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Multi-Architecture Deployments

OpenStack-Ansible supports deployments where either the control plane or compute nodes may comprise of several different CPU architectures

Mixed CPU architectures for compute nodes

OpenStack-Ansible supports having compute nodes of multiple architectures deployed in the same environment.

Deployments consisting entirely of x86_64 or aarch64 nodes do not need any special consideration and will work according to the normal OpenStack-Ansible documentation.

A deployment with a mixture of architectures, or adding a new architecture to an existing single architecure deployment requires some additional steps to be taken by both the deployer and end users to ensure that the behaviour is as desired.

Example - adding aarch64 nodes to an x86_64 deployment

  1. Install the operating system onto all the new compute nodes.

  2. Add the new compute nodes to openstack_user_config.yml.

  3. Ensure a host of each compute architecture is present in repo-infra_hosts in openstack_user_config.yml.

    This host will build python wheels for it’s own architecture which will speed up the deployment of many hosts. If you do not make a repository server for each architecture, ensure that measures are taken not to overload the opendev.org git servers, such as using local mirrors of all OpenStack service repos.

  4. Run the OpenStack-Ansible playbooks to deploy the required services.

  5. Add HW_ARCH_XXXX Trait to Every Compute Host in Openstack

    Although most CPU hardware traits such as instruction set extensions are detected and handled automatically in OpenStack, CPU architecture is not. It is necessary to manually add an architecture trait to the resource provider corresponding to every compute host. The required traits are:

    HW_ARCH_X86_64 for x86_64 Intel and AMD CPUs HW_ARCH_AARCH64 for aarch64 architecure CPUs

    (see: https://docs.openstack.org/os-traits/latest/reference/traits.html)

    openstack resource provider list
    openstack resource provider trait list <uuid-of-compute-host>
    openstack resource provider trait set --trait <existing-trait-1> --trait <existing-trait-2> ... --trait HW_ARCH_xxxxx <uuid-of-compute-host>


    The trait set command replaces all existing traits with the set provided, so you must specify all existing traits as well as the new trait.

  6. Configure Nova Scheduler to Check Architecture

    Two additional settings in /etc/nova/nova.conf in all Nova API instances:

    image_metadata_prefilter = True
    image_properties_default_architecture = x86_64

    The image_metadata_prefilter setting forces the Nova scheduler to match the hw_architecture property on Glance images with the corresponding HW_ARCH_XXX trait on compute host resource providers. This ensures that images explicitly tagged with a target architecture get scheduled hosts with a matching architecture.

    The image_properties_default_architecture setting would apply in an existing x86_64 architecture cloud where previously hw_architecture was not set on all Glance images. This avoids the need to retrospectively apply the property for all existing images which may be difficult as users may have their own tooling to create and upload images without applying the required property.


    Undocumented Behaviour Alert!

    Note that the image metadata prefilter and ImagePropertiesFilter are different and unrelated steps in the process Nova scheduler uses to determine candidate compute hosts. This section explains how to use them together.

    The image_metadata_prefilter only looks at the HW_ARCH_XXX traits on compute hosts and finds hardware that matches the required architecture. This only happens when the hw_architecture property is present on an image, and only if the required traits are manually added to compute hosts.

    The image_properties_default_architecture is used by the ImagePropertiesFilter which examines all the architectures supported by QEMU on each compute host; this includes software emulations of non-native architectures.

    If the full QEMU suite is installed on a compute host, that host will offer to run all architectures supported by the available qemu-system-* binaries. In this situation images without the hw_architecture property could be scheduled to a non native architecture host and emulated.

  7. Disable QEMU Emulation


    This step applies particularly to existing x86_64 environments when new aarch64 compute nodes are added and it cannot be assumed that the hw_architecure property is applied to all Glance images as the operator may not be in control of all image uploads.

    To avoid unwanted QEMU emulation of non native architectures it is necessary to ensure that only the native qemu-system-* binary is present on all compute nodes. The simplest way to do this for existing deployments is to use the system package manager to ensure that the unwanted binaries are removed.

    OpenStack-Ansible releases including 2023.1 and later will only install the native architecture qemu-system-*` binary so this step should not be required on newer releases.

  8. Upload images to Glance

    • Ideally the hw_architecture property is set for all uploaded images. It is mandatory to set this property for all architectures that do not match image_properties_default_architecture

    • It is recommended to set the property hw_firmware_type='uefi' for any images which require UEFI boot, even when this implicit with the aarch64 architecture. This is to avoid issues with NVRAM files in libvirt when deleting an instance.

Architecture emulation by Nova

Nova has the capability to allow emulation of one CPU architecture on a host with a different native CPU architecure, see https://docs.openstack.org/nova/latest/admin/hw-emulation-architecture.html for more details.

This OpenStack-Ansible documentation currently assumes that a deployer wishes to run images on a compute host with a native CPU architecure, and does not give an example configuration involving emulation.