OpenStack Nova Scheduling based on CPU architecture

If you have an OpenStack cloud with compute nodes with different CPU architectures, then this post will give you the needed information about how to tag your guest images, which enables the Nova scheduler to find a target with the correct CPU architecture.

Change history:
Date Change description
2017-08-25 The first release

TL;DR

Use hw_architecture instead of architecture or cpu_arch.

  • OpenStack CLI:

    $ openstack image set <IMG-UUID> --property hw_architecture=s390x
    
  • Ansible module os_image:

    - name: "Upload bootable s390x guest image into Glance."
      os_image:
        name: ubuntu_z
        filename: /tmp/ubuntu_s390x_disk.img
        container_format: bare
        disk_format: raw
        properties:
          hw_architecture: s390x
    

If that TL;DR doesn’t make sense to you, you’ll find a more exhaustive version below.

Use Case

Let’s assume you have set up an OpenStack cloud with compute nodes of different CPU architectures. For example, some Intel x86 compute nodes, some IBM Z s390x compute nodes (and maybe some IBM POWER ppc64 compute nodes).

An OpenStack cloud with compute nodes of different CPU architectures.

Those compute hosts with different CPU architectures need guest images which were built for those architectures. All an OpenStack user sees are the different guest images prepare by an OpenStack admin/operator. For the sake of example, maybe something like this:

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$ openstack image list
+--------------------------------------+----------+--------+
| ID                                   | Name     | Status |
+--------------------------------------+----------+--------+
| 7045d1c5-a927-4d49-9fe2-d3f63075591f | ubuntu_x | active |
| c5f206d1-d288-47fe-a67f-29a8b464bfc7 | ubuntu_p | active |
| 54c25961-d1ff-4d28-ac1a-41293ab9132b | ubuntu_z | active |
| ce7bea92-1e1d-4062-99bc-01a1db1a4b45 | rhel_x   | active |
| 46ecff6d-dd7f-4907-b7b3-57d0d7290e03 | rhel_z   | active |
| 4ef7df70-c5b8-43b9-9812-dfd4cd3e3c09 | suse_p   | active |
| ff208726-1ae5-4b40-84d4-571e06aff8e5 | suse_z   | active |
+--------------------------------------+----------+--------+

The openstack users usually don’t see the details of the different compute nodes. They don’t need to and shouldn’t. All they want is that this architecture specific image gets scheduled on the compute node which can fulfill their needs.

Guest image metadata properties

The most reasonable way I know to do this, is to add Glance image metadata, which specifies the CPU architecture of that image so that other OpenStack services can use that information. The one OpenStack service we need for that, is the Nova scheduler service. Specifically one of its filters, the ImagePropertiesFilter.

Unfortunately, the docs in Glance don’t make it that obvious which value you have to set. You will most likely stumble upon the metadata property architecture. The Glance docs [1] say this:

“The CPU architecture that must be supported by the hypervisor. For example, x86_64, arm, or ppc64.”

Sounds like the correct one for your use-case, right? The openstack CLI also says to use this [2].

It gets a bit more confusing when you want to use the Ansible module os_image [3] for your Infrastructure as Code (IaC). The example there uses cpu_arch:

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- os_image:
    name: cirros
    container_format: bare
    disk_format: qcow2
    state: present
    filename: cirros-0.3.0-x86_64-disk.img
    properties:
      cpu_arch: x86_64
      distro: ubuntu

I’m not sure if that metadata property has any effect anywhere. It hadn’t when I tested the Nova scheduling with it. A discussion on the OpenStack mailing list tried to find actions how to resolve this [4].

Impact to the Nova scheduler

Long story short, I don’t know the history of how it happened, but the Nova scheduler filter ImagePropertiesFilter which we want to use, needs the property to be named hw_architecture [5].

Apply the metadata property with:

$ openstack image set <IMG-UUID> --property hw_architecture=s390x

Double-check the property with:

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$ openstack image show <IMG-UUID>
+------------------+------------------------------------------------------+
| Field            | Value                                                |
+------------------+------------------------------------------------------+
| ...              | ...                                                  |
| container_format | bare                                                 |
| disk_format      | raw                                                  |
| id               | <IMG-UUID>                                           |
| min_disk         | 0                                                    |
| min_ram          | 0                                                    |
| name             | ubuntu_z                                             |
| properties       | hw_architecture='s390x'                              |
| schema           | /v2/schemas/image                                    |
| status           | active                                               |
+------------------+------------------------------------------------------+

When this is applied to the image, and a user launches that image, the Nova scheduler filters out hosts which don’t offer that CPU architecture. You can see the filtering happening in the Nova scheduler logs. The shortened example below starts with 2 compute nodes, one with x86 and the other one with s390x:

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$ grep filter /var/log/nova/nova-scheduler.log
DEBUG nova.filters [...] Starting with 2 host(s)
DEBUG nova.scheduler.filters.retry_filter [...] Re-scheduling is disabled host_passes
DEBUG nova.scheduler.filters.retry_filter [...] Re-scheduling is disabled host_passes
DEBUG nova.filters [...] Filter RetryFilter returned 2 host(s)
DEBUG nova.filters [...] Filter AvailabilityZoneFilter
DEBUG nova.filters [...] Filter RamFilter returned 2 host(s)
DEBUG nova.filters [...] Filter ComputeFilter returned 2 host(s)
DEBUG nova.filters [...] Filter ComputeCapabilitiesFilter returned 2 host(s)
DEBUG nova.scheduler.filters.image_props_filter [...]
    Instance contains properties ImageMetaProps(hw_architecture='s390x',...)
    that are not provided by the compute node
DEBUG nova.scheduler.filters.image_props_filter [...] (cmpx1, cmpx1)
    ram: 142990MB disk: 91136MB io_ops: 0 instances: 0
    does not support requested instance_properties
DEBUG nova.filters [...] Filter ImagePropertiesFilter returned 1 host(s)

You’ll notice that the ImagePropertiesFilter removed the one compute node which cannot fulfill the ImageMetaProps. We started with 2 hosts and at the end only one host is an eligible target host for the Instance, because it offers the prerequisite defined with hw_architecture. A full list of supported CPU architectures can be found at [6].

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