diskimage-builder/doc/source/user_guide/building_an_image.rst

Building An Image

Now that you have diskimage-builder properly installed <installation> you can get started by building your first disk image.

VM Image

Our first image is going to be a bootable vm image using one of the standard supported distribution elements <../elements> (Ubuntu or Fedora).

The following command will start our image build (distro must be either 'ubuntu' or 'fedora'):

disk-image-create <distro> vm

This will create a qcow2 file 'image.qcow2' which can then be booted.

Elements

It is important to note that we are passing in a list of elements <../elements> to disk-image-create in our above command. Elements are how we decide what goes into our image and what modifications will be performed.

Some elements provide a root filesystem, such as the ubuntu or fedora element in our example above, which other elements modify to create our image. At least one of these 'distro elements' must be specified when performing an image build. It's worth pointing out that there are many distro elements (you can even create your own), and even multiples for some of the distros. This is because there are often multiple ways to install a distro which are very different. For example: One distro element might use a cloud image while another uses a package installation tool to build a root filesystem for the same distro.

Other elements modify our image in some way. The 'vm' element in our example above ensures that our image has a bootloader properly installed. This is only needed for certain use cases and certain output formats and therefore it is not performed by default.

Output Formats

By default a qcow2 image is created by the disk-image-create command. Other output formats may be specified using the -t <format> argument. Multiple output formats can also be specified by comma separation. The supported output formats are:

• qcow2
• tar
• vhd
• docker
• raw

Filesystem Caveat

By default, disk-image-create uses a 4k byte-to-inode ratio when creating the filesystem in the image. This allows large 'whole-system' images to utilize several TB disks without exhausting inodes. In contrast, when creating images intended for tenant instances, this ratio consumes more disk space than an end-user would expect (e.g. a 50GB root disk has 47GB avail.). If the image is intended to run within a tens to hundrededs of gigabyte disk, setting the byte-to-inode ratio to the ext4 default of 16k will allow for more usable space on the instance. The default can be overridden by passing --mkfs-options like this:

disk-image-create --mkfs-options '-i 16384' <distro> vm