1FDISK(8) System Administration FDISK(8)
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6 fdisk - manipulate disk partition table
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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 GPT
22 (experimental for now), MBR, Sun, SGI and BSD partition tables.
23 fdisk does not use DOS-compatible mode and cylinders as display units
25 by default. The old deprecated DOS behavior can be enabled with the
26 '-c=dos -u=cylinders' command-line options.
27 Hard disks can be divided into one or more logical disks called parti‐
29 tions. This division is recorded in the partition table, found in sec‐
30 tor 0 of the disk. (In the BSD world one talks about `disk slices' and
31 a `disklabel'.)
32 Linux needs at least one partition, namely for its root file system.
34 It can use swap files and/or swap partitions, but the latter are more
35 efficient. So, usually one will want a second Linux partition dedi‐
36 cated as swap partition. On Intel-compatible hardware, the BIOS that
37 boots the system can often only access the first 1024 cylinders of the
38 disk. For this reason people with large disks often create a third
39 partition, just a few MB large, typically mounted on /boot, to store
40 the kernel image and a few auxiliary files needed at boot time, so as
41 to make sure that this stuff is accessible to the BIOS. There may be
42 reasons of security, ease of administration and backup, or testing, to
43 use more than the minimum number of partitions.
44
47 The device is usually /dev/sda, /dev/sdb or so. A device name refers
48 to the entire disk. Old systems without libata (a library used inside
49 the Linux kernel to support ATA host controllers and devices) make a
50 difference between IDE and SCSI disks. In such cases the device name
51 will be /dev/hd* (IDE) or /dev/sd* (SCSI).
52 The partition is a device name followed by a partition number. For
54 example, /dev/sda1 is the first partition on the first hard disk in the
55 system. See also Linux kernel documentation (the Documenta‐
56 tion/devices.txt file).
57
60 A BSD/SUN-type disklabel can describe 8 partitions, the third of which
61 should be a `whole disk' partition. Do not start a partition that
62 actually uses its first sector (like a swap partition) at cylinder 0,
63 since that will destroy the disklabel.
64 An IRIX/SGI-type disklabel can describe 16 partitions, the eleventh of
66 which should be an entire `volume' partition, while the ninth should be
67 labeled `volume header'. The volume header will also cover the parti‐
68 tion table, i.e., it starts at block zero and extends by default over
69 five cylinders. The remaining space in the volume header may be used
70 by header directory entries. No partitions may overlap with the volume
71 header. Also do not change its type or make some filesystem on it,
72 since you will lose the partition table. Use this type of label only
73 when working with Linux on IRIX/SGI machines or IRIX/SGI disks under
74 Linux.
75 A DOS-type partition table can describe an unlimited number of parti‐
77 tions. In sector 0 there is room for the description of 4 partitions
78 (called `primary'). One of these may be an extended partition; this is
79 a box holding logical partitions, with descriptors found in a linked
80 list of sectors, each preceding the corresponding logical partitions.
81 The four primary partitions, present or not, get numbers 1-4. Logical
82 partitions start numbering from 5.
83 In a DOS-type partition table the starting offset and the size of each
85 partition is stored in two ways: as an absolute number of sectors
86 (given in 32 bits), and as a Cylinders/Heads/Sectors triple (given in
87 10+8+6 bits). The former is OK -- with 512-byte sectors this will work
88 up to 2 TB. The latter has two problems. First, these C/H/S fields
89 can be filled only when the number of heads and the number of sectors
90 per track are known. And second, even if we know what these numbers
91 should be, the 24 bits that are available do not suffice. DOS uses
92 C/H/S only, Windows uses both, Linux never uses C/H/S.
93 If possible, fdisk will obtain the disk geometry automatically. This
95 is not necessarily the physical disk geometry (indeed, modern disks do
96 not really have anything like a physical geometry, certainly not some‐
97 thing that can be described in simplistic Cylinders/Heads/Sectors
98 form), but it is the disk geometry that MS-DOS uses for the partition
99 table.
100 Usually all goes well by default, and there are no problems if Linux is
102 the only system on the disk. However, if the disk has to be shared
103 with other operating systems, it is often a good idea to let an fdisk
104 from another operating system make at least one partition. When Linux
105 boots it looks at the partition table, and tries to deduce what (fake)
106 geometry is required for good cooperation with other systems.
107 Whenever a partition table is printed out, a consistency check is per‐
109 formed on the partition table entries. This check verifies that the
110 physical and logical start and end points are identical, and that each
111 partition starts and ends on a cylinder boundary (except for the first
112 partition).
113 Some versions of MS-DOS create a first partition which does not begin
115 on a cylinder boundary, but on sector 2 of the first cylinder. Parti‐
116 tions beginning in cylinder 1 cannot begin on a cylinder boundary, but
117 this is unlikely to cause difficulty unless you have OS/2 on your
118 machine.
119 A sync() and an ioctl(BLKRRPART) (reread partition table from disk) are
121 performed before exiting when the partition table has been updated.
122 Long ago it used to be necessary to reboot after the use of fdisk. I
123 do not think this is the case anymore -- indeed, rebooting too quickly
124 might cause loss of not-yet-written data. Note that both the kernel
125 and the disk hardware may buffer data.
126
129 The DOS 6.x FORMAT command looks for some information in the first sec‐
130 tor of the data area of the partition, and treats this information as
131 more reliable than the information in the partition table. DOS FORMAT
132 expects DOS FDISK to clear the first 512 bytes of the data area of a
133 partition whenever a size change occurs. DOS FORMAT will look at this
134 extra information even if the /U flag is given -- we consider this a
135 bug in DOS FORMAT and DOS FDISK.
136 The bottom line is that if you use cfdisk or fdisk to change the size
138 of a DOS partition table entry, then you must also use dd to zero the
139 first 512 bytes of that partition before using DOS FORMAT to format the
140 partition. For example, if you were using cfdisk to make a DOS parti‐
141 tion table entry for /dev/sda1, then (after exiting fdisk or cfdisk and
142 rebooting Linux so that the partition table information is valid) you
143 would use the command "dd if=/dev/zero of=/dev/sda1 bs=512 count=1" to
144 zero the first 512 bytes of the partition.
145 BE EXTREMELY CAREFUL if you use the dd command, since a small typo can
147 make all of the data on your disk useless.
148 For best results, you should always use an OS-specific partition table
150 program. For example, you should make DOS partitions with the DOS
151 FDISK program and Linux partitions with the Linux fdisk or Linux cfdisk
152 program.
153
156 -b sectorsize
157 Specify the sector size of the disk. Valid values are 512,
158 1024, 2048 or 4096. (Recent kernels know the sector size. Use
159 this only on old kernels or to override the kernel's ideas.)
160 Since util-linux-2.17, fdisk differentiates between logical and
161 physical sector size. This option changes both sector sizes to
162 sectorsize.
163 -c[=mode]
165 Specify the compatibility mode, 'dos' or 'nondos'. The default
166 is non-DOS mode. For backward compatibility, it is possible to
167 use the option without the <mode> argument -- then the default
168 is used. Note that the optional <mode> argument cannot be sepa‐
169 rated from the -c option by a space, the correct form is for
170 example '-c=dos'. This option is DEPRECATED.
171 -C cyls
173 Specify the number of cylinders of the disk. I have no idea why
174 anybody would want to do so. This option is DEPRECATED.
175 -H heads
177 Specify the number of heads of the disk. (Not the physical num‐
178 ber, of course, but the number used for partition tables.) Rea‐
179 sonable values are 255 and 16. This option is DEPRECATED.
180 -S sects
182 Specify the number of sectors per track of the disk. (Not the
183 physical number, of course, but the number used for partition
184 tables.) A reasonable value is 63. This option is DEPRECATED.
185 -h Print help and then exit.
187 -l List the partition tables for the specified devices and then
189 exit. If no devices are given, those mentioned in /proc/parti‐
190 tions (if that exists) are used.
191 -s partition...
193 Print the size (in blocks) of each given partition.
194 -u[=unit]
196 When listing partition tables, show sizes in 'sectors' or in
197 'cylinders'. The default is to show sizes in sectors. For
198 backward compatibility, it is possible to use the option without
199 the <units> argument -- then the default is used. Note that the
200 optional <unit> argument cannot be separated from the -u option
201 by a space, the correct form is for example '-u=cylinders'.
202 -v Print version number of fdisk program and exit.
204
207 FDISK_DEBUG=0xffff
208 enables debug output
209
212 cfdisk(8), sfdisk(8), mkfs(8), parted(8), partprobe(8), kpartx(8)
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215 The fdisk command is part of the util-linux package and is available
216 from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
217util-linux June 2012 FDISK(8)