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disklabel(8) read and write disk pack label
disklabel(5) disk pack label
disklabel, bounds_check_with_label, readdisklabel, setdisklabel, writedisklabel(9) disk label management routines

DISKLABEL(8) System Manager's Manual DISKLABEL(8)


disklabelread and write disk pack label


disklabel [-Acdtv] [-h | -p unit] [-T file] disk

disklabel -w [-Acdnv] [-T file] disk disktype [packid]

disklabel -e [-Acdnv] [-T file] disk

disklabel -E [-Acdnv] [-F|-f file] [-T file] disk

disklabel -R [-nv] [-F|-f file] disk protofile


The disklabel utility can be used to install, examine, or modify the label on a disk drive or pack. The disk label contains information about disk characteristics (size, type, etc.) and the partition layout, stored on the disk itself. It is used by the operating system to optimize disk I/O and locate the filesystems resident on the disk.
disklabel supports 15 configurable partitions, ‘a’ through ‘p’, excluding ‘c’. The ‘c’ partition describes the entire physical disk, is automatically created by the kernel, and cannot be modified or deleted by disklabel. By convention, the ‘a’ partition of the boot disk is the root partition, and the ‘b’ partition of the boot disk is the swap partition, but all other letters can be used in any order for any other partitions as desired.
The options are as follows:
Automatically allocate all the disk space in the OpenBSD portion of the disk in the recommended manner. See AUTOMATIC DISK ALLOCATION, below.
Clear the system's in-core copy of the label and update it based on the on-disk label.
Use the default label. This ignores any existing OpenBSD disk label on the disk.
Use the built-in command-driven label editor described below.
Edit an existing disk label using the editor specified in the EDITOR environment variable, or vi(1) if none is specified.
-F file
Write entries to file in fstab(5) format for any partitions for which mount point information is known. The entries will be written using disklabel UIDs. The -F flag is only valid when used in conjunction with the -E or -R flags. If file already exists, it will be overwritten.
-f file
The same as -F except that entries will be written using disk device names.
Print partition sizes in human readable format.
Make no permanent changes to the disklabel (useful for debugging purposes).
-p unit
Print partition sizes in unit instead of sectors. Valid units are b(ytes), c(ylinders), k(ilobytes), m(egabytes), g(igabytes) and t(erabytes).
Restore a disk label that was formatted in a prior operation and saved in an ASCII file.
-T file
Read the template for automatic allocation from file instead of using the builtin one. See AUTOMATIC DISK ALLOCATION below for the format.
Format the label as a disktab(5) entry.
Print additional information during operation (verbose mode).
Write a standard label on the designated drive.
Specify the disk to operate on. It can be specified by its full pathname, by an abbreviated disk form, or by its disklabel UID. In its abbreviated form, the path to the device, the ‘r’ denoting “raw device”, and the partition letter, can all be omitted. For example, the first IDE disk can be specified as either /dev/rwd0c, /dev/wd0c, or wd0.
Specify a disktype entry from the disktab(5) database.
Specify a pack identification string for the device (see below).
Used with the restore option (-R) to specify a file to read an ASCII label from.
The first form of the command (read) is used to examine the label on the named disk drive. It will display all of the parameters associated with the drive and its partition layout. The kernel's in-core copy of the label is displayed; if the disk has no label, or the partition types on the disk are incorrect, the kernel may have constructed or modified the label.
The second form of the command (write) is used to write a standard label on the designated drive. The drive parameters and partitions are taken from that file. If different disks of the same physical type are to have different partitions, it will be necessary to have separate disktab entries describing each, or to edit the label after installation as described below. The optional argument is a pack identification string, up to 16 characters long. The pack ID must be quoted if it contains blanks. The existing label will be updated via the in-core copy and any bootstrap code will be unaffected.
In the third form of the command (edit), the label is read from the in-core kernel copy and then supplied to an editor for changes. If no editor is specified in an EDITOR environment variable, vi(1) is used. When the editor terminates, the formatted label is reread and used to rewrite the disk label. Existing bootstrap code is unchanged.
The built-in label editor (fourth form) provides a simple interactive label editor. Some commands or prompts take an optional unit. Available units are ‘b’ for bytes, ‘c’ for cylinders, ‘k’ for kilobytes, ‘m’ for megabytes, ‘g’ for gigabytes, and ‘t’ for terabytes. If no unit is given, the default is to use sectors (usually 512 bytes).
Quantities are rounded to the nearest cylinder when units are specified for sizes (or offsets). At prompts that request a size, ‘*’ may be entered to indicate the rest of the available space, ‘%’ for percentage of total, and ‘&’ for percentage free. Commands may be aborted by entering ‘^D’ (Control-D). Entering ‘^D’ at the main ‘>’ prompt will exit the editor.
The editor commands are as follows:
? | h
Display help message with all available commands. There is also (simple) context-sensitive help available at most prompts.
Allocate all the disk space in the recommended manner. See AUTOMATIC DISK ALLOCATION, below.
a [part]
Add new partition. This option adds a new partition to the disk label. If no partition letter is specified (a-p), the user will be prompted for one.
Set OpenBSD disk boundaries. This option tells disklabel which parts of the disk it is allowed to modify. This option is probably only useful for ports with fdisk(8) partition tables where the ending sector in the MBR is incorrect. The user may enter ‘*’ at the “Size” prompt to indicate the entire size of the disk (minus the starting sector). This is useful for disks where the fdisk partition table is incapable of storing the real size. Note: data may become corrupted if boundaries are extended such that they overlap with other resident operating systems.
c [part]
Change the size of an existing partition. If no partition is specified, the user will be prompted for one. The new size may be in terms of the aforementioned units and may also be prefixed with ‘+’ or ‘-’ to change the size by a relative amount.
Sets the disk label to the default values as reported by the kernel. This simulates the case where there is no disk label.
d [part]
Delete an existing partition (or ‘*’ to delete all partitions). If no partition is specified, the user will be prompted for one.
Edit drive parameters. This option is used to set the following parameters: disk type, a descriptive label string, sectors/track, tracks/cylinder, sectors/cylinder, number of cylinders, total sectors, rpm, and interleave.
g [d | u]
Set disk geometry based on what the disk or user thinks (the user geometry is simply what the label said before disklabel made any changes).
Change the disklabel UID, specified as a 16-character hexadecimal string. If set to all zeros, a new UID will automatically be allocated when the disklabel is written to disk.
l [unit]
Print the disk label header.
Display this manual page. The manual page is piped through the pager specified by the PAGER environment variable or 'less' if PAGER is not set.
m [part]
Modify parameters for an existing partition. If no partition is specified, the user will be prompted for one. This option allows the user to change the filesystem type, starting offset, partition size, and mount point for the specified partition. If expert mode is enabled (see X below), then block fragment size, block size, and cylinders per group can also be modified. Note that not all parameters are configurable for non-BSD partitions.
n [part]
Name the mount point for an existing partition. If no partition is specified, the user will be prompted for one. This option is only valid if disklabel was invoked with the -f flag.
p [unit]
Print the current partition list. If a unit is given, the size and offsets are displayed in terms of the specified unit. If the unit is ‘*’ it is automatically determined by the size of the smallest partition.
Quit the editor. If any changes have been made, the user will be asked whether or not to save the changes to the on-disk label.
R [part]
Resize a partition in an automatically allocated label, compacting unused space between partitions with a higher offset. The last partition will be shrunk if necessary. Works only for automatically allocated labels with no spoofed partitions.
Recalculate free space. This command displays all the free areas on the disk and the total number of free sectors.
s [path]
Save the label to a file in ASCII format (suitable for loading via the -R option). If no path is specified, the user will be prompted for one.
Undo all changes made since entering the editor.
Undo (or redo) last change. Entering u once will undo the last change. Entering it again will restore the change.
Write the label to disk. This option will commit any changes to the on-disk label.
Toggle “expert mode”. By default, some settings are reserved for experts only (such as the block and fragment size on ffs partitions).
Exit the editor without saving any changes to the on-disk label.
Zero out the existing partition table and mountpoint information, leaving only the 'c' partition. The drive parameters are not changed.
In the restore form of the command (fifth form), the prototype file used to create the label should be in the same format as that produced when reading or editing a label. Comments are delimited by # and newline. Any existing bootstrap code will be unaffected.
The final three forms of disklabel are used to install bootstrap code on machines where the bootstrap is part of the label.
Note that when a disk has no real BSD disklabel, the kernel creates a default label so that the disk can be used. This default label will include other partitions found on the disk if they are supported on your architecture. For example, on systems that support fdisk(8) partitions the default label will also include DOS and Linux partitions. However, these entries are not dynamic, they are fixed at the time disklabel is run. That means that subsequent changes that affect non-OpenBSD partitions will not be present in the default label, though they may be updated by hand. To see the default label, run disklabel with the -d flag. disklabel can then be run with the -e flag and any entries pasted as desired from the default label into the real one.


The -A option and the editor command A automatically create a disklabel with a set of partitions suitable for a majority of OpenBSD installations. Any existing OpenBSD disklabel on the disk is ignored, but native partitions that would normally be spoofed are preserved in the disklabel, and are not modified during the allocation process.
Disk size determines the set of partitions which are created. Each partition is allocated space between a specified minimum and maximum. Initially, each partition is allocated its minimum space; remaining space is split between the partitions according to the given percentages, up to their maximum allowed space. Space left after all partitions have reached their maximum size is left unallocated. The sizes below are approximations, and may vary from architecture to architecture.
Disks > 7 Gigabytes
/		 5% of disk.  80M – 1G 
swap		 5% of disk.  80M – 2x max physical memory 
/tmp		 8% of disk. 120M – 4G 
/var		13% of disk.  80M – 2x size of crash dump 
/usr		 5% of disk. 900M – 2G 
/usr/X11R6	 3% of disk. 512M – 1G 
/usr/local	10% of disk.   2G – 10G 
/usr/src	 2% of disk.   1G – 2G 
/usr/obj	 4% of disk. 1.3G – 2G 
/home		45% of disk.   1G – 300G
Disks > 2 Gigabytes
/		 5% of disk. 800M – 2G 
swap		10% of disk.  80M – 2x max physical memory 
/usr		78% of disk. 900M – 3G 
/home		 7% of disk. 256M – 2G
Disks > 700 Megabytes
/		95% of disk. 700M – 4G 
swap		 5% of disk.   1M – 2x max physical memory
A template for the automatic allocation can be passed to disklabel using the -T option. The template consists of one line per partition, with each line giving mountpoint, min-max size range, and percentage of disk, space-separated. Max can be unlimited by specifying '*'. If only mountpoint and min size are given, the partition is created with that exact size.
/		250M 
swap		80M-256M 10% 
/tmp		120M-4G	8% 
/var		80M-4G	13% 
/usr		900M-2G	5% 
/usr/X11R6	512M-1G	3% 
/usr/local	2G-10G	10% 
/usr/src	1G-2G	2% 
/usr/obj	1.3G-2G	4% 
/home		1G-*	45%


Disk description file.
Primary bootstrap.
Secondary bootstrap.


Display the in-core label for sd0 as obtained via /dev/rsd0c:
# disklabel sd0
Create a label for sd0 based on information for “sd2212” found in /etc/disktab. Any existing bootstrap code will be clobbered.
# disklabel -w /dev/rsd0c sd2212 foo
Read the on-disk label from a disk with DUID 3eb7f9da875cb9ee, edit it and reinstall in-core as well as on-disk. Existing bootstrap code is unaffected.
# disklabel -E 3eb7f9da875cb9ee
Restore the on-disk and in-core label for sd0 from information in mylabel. Existing bootstrap code is unaffected.
# disklabel -R sd0 mylabel


The kernel device drivers will not allow the size of a disk partition to be decreased or the offset of a partition to be changed while it is open. Some device drivers create a label containing only a single large partition if a disk is unlabeled; thus, the label must be written to the ‘a’ partition of the disk while it is open. This sometimes requires the desired label to be set in two steps, the first one creating at least one other partition, and the second setting the label on the new partition while shrinking the ‘a’ partition.
On some machines the bootstrap code may not fit entirely in the area allocated for it by some filesystems. As a result, it may not be possible to have filesystems on some partitions of a “bootable” disk. When installing bootstrap code, disklabel checks for these cases. If the installed boot code would overlap a partition of type FS_UNUSED it is marked as type FS_BOOT. The newfs(8) utility will disallow creation of filesystems on FS_BOOT partitions. Conversely, if a partition has a type other than FS_UNUSED or FS_BOOT, disklabel will not install bootstrap code that overlaps it.


disklabel(5), disktab(5), scan_ffs(8)


The maximum disk and partition size is 64PB.
On some machines, such as Sparc64, partition tables may not exhibit the full functionality described above.
December 8, 2016 OpenBSD-current