Creating instance images

Bare Metal provisioning requires two sets of images: the deploy images and the user images. The deploy images are used by the Bare Metal service to prepare the bare metal server for actual OS deployment. Whereas the user images are installed on the bare metal server to be used by the end user. There are two types of user images:

partition images

contain only the contents of the root partition. Additionally, two more images are used together with them when booting from network: an image with a kernel and with an initramfs.

Warning

To use partition images, Grub2 must be installed in the image. This is not a recommended path.

whole disk images

contain a complete partition table with one or more partitions.

Warning

The kernel/initramfs pair must not be used with whole disk images, otherwise they’ll be mistaken for partition images. Whole disk images are the recommended type of images to use.

Many distributions publish their own cloud images. These are usually whole disk images that are built for legacy boot mode (not UEFI), with Ubuntu being an exception (they publish images that work in both modes).

Supported Disk Image Formats

The following formats are tested by Ironic and are expected to work as long as no unknown or unsafe special features are being used

  • raw - A file containing bytes as they would exist on a disk or other block storage device. This is the simplest format.

  • qcow2 - An updated file format based upon the QEMU Copy-on-Write format.

A special mention exists for iso formatted “CD” images. While Ironic uses the ISO9660 filesystems in some of it’s processes for aspects such as virtual media, it does not support writing them to the remote block storage device.

Image formats we believe may work due to third party reports, but do not test:

  • vmdk - A file format derived from the image format originally created by VMware for their hypervisor product line. Specifically we believe a single file VMDK formatted image should work. As there are are several subformats, some of which will not work and may result in unexpected behavior such as failed deployments.

  • vdi - A file format used by Oracle VM Virtualbox hypervisor.

As Ironic does not support these formats, their usage is normally blocked due security considerations by default. Please consult with your Ironic Operator.

It is important to highlight that Ironic enforces and matches the file type based upon signature, and not file extension. If there is a mismatch, the input and or remote service records such as in the Image service must be corrected.

disk-image-builder

The disk-image-builder can be used to create user images required for deployment and the actual OS which the user is going to run.

  • Install diskimage-builder package (use virtualenv, if you don’t want to install anything globally):

    # pip install diskimage-builder

  • Build the image your users will run (Ubuntu image has been taken as an example):

    • Partition images

      $ disk-image-create ubuntu baremetal dhcp-all-interfaces grub2 -o my-image

    • Whole disk images

      $ disk-image-create ubuntu vm dhcp-all-interfaces -o my-image

      … with an EFI partition:

      $ disk-image-create ubuntu vm block-device-efi dhcp-all-interfaces -o my-image

The partition image command creates my-image.qcow2, my-image.vmlinuz and my-image.initrd files. The grub2 element in the partition image creation command is only needed if local boot will be used to deploy my-image.qcow2, otherwise the images my-image.vmlinuz and my-image.initrd will be used for PXE booting after deploying the bare metal with my-image.qcow2. For whole disk images only the main image is used.

If you want to use Fedora image, replace ubuntu with fedora in the chosen command.

Virtual machine

Virtual machine software can also be used to build user images. There are different software options available, qemu-kvm is usually a good choice on linux platform, it supports emulating many devices and even building images for architectures other than the host machine by software emulation. VirtualBox is another good choice for non-linux host.

The procedure varies depending on the software used, but the steps for building an image are similar, the user creates a virtual machine, and installs the target system just like what is done for a real hardware. The system can be highly customized like partition layout, drivers or software shipped, etc.

Usually libvirt and its management tools are used to make interaction with qemu-kvm easier, for example, to create a virtual machine with virt-install:

$ virt-install --name centos8 --ram 4096 --vcpus=2 -f centos8.qcow2 \
> --cdrom CentOS-8-x86_64-1905-dvd1.iso

Graphic frontend like virt-manager can also be utilized.

The disk file can be used as user image after the system is set up and powered off. The path of the disk file varies depending on the software used, usually it’s stored in a user-selected part of the local file system. For qemu-kvm or GUI frontend building upon it, it’s typically stored at /var/lib/libvirt/images.