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4.5. Scheduling hosts

The Compute scheduling service determines on which host (or host aggregate), an instance will be placed. As an administrator, you can influence where the scheduler will place an instance. For example, you might want to limit scheduling to hosts in a certain group or with the right RAM.

You can configure the following components:

  • Filters - Determine the initial set of hosts on which an instance might be placed (see 「Configuring scheduling filters」).
  • Weights - When filtering is complete, the resulting set of hosts are prioritized using the weighting system. The highest weight has the highest priority (see 「Configuring scheduling weights」).
  • Scheduler service - There are a number of configuration options in the /var/lib/config-data/puppet-generated/<nova_container>/etc/nova/nova.conf file (on the scheduler host), which determine how the scheduler executes its tasks, and handles weights and filters.

In the following diagram, both host 1 and 3 are eligible after filtering. Host 1 has the highest weight and therefore has the highest priority for scheduling.

Scheduling Hosts

4.5.1. Configuring scheduling filters

You define the filters you want the scheduler to use by adding or removing filters from the NovaSchedulerDefaultFilters parameter in your Compute environment file.

The default configuration runs the following filters in the scheduler:

  • RetryFilter
  • AvailabilityZoneFilter
  • ComputeFilter
  • ComputeCapabilitiesFilter
  • ImagePropertiesFilter
  • ServerGroupAntiAffinityFilter
  • ServerGroupAffinityFilter

Some filters use information in parameters passed to the instance in:

The following table lists all the available filters.

表4.7 Scheduling Filters



Only passes hosts in host aggregates whose metadata matches the instance’s image metadata; only valid if a host aggregate is specified for the instance. For more information, see 「Creating an image」.


Metadata in the host aggregate must match the host’s flavor metadata. For more information, see 「Updating flavor metadata」.


This filter can only be specified in the same NovaSchedulerDefaultFilters parameter as ComputeCapabilitiesFilter when you scope your flavor extra_specs keys by prefixing them with the correct namespace:

  • ComputeCapabilitiesFilter namespace = "capabilities:"
  • AggregateInstanceExtraSpecsFilter namespace = "aggregate_instance_extra_specs:"


A host with the specified filter_tenant_id can only contain instances from that tenant (project).


The tenant can still place instances on other hosts.


Passes all available hosts (however, does not disable other filters).


Filters using the instance’s specified availability zone.


Ensures Compute metadata is read correctly. Anything before the : is read as a namespace. For example, quota:cpu_period uses quota as the namespace and cpu_period as the key.


Passes only hosts that are operational and enabled.


Enables an instance to build on a host that is different from one or more specified hosts. Specify different hosts using the nova boot option --different_host option.


Only passes hosts that match the instance’s image properties. For more information, see 「Creating an image」.


Passes only isolated hosts running isolated images that are specified using isolated_hosts and isolated_images (comma-separated values).


Recognises and uses an instance’s custom JSON filters:

  • Valid operators are: =, <, >, in, ⇐, >=, not, or, and
  • Recognised variables are: $free_ram_mb, $free_disk_mb, $total_usable_ram_mb, $vcpus_total, $vcpus_used

The filter is specified as a query hint in the nova boot command. For example:

--hint query='['>=', '$free_disk_mb', 200 * 1024]'


Use this filter to limit scheduling to Compute nodes that report the metrics configured by using metrics/weight_setting.

To use this filter, add the following configuration to your Compute environment file:

        value: 'cpu.virt_driver'

By default, the Compute scheduling service updates the metrics every 60 seconds. To ensure the metrics are up-to-date, you can increase the frequency at which the metrics data is refreshed using the update_resources_interval configuration option. For example, use the following configuration to refresh the metrics data every 2 seconds:

        value: '2'


Filters out hosts based on its NUMA topology. If the instance has no topology defined, any host can be used. The filter tries to match the exact NUMA topology of the instance to those of the host (it does not attempt to pack the instance onto the host). The filter also looks at the standard over-subscription limits for each NUMA node, and provides limits to the compute host accordingly.


Filters out hosts that have failed a scheduling attempt; valid if scheduler_max_attempts is greater than zero (defaults to "3").


Passes one or more specified hosts; specify hosts for the instance using the --hint same_host option for nova boot.


Only passes hosts for a specific server group:

  • Give the server group the affinity policy (nova server-group-create --policy affinity groupName).
  • Build the instance with that group (nova boot option --hint group=UUID)


Only passes hosts in a server group that do not already host an instance:

  • Give the server group the anti-affinity policy (nova server-group-create --policy anti-affinity groupName).
  • Build the instance with that group (nova boot option --hint group=UUID).


Only passes hosts on the specified IP subnet range specified by the instance’s cidr and build_new_host_ip hints. Example:

--hint build_near_host_ip= --hint cidr=/24