In OpenStack, flavors define the compute, memory, and storage capacity of nova computing instances. To put it simply, a flavor is an available hardware configuration for a server. It defines the size of a virtual server that can be launched.


Flavors can also determine on which compute host a flavor can be used to launch an instance. For information about customizing flavors, refer to Manage Flavors.


A flavor consists of the following parameters:

Flavor ID

Unique ID (integer or UUID) for the new flavor. This property is required. If specifying ‘auto’, a UUID will be automatically generated.


Name for the new flavor. This property is required.

Historically, names were given a format XX.SIZE_NAME. These are typically not required, though some third party tools may rely on it.


Number of virtual CPUs to use. This property is required.

Memory MB

Amount of RAM to use (in megabytes). This property is required.

Root Disk GB

Amount of disk space (in gigabytes) to use for the root (/) partition. This property is required.

The root disk is an ephemeral disk that the base image is copied into. When booting from a persistent volume it is not used. The 0 size is a special case which uses the native base image size as the size of the ephemeral root volume. However, in this case the filter scheduler cannot select the compute host based on the virtual image size. As a result, 0 should only be used for volume booted instances or for testing purposes. Volume-backed instances can be enforced for flavors with zero root disk via the os_compute_api:servers:create:zero_disk_flavor policy rule.

Ephemeral Disk GB

Amount of disk space (in gigabytes) to use for the ephemeral partition. This property is optional. If unspecified, the value is 0 by default.

Ephemeral disks offer machine local disk storage linked to the lifecycle of a VM instance. When a VM is terminated, all data on the ephemeral disk is lost. Ephemeral disks are not included in any snapshots.


Amount of swap space (in megabytes) to use. This property is optional. If unspecified, the value is 0 by default.


This value was only applicable when using the xen compute driver with the nova-network network driver. Since nova-network has been removed, this no longer applies and should not be specified. It will likely be removed in a future release. neutron users should refer to the neutron QoS documentation

Is Public

Boolean value that defines whether the flavor is available to all users or private to the project it was created in. This property is optional. In unspecified, the value is True by default.

By default, a flavor is public and available to all projects. Private flavors are only accessible to those on the access list for a given project and are invisible to other projects.

Extra Specs

Key and value pairs that define on which compute nodes a flavor can run. These are optional.

Extra specs are generally used as scheduler hints for more advanced instance configuration. The key-value pairs used must correspond to well-known options. For more information on the standardized extra specs available, see below


A free form description of the flavor. Limited to 65535 characters in length. Only printable characters are allowed. Available starting in microversion 2.55.

Extra Specs


This is now documented in Extra Specs, so this should be removed and the documentation moved to those specs.

Hardware video RAM

Specify hw_video:ram_max_mb to control the maximum RAM for the video image. Used in conjunction with the hw_video_ram image property. hw_video_ram must be less than or equal to hw_video:ram_max_mb.

This is currently supported by the libvirt and the vmware drivers.

See https://libvirt.org/formatdomain.html#elementsVideo for more information on how this is used to set the vram attribute with the libvirt driver.

See https://pubs.vmware.com/vi-sdk/visdk250/ReferenceGuide/vim.vm.device.VirtualVideoCard.html for more information on how this is used to set the videoRamSizeInKB attribute with the vmware driver.

Watchdog behavior

For the libvirt driver, you can enable and set the behavior of a virtual hardware watchdog device for each flavor. Watchdog devices keep an eye on the guest server, and carry out the configured action, if the server hangs. The watchdog uses the i6300esb device (emulating a PCI Intel 6300ESB). If hw:watchdog_action is not specified, the watchdog is disabled.

To set the behavior, use:

$ openstack flavor set FLAVOR-NAME --property hw:watchdog_action=ACTION

Valid ACTION values are:

  • disabled: (default) The device is not attached.

  • reset: Forcefully reset the guest.

  • poweroff: Forcefully power off the guest.

  • pause: Pause the guest.

  • none: Only enable the watchdog; do nothing if the server hangs.


Watchdog behavior set using a specific image’s properties will override behavior set using flavors.

Random-number generator

If a random-number generator device has been added to the instance through its image properties, the device can be enabled and configured using:

$ openstack flavor set FLAVOR-NAME \
    --property hw_rng:allowed=True \
    --property hw_rng:rate_bytes=RATE-BYTES \
    --property hw_rng:rate_period=RATE-PERIOD


  • RATE-BYTES: (integer) Allowed amount of bytes that the guest can read from the host’s entropy per period.

  • RATE-PERIOD: (integer) Duration of the read period in milliseconds.

Performance Monitoring Unit (vPMU)

If nova is deployed with the libvirt virt driver and libvirt.virt_type is set to qemu or kvm, a vPMU can be enabled or disabled for an instance using the hw:pmu extra_spec or the hw_pmu image property. The supported values are True or False. If the vPMU is not explicitly enabled or disabled via the flavor or image, its presence is left to QEMU to decide.

$ openstack flavor set FLAVOR-NAME --property hw:pmu=True|False

The vPMU is used by tools like perf in the guest to provide more accurate information for profiling application and monitoring guest performance. For realtime workloads, the emulation of a vPMU can introduce additional latency which may be undesirable. If the telemetry it provides is not required, such workloads should set hw:pmu=False. For most workloads the default of unset or enabling the vPMU hw:pmu=True will be correct.

Hiding hypervisor signature

Some hypervisors add a signature to their guests. While the presence of the signature can enable some paravirtualization features on the guest, it can also have the effect of preventing some drivers from loading. Hiding the signature by setting this property to true may allow such drivers to load and work.


As of the 18.0.0 Rocky release, this is only supported by the libvirt driver.

Prior to the 21.0.0 Ussuri release, this was called hide_hypervisor_id. An alias is provided to provide backwards compatibility.

$ openstack flavor set FLAVOR-NAME \
    --property hw:hide_hypervisor_id=VALUE


  • VALUE: (string) ‘true’ or ‘false’. ‘false’ is equivalent to the property not existing.

Secure Boot

Secure Boot can help ensure the bootloader used for your instances is trusted, preventing a possible attack vector.

$ openstack flavor set FLAVOR-NAME \
    --property os:secure_boot=SECURE_BOOT_OPTION

Valid SECURE_BOOT_OPTION values are:

  • required: Enable Secure Boot for instances running with this flavor.

  • disabled or optional: (default) Disable Secure Boot for instances running with this flavor.


Supported by the Hyper-V and libvirt drivers.

Changed in version 23.0.0: (Wallaby)

Added support for secure boot to the libvirt driver.

Custom resource classes and standard resource classes to override

Specify custom resource classes to require or override quantity values of standard resource classes.

The syntax of the extra spec is resources:<resource_class_name>=VALUE (VALUE is integer). The name of custom resource classes must start with CUSTOM_. Standard resource classes to override are VCPU, MEMORY_MB or DISK_GB. In this case, you can disable scheduling based on standard resource classes by setting the value to 0.

For example:


  • resources:VCPU=0

See Create flavors for use with the Bare Metal service for more examples.

New in version 16.0.0: (Pike)

Required traits

Required traits allow specifying a server to build on a compute node with the set of traits specified in the flavor. The traits are associated with the resource provider that represents the compute node in the Placement API. See the resource provider traits API reference for more details: https://docs.openstack.org/api-ref/placement/#resource-provider-traits

The syntax of the extra spec is trait:<trait_name>=required, for example:

  • trait:HW_CPU_X86_AVX2=required

  • trait:STORAGE_DISK_SSD=required

The scheduler will pass required traits to the GET /allocation_candidates endpoint in the Placement API to include only resource providers that can satisfy the required traits. In 17.0.0 the only valid value is required. In 18.0.0 forbidden is added (see below). Any other value will be considered invalid.

The FilterScheduler is currently the only scheduler driver that supports this feature.

Traits can be managed using the osc-placement plugin.

New in version 17.0.0: (Queens)

Forbidden traits

Forbidden traits are similar to required traits, described above, but instead of specifying the set of traits that must be satisfied by a compute node, forbidden traits must not be present.

The syntax of the extra spec is trait:<trait_name>=forbidden, for example:

  • trait:HW_CPU_X86_AVX2=forbidden

  • trait:STORAGE_DISK_SSD=forbidden

The FilterScheduler is currently the only scheduler driver that supports this feature.

Traits can be managed using the osc-placement plugin.

New in version 18.0.0: (Rocky)

Numbered groupings of resource classes and traits

Specify numbered groupings of resource classes and traits.

The syntax is as follows (N and VALUE are integers):


A given numbered resources or trait key may be repeated to specify multiple resources/traits in the same grouping, just as with the un-numbered syntax.

Specify inter-group affinity policy via the group_policy key, which may have the following values:

  • isolate: Different numbered request groups will be satisfied by different providers.

  • none: Different numbered request groups may be satisfied by different providers or common providers.


If more than one group is specified then the group_policy is mandatory in the request. However such groups might come from other sources than flavor extra_spec (e.g. from Neutron ports with QoS minimum bandwidth policy). If the flavor does not specify any groups and group_policy but more than one group is coming from other sources then nova will default the group_policy to none to avoid scheduler failure.

For example, to create a server with the following VFs:

  • One SR-IOV virtual function (VF) on NET1 with bandwidth 10000 bytes/sec

  • One SR-IOV virtual function (VF) on NET2 with bandwidth 20000 bytes/sec on a different NIC with SSL acceleration

It is specified in the extra specs as follows:


See Granular Resource Request Syntax for more details.

New in version 18.0.0: (Rocky)