225 The IRIX/SGI-type disklabel is currently not supported by the kernel.
227 Moreover, IRIX/SGI header directories are not fully supported yet.
228 The option `dump partition table to file' is missing.
230
232 cfdisk(8), sfdisk(8), mkfs(8), parted(8), partprobe(8), kpartx(8)
233
235 The fdisk command is part of the util-linux-ng package and is available
236 from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.
237Linux 2.0 11 June 1998 FDISK(8)