Monday, August 17, 2009

fdisk

NAMEfdisk - Partition table manipulator for Linux
SYNOPSISfdisk [-u] [-b sectorsize][-C cyls] [-H heads] [-S sects] device
fdisk -l [-u] [device ...]
fdisk -s partition ...
fdisk -v
DESCRIPTIONHard disks can be divided into one or more logical disks calledpartitions.This division is described in thepartition tablefound in sector 0 of the disk.
In the BSD world one talks about `disk slices' and a `disklabel'.
Linux needs at least one partition, namely for its root file system.It can use swap files and/or swap partitions, but the latter are moreefficient. So, usually one will want a second Linux partitiondedicated as swap partition.On Intel compatible hardware, the BIOS that boots the systemcan often only access the first 1024 cylinders of the disk.For this reason people with large disks often create a third partition,just a few MB large, typically mounted on/boot,to store the kernel image and a few auxiliary files needed at boot time,so as to make sure that this stuff is accessible to the BIOS.There may be reasons of security, ease of administration and backup,or testing, to use more than the minimum number of partitions.
fdisk(in the first form of invocation)is a menu driven program for creation and manipulation ofpartition tables.It understands DOS type partition tables and BSD or SUN type disklabels.
Thedeviceis usually one of the following:/dev/hda/dev/hdb/dev/sda/dev/sdb(/dev/hd[a-h] for IDE disks, /dev/sd[a-p] for SCSI disks,/dev/ed[a-d] for ESDI disks, /dev/xd[ab] for XT disks).A device name refers to the entire disk.
Thepartitionis adevicename followed by a partition number. For example,/dev/hda1is the first partition on the first IDE hard disk in the system.Disks can have up to 15 partitions.See also/usr/src/linux/Documentation/devices.txt.
A BSD/SUN type disklabel can describe 8 partitions,the third of which should be a `whole disk' partition.Do not start a partition that actually uses its first sector(like a swap partition) at cylinder 0, since that willdestroy the disklabel.
An IRIX/SGI type disklabel can describe 16 partitions,the eleventh of which should be an entire `volume' partition,while the ninth should be labeled `volume header'.The volume header will also cover the partition table, i.e.,it starts at block zero and extends by default over five cylinders.The remaining space in the volume header may be used by headerdirectory entries. No partitions may overlap with the volume header.Also do not change its type and make some file system on it, sinceyou will lose the partition table. Use this type of label only whenworking with Linux on IRIX/SGI machines or IRIX/SGI disks under Linux.
A DOS type partition table can describe an unlimited numberof partitions. In sector 0 there is room for the descriptionof 4 partitions (called `primary'). One of these may be anextended partition; this is a box holding logical partitions,with descriptors found in a linked list of sectors, eachpreceding the corresponding logical partitions.The four primary partitions, present or not, get numbers 1-4.Logical partitions start numbering from 5.
In a DOS type partition table the starting offset and the sizeof each partition is stored in two ways: as an absolute numberof sectors (given in 32 bits) and as a Cylinders/Heads/Sectorstriple (given in 10+8+6 bits). The former is OK - with 512-bytesectors this will work up to 2 TB. The latter has two differentproblems. First of all, these C/H/S fields can be filled onlywhen the number of heads and the number of sectors per trackare known. Secondly, even if we know what these numbers should be,the 24 bits that are available do not suffice.DOS uses C/H/S only, Windows uses both, Linux never uses C/H/S.
If possible,fdiskwill obtain the disk geometry automatically. This is notnecessarily the physical disk geometry (indeed, modern disks do notreally have anything like a physical geometry, certainly not somethingthat can be described in simplistic Cylinders/Heads/Sectors form),but is the disk geometry that MS-DOS uses for the partition table.
Usually all goes well by default, and there are no problems ifLinux is the only system on the disk. However, if the disk hasto be shared with other operating systems, it is often a good ideato let an fdisk from another operating system make at least onepartition. When Linux boots it looks at the partition table, andtries to deduce what (fake) geometry is required for goodcooperation with other systems.
Whenever a partition table is printed out, a consistency check is performedon the partition table entries. This check verifies that the physical andlogical start and end points are identical, and that the partition startsand ends on a cylinder boundary (except for the first partition).
Some versions of MS-DOS create a first partition which does not beginon a cylinder boundary, but on sector 2 of the first cylinder.Partitions beginning in cylinder 1 cannot begin on a cylinder boundary, butthis is unlikely to cause difficulty unless you have OS/2 on your machine.
A sync() and a BLKRRPART ioctl() (reread partition table from disk)are performed before exiting when the partition table has been updated.Long ago it used to be necessary to reboot after the use of fdisk.I do not think this is the case anymore - indeed, rebooting too quicklymight cause loss of not-yet-written data. Note that both the kerneland the disk hardware may buffer data.

DOS 6.x WARNING
The DOS 6.x FORMAT command looks for some information in the firstsector of the data area of the partition, and treats this informationas more reliable than the information in the partition table. DOSFORMAT expects DOS FDISK to clear the first 512 bytes of the data areaof a partition whenever a size change occurs. DOS FORMAT will look atthis extra information even if the /U flag is given -- we considerthis a bug in DOS FORMAT and DOS FDISK.
The bottom line is that if you use cfdisk or fdisk to change the size of aDOS partition table entry, then you must also useddto zero the first 512 bytes of that partition before using DOS FORMAT toformat the partition. For example, if you were using cfdisk to make a DOSpartition table entry for /dev/hda1, then (after exiting fdisk or cfdiskand rebooting Linux so that the partition table information is valid) youwould use the command "dd if=/dev/zero of=/dev/hda1 bs=512 count=1" to zerothe first 512 bytes of the partition.
BE EXTREMELY CAREFULif you use theddcommand, 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 tableprogram. For example, you should make DOS partitions with the DOS FDISKprogram and Linux partitions with the Linux fdisk or Linux cfdisk program.

OPTIONS
-b sectorsize
Specify the sector size of the disk. Valid values are 512, 1024, or 2048.(Recent kernels know the sector size. Use this only on old kernels orto override the kernel's ideas.)
-C cyls
Specify the number of cylinders of the disk.I have no idea why anybody would want to do so.
-H heads
Specify the number of heads of the disk. (Not the physical number,of course, but the number used for partition tables.)Reasonable values are 255 and 16.
-S sects
Specify the number of sectors per track of the disk.(Not the physical number, of course, but the number used forpartition tables.)A reasonable value is 63.
-l
List the partition tables for the specified devices and then exit.If no devices are given, those mentioned in/proc/partitions(if that exists) are used.
-u
When listing partition tables, give sizes in sectors insteadof cylinders.
-s partition
Thesizeof the partition (in blocks) is printed on the standard output.
-v
Print version number offdiskprogram and exit.
Post a Comment

You might also like :

Related Posts with Thumbnails