partition table maintenance
fdisk makes changes to the MBR and GPT
tables found on some disks. On such disks sector 0 contains a 4-entry MBR
partition table that divides the space. The GPT partition table, when
present, starts at sector 1.
Each entry has a type. A type of 0xA6 indicates an OpenBSD MBR partition, which starts with its own 16-entry partition table known as a disklabel(5). This can be edited using disklabel(8).
Caution is advised when editing these tables, since some platforms use tricks to boot and may rely on specific configurations created at install time.
The options are as follows:
- Modifies existing GPT partitions to allocate all possible space to an
OpenBSD partition and the optional
-bpartition. The free space is maximized by deleting all existing partitions except the boot partitions ‘APFS ISC’, ‘APFS’, ‘APFS Recovry’, ‘BIOS Boot’, ‘HiFive FSBL’ and ‘HiFive BBL’.
If one of ‘APFS ISC’, ‘APFS’, or ‘APFS Recovry’ is detected then any existing ‘EFI system’ will also be preserved. If the preserved ‘EFI system’ partition has fewer blocks than the number requested with
-b, the modification of the GPT is aborted.
- Configures a special boot partition of the specified number of blocks,
offset and type. offset defaults to the first
available block, and type defaults to
The OpenBSD partition will follow the boot partition and use the remaining available space.
Can only be used when initializing disk with
-gis specified, only the blocks value is used.
- Specifies an alternate BIOS geometry for
fdiskto use. By default, an automatic calculation of disk size will be built using heuristics. These figures are taken from the in-core disklabel (see disklabel(8)), or values that /boot has passed to the kernel.
- Use the
fdiskinteractive editor to modify the partition table. See COMMAND MODE, below, for more information.
- Specifies an alternate MBR template file. The default file is /usr/mdec/mbr.
- A default GPT, including a protective MBR, will be written to
disk. The default GPT has a single
OpenBSD partition containing all available space
not allocated by
- A default MBR will be written to disk. The default
MBR will be read from the file specified by
-f, the default file /usr/mdec/mbr or the built-in default. Normally the default MBR has a single OpenBSD partition containing all available space not allocated by
- Treat disk as if it has the specified number of blocks.
- Update MBR bootcode, preserving existing MBR partition table. The MBR bootcode extends from offset 0x000 to the start of the MBR partition table at offset 0x1BE. This is useful for writing new MBR bootcode onto an existing drive, and is equivalent to the DOS command “FDISK /MBR”. Note that this option will overwrite the NT disk signature, if present.
- Print the contents of the MBR, the Primary GPT and the Secondary GPT. Also print more detailed GPT header and partition entry information.
- Avoid asking yes/no questions when not desirable.
- Specify the disk to operate on. It can be specified either by its full pathname or an abbreviated disk form. In its abbreviated form, the path to the device and the ‘r’ denoting "raw device" are omitted and the partition letter is optional. For example, the first IDE disk can be specified as either /dev/rwd0c, wd0c, or wd0.
When called with no special flags,
prints the partition table of the disk.
If disk does not contain a valid GPT, the contents of the MBR are displayed. For example:
# fdisk sd0 Disk: sd0 geometry: 121601/255/63 [1953525168 Sectors] Offset: 0 Signature: 0xAA55 Starting Ending LBA Info: #: id C H S - C H S [ start: size ] ------------------------------------------------------------------------ 0: 0B 0 1 1 - 26108 0 63 [ 63: 419425020 ] FAT32 1: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused 2: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused *3: A6 26108 1 1 - 121600 254 63 [ 419425083: 1534094982 ] OpenBSD
This 1953525168 sector (931GB) disk drive is divided into two MBR partitions that span the whole disk. The first MBR partition is a 200GB FAT32 partition; the second is a 731GB OpenBSD MBR partition using the remainder of the disk. The fields of the output are:
- Number identifying each MBR partition table entry. There are a total of four slots. ‘*’ denotes the MBR partition which is declared bootable.
- MBR partition type identifier. OpenBSD reserves the magic number hexadecimal A6 (166 decimal).
- These fields provide the starting and ending address of the MBR partition in BIOS geometry.
- These fields provide the starting sector and size in sectors of the MBR partition in linear block addresses.
NOTE: The BIOS geometry sectors field (C/H/S) is “1 based”, but the LBA "start" field is “0 based”.
The CHS values will need to be in the BIOS's geometry for the system to be able to boot and use the drive correctly. These values must be kept correctly synchronized or a variety of problems develop which are very difficult to diagnose.
If disk contains a protective MBR and a valid GPT, the contents of the GPT are displayed. For example:
Disk: sd2 Usable LBA: 64 to 7765952 [7766016 Sectors] #: type [ start: size ] ------------------------------------------------------------------------ 1: EFI Sys [ 64: 960 ] 3: OpenBSD [ 1024: 7764929 ]
This 7766016 sector disk drive is divided into two partitions that span the whole disk. The first partition is a 960 sector EFI Sys partition; the second is a 7764929 sector OpenBSD partition using the remaining space. The fields of the output are:
- Number identifying each partition table entry.
- The partition type identifier. If the type is recognized, the name of the type is displayed. Otherwise the type GUID is displayed.
- The sector the partition starts on.
- The number of sectors in the partition.
-v option is specified, the disk
GUID and each partition's GUID and name are also displayed.
In either the MBR or GPT case the OpenBSD partition shown above is subdivided further using the functionality provided by disklabel(8), which provides OpenBSD partitions.
# /dev/rsd0c: type: SCSI disk: SCSI disk label: WDC WD10EADS-65L duid: 085ef8d68623f5b3 flags: bytes/sector: 512 sectors/track: 63 tracks/cylinder: 255 sectors/cylinder: 16065 cylinders: 121601 total sectors: 1953525168 boundstart: 419425083 boundend: 1953520065 drivedata: 0 16 partitions: # size offset fstype [fsize bsize cpg] a: 2097125 419425083 4.2BSD 2048 16384 1 b: 4715520 421522208 swap c: 1953525168 0 unused d: 8388608 426237728 4.2BSD 2048 16384 1 e: 16771072 434626336 4.2BSD 2048 16384 1 f: 4194304 451397408 4.2BSD 2048 16384 1 g: 2097152 455591712 4.2BSD 2048 16384 1 h: 20971520 457688864 4.2BSD 2048 16384 1 i: 419425020 63 MSDOS j: 4194304 478660384 4.2BSD 2048 16384 1 k: 4194304 482854688 4.2BSD 2048 16384 1 l: 629145536 487049024 4.2BSD 4096 32768 1
These OpenBSD partitions are then mounted as follows using /etc/fstab:
/dev/sd0a / ffs rw,softdep 1 1 /dev/sd0d /tmp ffs rw,softdep,nodev,nosuid 1 2 /dev/sd0e /var ffs rw,softdep,nodev,nosuid 1 2 /dev/sd0f /usr ffs rw,softdep,nodev 1 2 /dev/sd0g /usr/X11R6 ffs rw,softdep,nodev 1 2 /dev/sd0h /usr/local ffs rw,softdep,nodev 1 2 /dev/sd0i /mnt/example msdos rw,nodev,nosuid 1 2 /dev/sd0j /usr/src ffs rw,softdep,nodev,nosuid 1 2 /dev/sd0k /usr/obj ffs rw,softdep,nodev,nosuid 1 2 /dev/sd0l /home ffs rw,softdep,nodev,nosuid 1 2
-e flag causes
fdisk to enter an interactive command mode. The
prompt contains information about the state of the edit process.
Where disk is the name of the disk being edited, ‘*’ means that the in-memory copy of the partition table has been modified, but not yet written to disk and 1 is the number of the boot block being edited. This number will be 2 when editing an extended MBR partition, 3 when editing an extended MBR partition within an extended MBR partition, and so on.
The list of commands and their explanations are given below. Commands may be abbreviated provided enough characters are given to ensure unambiguity.
- A synonym for
- Display a list of commands that
fdiskunderstands in the interactive edit mode.
- Display this manual page.
- Initialize the currently selected, in-memory copy of the boot block.
By default an MBR is initialized. If
gptis specified, a protective MBR and a GPT are initialized.
- Edit a given table entry in the memory copy of the current boot block. Sizes may be adjusted in BIOS geometry mode (MBR only) or using sector offsets and sizes. A unit ‘b’, ‘k’, ‘m’, ‘g’, or ‘t’ may be appended to indicate bytes, kilobytes, megabytes, gigabytes, or terabytes. The special size value ‘*’ will cause the partition to be sized to use the remainder of the disk.
- Make the given MBR partition table entry bootable and mark all others as not bootable (only one entry can be marked bootable). The bootable partition is denoted with ‘*’. If a value of 0 is given, the MBR partition is marked as not bootable, but no other MBR partitions are touched.
- Update the machine MBR bootcode and 0xAA55 signature in the memory copy of the currently selected MBR. Note that this option will overwrite an NT disk signature, if present. Not available when editing a GPT.
- Select and load into memory the MBR pointed to by the extended MBR partition table entry in the current boot block. Not available when editing a GPT.
- Change the partition identifier of the given partition table entry. This command is particularly useful for reassigning an existing partition to OpenBSD.
- Swap two partition entries.
- Print the currently selected in-memory copy of the partition table to the terminal. A unit ‘b’, ‘k’, ‘m’, ‘g’, or ‘t’ may be appended to indicate bytes, kilobytes, megabytes, gigabytes, or terabytes. Otherwise the number of sectors is printed.
- Write the in-memory copy of the partition table to disk.
- Exit the current level of
fdisk, either returning to the previously selected in-memory copy of a MBR, or exiting the program if there is none.
- Exit the current level of
fdisk, either returning to the previously selected in-memory copy of a MBR, or exiting the program if there is none. Unlike exit it does write the modified block out.
- Quit program without saving current changes.
- default MBR template
fstab(5), boot_amd64(8), boot_i386(8), boot_macppc(8), disklabel(8)
Hand crafted disk layouts are highly error prone. It is common practice, though by no means required, that MBR partitions start on a cylinder boundary (generally head 0, sector 1, but head 1, sector 1 for track 0), and that MBR partitions also end at cylinder boundaries.