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 [-u] [-b sectorsize] [-C cyls] [-H heads] [-S sects] device
10 fdisk -l [-u] [device ...]
12 fdisk -s partition ...
14 fdisk -v
16
18 Hard disks can be divided into one or more logical disks called parti‐
19 tions. This division is described in the partition table found in sec‐
20 tor 0 of the disk.
21 In the BSD world one talks about `disk slices' and a `disklabel'.
23 Linux needs at least one partition, namely for its root file system.
25 It can use swap files and/or swap partitions, but the latter are more
26 efficient. So, usually one will want a second Linux partition dedicated
27 as swap partition. On Intel compatible hardware, the BIOS that boots
28 the system can often only access the first 1024 cylinders of the disk.
29 For this reason people with large disks often create a third partition,
30 just a few MB large, typically mounted on /boot, to store the kernel
31 image and a few auxiliary files needed at boot time, so as to make sure
32 that this stuff is accessible to the BIOS. There may be reasons of
33 security, ease of administration and backup, or testing, to use more
34 than the minimum number of partitions.
35 fdisk (in the first form of invocation) is a menu driven program for
37 creation and manipulation of partition tables. It understands DOS type
38 partition tables and BSD or SUN type disklabels.
39 fdisk doesn't understand GUID Partition Table (GPT) and it is not
41 designed for large partitions. In particular case use more advanced GNU
42 parted(8).
43 The device is usually one of the following:
45 /dev/hda
46 /dev/hdb
47 /dev/sda
48 /dev/sdb
49 (/dev/hd[a-h] for IDE disks, /dev/sd[a-p] for SCSI disks, /dev/ed[a-d]
50 for ESDI disks, /dev/xd[ab] for XT disks). A device name refers to the
51 entire disk.
52 The partition is a device name followed by a partition number. For
54 example, /dev/hda1 is the first partition on the first IDE hard disk in
55 the system. Disks can have up to 15 partitions. See also
56 /usr/src/linux/Documentation/devices.txt.
57 A BSD/SUN type disklabel can describe 8 partitions, the third of which
59 should be a `whole disk' partition. Do not start a partition that
60 actually uses its first sector (like a swap partition) at cylinder 0,
61 since that will destroy the disklabel.
62 An IRIX/SGI type disklabel can describe 16 partitions, the eleventh of
64 which should be an entire `volume' partition, while the ninth should be
65 labeled `volume header'. The volume header will also cover the parti‐
66 tion table, i.e., it starts at block zero and extends by default over
67 five cylinders. The remaining space in the volume header may be used
68 by header directory entries. No partitions may overlap with the volume
69 header. Also do not change its type and make some file system on it,
70 since you will lose the partition table. Use this type of label only
71 when working with Linux on IRIX/SGI machines or IRIX/SGI disks under
72 Linux.
73 A DOS type partition table can describe an unlimited number of parti‐
75 tions. In sector 0 there is room for the description of 4 partitions
76 (called `primary'). One of these may be an extended partition; this is
77 a box holding logical partitions, with descriptors found in a linked
78 list of sectors, each preceding the corresponding logical partitions.
79 The four primary partitions, present or not, get numbers 1-4. Logical
80 partitions start numbering from 5.
81 In a DOS type partition table the starting offset and the size of each
83 partition is stored in two ways: as an absolute number of sectors
84 (given in 32 bits) and as a Cylinders/Heads/Sectors triple (given in
85 10+8+6 bits). The former is OK - with 512-byte sectors this will work
86 up to 2 TB. The latter has two different problems. First of all, these
87 C/H/S fields can be filled only when the number of heads and the number
88 of sectors per track are known. Secondly, even if we know what these
89 numbers should be, the 24 bits that are available do not suffice. DOS
90 uses C/H/S only, Windows uses both, Linux never uses C/H/S.
91 If possible, fdisk will obtain the disk geometry automatically. This
93 is not necessarily the physical disk geometry (indeed, modern disks do
94 not really have anything like a physical geometry, certainly not some‐
95 thing that can be described in simplistic Cylinders/Heads/Sectors
96 form), but is the disk geometry that MS-DOS uses for the partition ta‐
97 ble.
98 Usually all goes well by default, and there are no problems if Linux is
100 the only system on the disk. However, if the disk has to be shared with
101 other operating systems, it is often a good idea to let an fdisk from
102 another operating system make at least one partition. When Linux boots
103 it looks at the partition table, and tries to deduce what (fake) geome‐
104 try is required for good cooperation with other systems.
105 Whenever a partition table is printed out, a consistency check is per‐
107 formed on the partition table entries. This check verifies that the
108 physical and logical start and end points are identical, and that the
109 partition starts and ends on a cylinder boundary (except for the first
110 partition).
111 Some versions of MS-DOS create a first partition which does not begin
113 on a cylinder boundary, but on sector 2 of the first cylinder. Parti‐
114 tions beginning in cylinder 1 cannot begin on a cylinder boundary, but
115 this is unlikely to cause difficulty unless you have OS/2 on your
116 machine.
117 A sync() and a BLKRRPART ioctl() (reread partition table from disk) are
119 performed before exiting when the partition table has been updated.
120 Long ago it used to be necessary to reboot after the use of fdisk. I
121 do not think this is the case anymore - indeed, rebooting too quickly
122 might cause loss of not-yet-written data. Note that both the kernel and
123 the disk hardware may buffer data.
124
127 The DOS 6.x FORMAT command looks for some information in the first sec‐
128 tor of the data area of the partition, and treats this information as
129 more reliable than the information in the partition table. DOS FORMAT
130 expects DOS FDISK to clear the first 512 bytes of the data area of a
131 partition whenever a size change occurs. DOS FORMAT will look at this
132 extra information even if the /U flag is given -- we consider this a
133 bug in DOS FORMAT and DOS FDISK.
134 The bottom line is that if you use fdisk to change the size of a DOS
136 partition table entry, then you must also use dd to zero the first 512
137 bytes of that partition before using DOS FORMAT to format the parti‐
138 tion. For example, if you were using disk to make a DOS partition ta‐
139 ble entry for /dev/hda1, then (after exiting fdisk and rebooting Linux
140 so that the partition table information is valid) you would use the
141 command "dd if=/dev/zero of=/dev/hda1 bs=512 count=1" to zero the first
142 512 bytes of the partition.
143 BE EXTREMELY CAREFUL if you use the dd command, since a small typo can
145 make all of the data on your disk useless.
146 For best results, you should always use an OS-specific partition table
148 program. For example, you should make DOS partitions with the DOS
149 FDISK program and Linux partitions with the Linux fdisk program.
150
153 -b sectorsize
154 Specify the sector size of the disk. Valid values are 512, 1024,
155 or 2048. (Recent kernels know the sector size. Use this only on
156 old kernels or to override the kernel's ideas.)
157 -C cyls
159 Specify the number of cylinders of the disk. I have no idea why
160 anybody would want to do so.
161 -H heads
163 Specify the number of heads of the disk. (Not the physical num‐
164 ber, of course, but the number used for partition tables.) Rea‐
165 sonable values are 255 and 16.
166 -S sects
168 Specify the number of sectors per track of the disk. (Not the
169 physical number, of course, but the number used for partition
170 tables.) A reasonable value is 63.
171 -l List the partition tables for the specified devices and then
173 exit. If no devices are given, those mentioned in /proc/parti‐
174 tions (if that exists) are used.
175 -u When listing partition tables, give sizes in sectors instead of
177 cylinders.
178 -s partition
180 The size of the partition (in blocks) is printed on the standard
181 output.
182 -v Print version number of fdisk program and exit.
184
186 There are several *fdisk programs around. Each has its problems and
187 strengths. Try them in the order parted, fdisk, sfdisk.
188 The IRIX/SGI type disklabel is currently not supported by the kernel.
190 Moreover, IRIX/SGI header directories are not fully supported yet.
191 The option `dump partition table to file' is missing.
193
195 mkfs(8), parted(8), sfdisk(8)
196Linux 2.0 11 June 1998 FDISK(8)