hw_machine_type - Configuring and updating QEMU instance machine types¶
New in version 12.0.0: (Liberty)
Changed in version 23.0.0: (Wallaby)
The libvirt driver now records the machine type of an instance at start up
allowing the [libvirt]hw_machine_type
configurable to change over time
without impacting existing instances.
Added nova-manage
commands to control the machine_type of an instance.
Note
The following only applies to environments using libvirt compute hosts.
Introduction¶
QEMU’s machine type concept can be thought of as a virtual chipset that
provides certain default devices (e.g. PCIe graphics card, Ethernet controller,
SATA controller, etc). QEMU supports two main variants of “machine type” for
x86 hosts: (a) pc
, which corresponds to Intel’s I440FX chipset (released in
1996) and (b) q35
, which corresponds to Intel’s 82Q35 chipset (released in
2007). For AArch64 hosts, the machine type is called: virt
.
The pc
machine type is considered legacy, and does not support many modern
features. Although at this time of writing, upstream QEMU has not reached an
agreement to remove new versioned variants of the pc
machine type, some
long-term stable Linux distributions (CentOS, RHEL, possibly others) are moving
to support q35
only.
Configure¶
For end users the machine type of an instance is controlled by the selection of an image with the hw_machine_type image metadata property set.
$ openstack image set --property hw_machine_type=q35 $IMAGE
The libvirt virt driver supports the configuration of a per compute host
default machine type via the libvirt.hw_machine_type
option. Providing a default machine type per host architecture to be used when
no corresponding hw_machine_type
image property is provided for the
instance.
When this option is not defined the libvirt driver relies on the following hardcoded dictionary of default machine types per architecture:
default_mtypes = {
obj_fields.Architecture.ARMV7: "virt",
obj_fields.Architecture.AARCH64: "virt",
obj_fields.Architecture.S390: "s390-ccw-virtio",
obj_fields.Architecture.S390X: "s390-ccw-virtio",
obj_fields.Architecture.I686: "pc",
obj_fields.Architecture.X86_64: "pc",
}
Update¶
Prior to the Wallaby (23.0.0) release the
libvirt.hw_machine_type
option had to remain static once
set for the lifetime of a deployment. This was due to the machine type of
instances without a hw_machine_type
image property using the newly
configured machine types after a hard reboot or migration This could in turn
break the internal ABI of the instance when changing between underlying machine
types such as pc
to q35
.
From the Wallaby (23.0.0) release it is now possible to change the
libvirt.hw_machine_type
config once all instances have a
machine type recorded within the system metadata of the instance.
To allow this the libvirt driver will now attempt to record the machine type
for any instance that doesn’t already have it recorded during start up of the
compute service or initial spawn of an instance. This should ensure a machine
type is recorded for all instances after an upgrade to Wallaby that are not in
a SHELVED_OFFLOADED
state.
To record a machine type for instances in a SHELVED_OFFLOADED
state after
an upgrade to Wallaby a new nova-manage command has been introduced
to initially record the machine type of an instance.
$ nova-manage libvirt update_machine_type $instance $machine_type
This command can also be used later to update the specific machine type used by the instance. An additional nova-manage command is also available to fetch the machine type of a specific instance:
$ nova-manage libvirt get_machine_type $instance
To confirm that all instances within an environment or a specific cell have had a machine type recorded another nova-manage command can be used:
$ nova-manage libvirt list_unset_machine_type
The logic behind this command is also used by a new nova-status upgrade check that will fail with a warning when instances without a machine type set exist in an environment.
$ nova-status upgrade check
Once it has been verified that all instances within the environment or specific
cell have had a machine type recorded then the
libvirt.hw_machine_type
can be updated without impacting
existing instances.