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

Disk Image Layout

When generating a block image (e.g. qcow2 or raw), by default one image with one partition holding all files is created.

The appropriate functionality to use multiple partitions and even LVMs is currently under development; therefore the possible configuration is currently limited, but will get more flexible as soon as all the functionality is implemented.

The configuration is done by means of the environment variable DIB_BLOCK_DEVICE_CONFIG. This variable must hold JSON structured configuration data.

In future this will be a list of four elements, each describing one level of block device setup - but because currently only the lowest level is implemented, it contains only the configuration of the first level of block device setup

The default is:

DIB_BLOCK_DEVICE_CONFIG='[
    [["local_loop",
      {"name": "rootdisk"}]]]'

In general each module is configured in the way, that the first element in the list is the name of the module (e.g. local_loop) followed by a dictionary of parameters (here {"name": "rootdisk"}).

Level 0

Module: Local Loop

This module generates a local image file and uses the loop device to create a block device from it. The symbolic name for this module is local_loop.

Configuration options:

name

(mandatory) The name of the image. This is used as the name for the image in the file system and also as a symbolic name to be able to reference this image (e.g. to create a partition table on this disk).

size

(optional) The size of the disk. The size can be expressed using unit names like TiB (1024^4 bytes) or GB (1000^3 bytes). Examples: 2.5GiB, 12KB. If the size is not specified here, the size as given to disk-image-create (--image-size) or the automatically computed size is used.

directory

(optional) The directory where the image is created.

Example:

DIB_BLOCK_DEVICE_CONFIG='[
     [["local_loop",
       {"name": "rootdisk"}],
      ["local_loop",
       {"name": "datadisk",
        "size": "7.5GiB",
        "directory": "/var/tmp"}]]]'

This creates two image files and uses the loop device to use them as block devices. One image file called rootdisk is created with default size in the default temp directory. The second image has the size of 7.5GiB and is created in the /var/tmp folder.

Please note that due to current implementation restrictions it is only allowed to specify one local loop image.

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

Speedups

If you have 4GB of available physical RAM (as reported by /proc/meminfo MemTotal), or more, diskimage-builder will create a tmpfs mount to build the image in. This will improve image build time by building it in RAM. By default, the tmpfs file system uses 50% of the available RAM. Therefore, the RAM should be at least the double of the minimum tmpfs size required. For larger images, when no sufficient amount of RAM is available, tmpfs can be disabled completely by passing --no-tmpfs to disk-image-create. ramdisk-image-create builds a regular image and then within that image creates ramdisk. If tmpfs is not used, you will need enough room in /tmp to store two uncompressed cloud images. If tmpfs is used, you would still need /tmp space for one uncompressed cloud image and about 20% of that image for working files.