Build disk images with Elemental

Requirements:

• qemu-img utility
• elemental binary
• elemental runtime dependencies

The suggested approach is based on using the Elemental installer (elemental install command) to run the installation from a Linux to a loop device. The loop device can be a raw image created with qemu-img create that can easily be converted to other formats after the installation by using qemu-img convert.

Get Elemental

Elemental binary can be downloaded from the github releases page.

The golang binary can be used as is, however take into account that some linux utilities are expected to be present in the host. More specific elemental expects to find common linux utilities to operate over block devices: rsync, parted, blkid, lsblk, udevadm, resize2fs, tune2fs, mkfs.ext2, etc.

Prepare the loop device

Preparing the a loop device for the installation is simple and straight forward.

# Create a raw image of 32G
> qemu-img create -f raw diks.img 32G

# Set the disk image as a loop device
> sudo losetup -f --show disk.img
<device>

Run elemental installation

Execute the elemental installation as described in TODO:

> sudo elemental install --force-efi --docker-image <image> <device>

Where <image> is the cOS derivative container image we want to use for the disk creation and <device> is the loop device previously created with losetup (e.g. /dev/loop0).

Convert the RAW image to desired format

Once the installation is done just unsetting the loop device and converting the image to the desired format is missing:

# Unset the loop device
> sudo losetup -d <device>

# Convert the RAW image to qcow2
> qemu-img convert -f raw -O qcow2 disk.img disk.qcow2

QEMU supports a wide range of formats including common ones such as vdi, vmdk or vhdx.

The result can be easily tested on QEMU with:

> qemu -m 4096 -hda disk.qcow2 -bios /usr/share/qemu/ovmf-x86_64.bin

Note the firmware image path varies depending on the host distro, the path provided in this example is based on openSUSE Leap.