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4.4. Managing host aggregates

A single Compute deployment can be partitioned into logical groups for performance or administrative purposes. OpenStack uses the following terms:

  • Host aggregates - A host aggregate creates logical units in a OpenStack deployment by grouping together hosts. Aggregates are assigned Compute hosts and associated metadata; a host can be in more than one host aggregate. Only administrators can see or create host aggregates.

    An aggregate’s metadata is commonly used to provide information for use with the Compute scheduler (for example, limiting specific flavors or images to a subset of hosts). Metadata specified in a host aggregate will limit the use of that host to any instance that has the same metadata specified in its flavor.

    Administrators can use host aggregates to handle load balancing, enforce physical isolation (or redundancy), group servers with common attributes, or separate out classes of hardware. When you create an aggregate, a zone name must be specified, and it is this name which is presented to the end user.

  • Availability zones - An availability zone is the end-user view of a host aggregate. An end user cannot view which hosts make up the zone, nor see the zone’s metadata; the user can only see the zone’s name.

    End users can be directed to use specific zones which have been configured with certain capabilities or within certain areas.

4.4.1. Enabling host aggregate scheduling

By default, host-aggregate metadata is not used to filter instance usage. You must update the Compute scheduler’s configuration to enable metadata usage:

  1. Open your Compute environment file.
  2. Add the following values to the NovaSchedulerDefaultFilters parameter, if they are not already present:

    • AggregateInstanceExtraSpecsFilter for host aggregate metadata.


      Scoped specifications must be used for setting flavor extra_specs when specifying both AggregateInstanceExtraSpecsFilter and ComputeCapabilitiesFilter filters as values of the same NovaSchedulerDefaultFilters parameter, otherwise the ComputeCapabilitiesFilter will fail to select a suitable host. See 表4.7「Scheduling Filters」 for further details.

    • AvailabilityZoneFilter for availability zone host specification when launching an instance.
  3. Save the configuration file.
  4. Deploy the overcloud.