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Manual Pages  — MKIMG


mkimg – utility to make disk images



mkimg [-H heads] [-P blksz] [-S secsz] [-T tracksz] [-b bootcode] [-c min_capacity] [-C max_capacity] [--capacity capacity] [-f format] [-o outfile] [-a active] [-v] [-y] [-s scheme [-p partition ...]]
mkimg --formats | --schemes | --version


The mkimg utility creates a disk image from the raw partition contents specified with the partition argument(s) and using the partitioning scheme specified with the scheme argument. The disk image is written to stdout by default or the file specified with the outfile argument. The image file is a raw disk image by default, but the format of the image file can be specified with the format argument.

The disk image can be made bootable by specifying the scheme-specific boot block contents with the bootcode argument and, depending on the scheme, with a boot partition. The contents of such a boot partition is provided like any other partition and the mkimg utility does not treat it any differently from other partitions.

Some partitioning schemes need a disk geometry and for those the mkimg utility accepts the tracksz and heads arguments, specifying the number of sectors per track and the number of heads per cylinder (resp.)

Both the logical and physical sector size can be specified and for that the mkimg utility accepts the secsz and blksz arguments. The secsz argument is used to specify the logical sector size. This is the sector size reported by a disk when queried for its capacity. Modern disks use a larger sector size internally, referred to as block size by the mkimg utility and this can be specified by the blksz argument. The mkimg utility will use the (physical) block size to determine the start of partitions and to round the size of the disk image.

The -c option can be used to specify a minimal capacity for the disk image. Use this option without the -s and -p options to create an empty disk image with the given (virtual) size. An empty partition table can be written to the disk when specifying a partitioning scheme with the -s option, but without specifying any partitions. When the size required for all the partitions is larger than the given capacity, then the disk image will be larger than the capacity given.

The -C option specifies a maximum capacity for the disk image. If the combined sizes of the given partitions exceed the size given with -C, image creation fails.

The --capacity option is a shorthand to specify the minimum and maximum capacity at the same time.

The -v option increases the level of output that the mkimg utility prints.

The -y option is used for testing purposes only and is not to be used in production. When present, the mkimg utility will generate predictable values for Universally Unique Identifiers (UUIDs) and time stamps so that consecutive runs of the mkimg utility will create images that are identical.

The active option marks a partition as active, if the partitioning scheme supports it. Currently, only the mbr scheme supports this concept. By default, mkimg will only mark the first partition as active when boot code is specified. Use the active option to override the active partition. The number specified corresponds to the number after the 's' in the partition's geom(8) name. No partitions are marked active when the value is 0.

A set of long options exist to query about the mkimg utility itself. Options in this set should be given by themselves because the mkimg utility exits immediately after providing the requested information. The version of the mkimg utility is printed when the --version option is given. The list of supported output formats is printed when the --formats option is given and the list of supported partitioning schemes is printed when the --schemes option is given. Both the format and scheme lists a space-separated lists for easy handling in scripts.

For a more descriptive list of supported partitioning schemes or supported output format, or for a detailed description of how to specify partitions, run the mkimg utility without any arguments. This will print a usage message with all the necessary details.


The mkimg utility supports a number of output file formats. A short description of these is given below.


QCOW stands for "QEMU Copy On Write". It's a sparse file format akin to VHD and VMDK and QCOW represents the first version. QCOW2 represents version 2 of the file format. Version 2 is not backward compatible with version 1 and adds support for snapshots among other things. The QCOW file formats are natively supported by QEMU and Xen. To write QCOW, specify -f qcow on the command line. To write version 2 QCOW, specify -f qcow2 on the command line. The preferred file extension is ".qcow" and ".qcow2" for QCOW and QCOW2 (resp.), but ".qcow" is sometimes used for version 2 files as well.

RAW file format

This file format is a sector by sector representation of an actual disk. There is no extra information that describes or relates to the format itself. The size of the file is the size of the (virtual) disk. This file format is suitable for being copyied onto a disk with utilities like dd. To write a raw disk file, either omit the -f option, or specify -f raw on the command line. The preferred file extension is one of ".img" or ".raw", but there's no real convention for it.

Dynamic VHD and Fixed VHD

Microsoft's "Virtual Hard Disk" file formats. The dynamic format is a sparse format akin to QCOW and VMDK. The fixed format is effectively a raw format with a footer appended to the file and as such it's often indistinguishable from the raw format. The fixed file format has been added to support Microsoft's Azure platform and due to inconsistencies in interpretation of the footer is not compatible with utilities like qemu when it is specifically instructed to interpreted the file as a VHD file. By default qemu will treat the file as a raw disk file, which mostly works fine. To have mkimg create a dynamic VHD file, specify -f vhd on the command line. To create a fixed VHD file for use by Azure, specify -f vhdf on the command line. The preferred file extension is ".vhd".

Dynamic VHDX

Microsoft's "Virtual Hard Disk v2" file formats, the successor to VHD. VHDX is the required format for the 2nd generation Hyper-V VMs. To have mkimg create a dynamic VHDX file, specify -f vhdx on the command line. The preferred file extension is ".vhdx".


VMware's "Virtual Machine Disk" file format. It's a sparse file format akin to QCOW and VHD and supported by many virtualization solutions. To create a VMDK file, specify -f vmdk on the command line. The preferred file extension is ".vmdk".

Not all virtualization solutions support all file formats, but often those virtualization environments have utilities to convert from one format to another. Note however that conversion may require that the virtual disk size is changed to match the constraints of the output format and this may invalidate the contents of the disk image. For example, the GUID Partition Table (GPT) scheme has a header in the last sector on the disk. When changing the disk size, the GPT must be changed so that the last header is moved accordingly. This is typically not part of the conversion process. If possible, use an output format specifically for the environment in which the file is intended to be used.


  Directory to put temporary files in; default is /tmp.


To create a bootable disk image that is partitioned using the GPT scheme and containing a root file system that was previously created using makefs(8) and also containing a swap partition, run the mkimg utility as follows:

    % mkimg -s gpt -b /boot/pmbr -p freebsd-boot:=/boot/gptboot

-p freebsd-ufs:=root-file-system.ufs -p freebsd-swap::1G -o gpt.img

The command line given above results in a raw image file. This is because no output format was given. To create a VMDK image for example, add the -f vmdk argument to the mkimg utility and name the output file accordingly.

A nested partitioning scheme is created by running the mkimg utility twice. The output of the first will be fed as the contents of a partition to the second. This can be done using a temporary file, like so:

    % mkimg -s bsd -b /boot/boot -p freebsd-ufs:=root-file-system.ufs

-p freebsd-swap::1G -o /tmp/bsd.img

    % mkimg -s mbr -b /boot/mbr -p freebsd:=/tmp/bsd.img -o mbr-bsd.img

Alternatively, the mkimg utility can be run in a cascaded fashion, whereby the output of the first is fed directly into the second. To do this, run the mkimg utility as follows:

    % mkimg -s mbr -b /boot/mbr -p freebsd:-'mkimg -s bsd -b /boot/boot

-p freebsd-ufs:=root-file-system.ufs -p freebsd-swap::1G' -o mbr-bsd.img

To accommodate the need to have partitions named or numbered in a certain way, the mkimg utility allows for the specification of empty partitions. For example, to create an image that is compatible with partition layouts found in /etc/disktab, the 'd' partition often needs to be skipped. This is accomplished by inserting an unused partition after the first 2 partition specifications. It is worth noting at this time that the BSD scheme will automatically skip the 'c' partition by virtue of it referring to the entire disk. To create an image that is compatible with the qp120at disk, use the mkimg utility as follows:

    % mkimg -s bsd -b /boot/boot -p freebsd-ufs:=root-file-system.ufs

-p freebsd-swap::20M -p- -p- -p- -p- -p freebsd-ufs:=usr-file-system.ufs -o bsd.img

For partitioning schemes that feature partition labels, the mkimg utility supports assigning labels to the partitions specified. In the following example the file system partition is labeled as 'backup':

    % mkimg -s gpt -p freebsd-ufs/backup:=file-system.ufs -o gpt.img


dd(1), gpart(8), makefs(8), mdconfig(8), newfs(8)


The mkimg utility first appeared in FreeBSD 10.1 .


The mkimg utility and manpage were written by Marcel Moolenaar <Mt marcel@FreeBSD.org>.

MKIMG (1) June 8, 2020

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