| FDISK(8) | System Manager's Manual | FDISK(8) |
| fdisk | [-aBFfIiSuv] [-0 | -1 | -2 | -3 | -E number [-s id/start/size[/bootmenu]]] [-r file | -w file] [-A ptn_alignment[/ptn_0_offset]] [-b cylinders/heads/sectors] [-c bootcode] [-T disktype] [-t disktab] [device] |
| fdisk | -l |
The MBR contains bootable code, a partition table, an indication of which partition is ‘active', and (optionally, depending on the boot code) a menu for selecting a partition to be booted. There can be at most 4 partitions defined in sector 0, one of which can be an extended partition which can be split into any number of sub-partitions (then called logical partitions).
The boot code in the MBR is usually invoked by the BIOS or firmware, and the MBR passes control to the next stage boot code stored in the first sector of the partition to be booted (the partition boot record or PBR).
After booting, NetBSD does not use the partitioning done by fdisk, instead it uses a NetBSD disklabel saved in sector 1 of the NetBSD partition. See mbrlabel(8) for a way of using information from the MBR to construct a NetBSD disklabel.
The standard MBR boot code will only boot the ‘active' partition. However, NetBSD contains additional boot programs which allow the user to interactively select which of the partitions to boot. The ‘mbr_ext' code will also boot NetBSD from an extended partition but will not work on old systems that do not support LBA reads, the ‘mbr_com0' and ‘mbr_com0_9600' will read and write from a serial port. At the start the fdisk program will determine whether the disk sector 0 is valid as a boot sector. (This is determined by checking the magic number.) If not, fdisk will initialise the boot code as well as the partition table. During this, all four partitions will be marked empty.
The flags -a, -i or -u are used to indicate that the partition data is to be updated. The fdisk program will enter an interactive conversational mode. This mode is designed not to change any data unless you explicitly tell it to; fdisk selects defaults for its questions to guarantee that behaviour.
If partition data is going to be updated and the disk carries GUID Partition Tables, fdisk will remove both primary and backup GPT headers from the disk. See gpt(8) for information on how to manipulate GUID Partition Tables.
fdisk will calculate the correct cylinder, head, and sector values for any partition you edit. If you specify -v you will be asked whether you want to specify them yourself.
Finally, when all the data for the first sector has been accumulated, fdisk will ask if you really want to write the new partition table. Only if you reply affirmatively to this question will fdisk write anything to the disk.
Available options:
Using the -f flag with -u makes it impossible to specify the starting and ending cylinder, head, and sector fields (only start and size can be specified by -s option). They will be automatically computed using the BIOS geometry.
In a non-interactive mode (specified by -f option), partition data should be specified by -s option. A partition selection option (-0, -1, -2, -3, or -E number) should also be specified to select a partition slot to be updated.
fdisk will not allow you to create partitions which overlap. If -u and -s are specified in a non-interactive mode then the details of the specified partition will be changed. Any other partitions which overlap the requested part of the disk will be silently deleted.
If bootmenu is specified for any partition fdisk will determine whether the installed boot code supports the bootselect code, if it doesn't you will be asked whether you want to install the required boot code. To remove a bootmenu label, simply press <space> followed by <return>.
Using -v with -u allows the user to change more parameters than normally permitted.
When called with no arguments, it prints the partition table. An example follows:
Disk: /dev/rwd0d
NetBSD disklabel disk geometry:
cylinders: 16383, heads: 16, sectors/track: 63 (1008 sectors/cylinder)
total sectors: 40032696
BIOS disk geometry:
cylinders: 1023, heads: 255, sectors/track: 63 (16065 sectors/cylinder)
total sectors: 40032696
Partition table:
0: NetBSD (sysid 169)
bootmenu: net 1.5.
start 4209030, size 8289540 (4048 MB, Cyls 262-778), Active
1: Primary DOS with 32 bit FAT (sysid 11)
bootmenu: win98
start 63, size 4208967 (2055 MB, Cyls 0-262)
2: NetBSD (sysid 169)
bootmenu: current
start 32515560, size 7517136 (3670 MB, Cyls 2024-2491/234/40)
3: Ext. partition - LBA (sysid 15)
start 12498570, size 20016990 (9774 MB, Cyls 778-2024)
Extended partition table:
E0: NetBSD (sysid 169)
bootmenu: test
start 12498633, size 12305727 (6009 MB, Cyls 778-1544)
E1: Primary DOS with 32 bit FAT (sysid 11)
start 24804423, size 4096512 (2000 MB, Cyls 1544-1799)
E2: Primary DOS with 32 bit FAT (sysid 11)
start 28900998, size 3614562 (1765 MB, Cyls 1799-2024)
Bootselector enabled, infinite timeout.
First active partition: 0
This example disk is divided into four partitions, the last of which is an extended partition. The logical partitions of the extended partition are also shown. In this case there is no free space in either the disk or in the extended partition.
The various fields in each partition entry are:
If the -v flag is specified, the beginning and end of each partition are also displayed as follows: beg: cylinder cylinder, head head, sector sector
Note: these numbers are read from the bootblock, so are the values calculated by a previous run of fdisk.
fdisk attempts to check whether each partition is bootable, by checking the magic number and some other characteristics of the first sector of each partition (the PBR). If the partition does not appear to be bootable, fdisk will print a line containing “PBR is not bootable” followed by an error message. If the partition is bootable, and if the -v flag is specified, fdisk will print “PBR appears to be bootable”. If the -v flag is specified more than once, fdisk will print the heading “Information from PBR:” followed by one or more lines of information gleaned from the PBR; this additional information may be incorrect or misleading, because different operating systems use different PBR formats. Note that, even if no errors are reported, an attempt to boot from the partition might fail. NetBSD partitions may be made bootable using installboot(8).
Traditionally the partition boundaries should be on cylinder boundaries using the BIOS geometry, with the exception of the first partition, which traditionally begins in the second track of the first cylinder (cylinder 0, head 1, sector 1). Although the BIOS geometry is typically different from the geometry reported by the drive, neither will match the actual physical geometry for modern disks (the actual geometry will vary across the disk). Keeping the partition boundaries on cylinder boundaries makes partitioning a driver easier as only relatively small numbers need be entered.
The automatic calculation of the starting cylinder and other parameters uses a set of figures that represent what the BIOS thinks is the geometry of the drive. The default values should be correct for the system on which fdisk is run; however, if you move the disk to a different system, the BIOS of that system might use a different geometry translation.
If you run the equivalent of fdisk on a different operating system then the bootmenu strings associated with extended partitions may be lost.
Editing an existing partition is risky, and may cause you to lose all the data in that partition.
You should run this program interactively once or twice to see how it works. This is completely safe as long as you answer the last question in the negative. You can also specify -w file to write the output to a file and later specify -r file to read back the updated information. This can be done without having write access to the disk volume.
fdisk -u /dev/rwd0d
Change active MBR partition of /dev/rwd0d in interactive mode:
fdisk -a /dev/rwd0d
Install MBR bootcode /usr/mdec/mbr_bootsel into /dev/rwd0d:
fdisk -c /usr/mdec/mbr_bootsel /dev/rwd0d
Set MBR partition data for slot 0 of /dev/rwd0d specifying values without prompt:
fdisk -f -u -0 -s 169/63/2097089 /dev/rwd0d
Make partition slot 0 of /dev/rwd0d active without prompt:
fdisk -f -a -0 /dev/rwd0d
Initialize and create MBR partition data using bootcode destdir/usr/mdec/mbr without prompt against 1GB disk image file diskimg:
fdisk -f -i -b 130/255/63 -c destdir/usr/mdec/mbr -F diskimg
Create MBR partition data for slot 0 which has an active NetBSD partition using whole disk without prompt against 1GB disk image file diskimg:
fdisk -f -a -u -0 -s 169/63/2097089 -F diskimg
There are subtleties that the program detects that are not explained in this manual page.
| December 1, 2011 | NetBSD 6.1 |