Host Aggregates

Host aggregates can be regarded as a mechanism to further partition an availability zone; while availability zones are visible to users, host aggregates are only visible to administrators. Host aggregates started out as a way to use Xen hypervisor resource pools, but has been generalized to provide a mechanism to allow administrators to assign key-value pairs to groups of machines. Each node can have multiple aggregates, each aggregate can have multiple key-value pairs, and the same key-value pair can be assigned to multiple aggregate. This information can be used in the scheduler to enable advanced scheduling, to set up xen hypervisor resources pools or to define logical groups for migration.

Availability Zones (AZs)

Availability Zones are the end-user visible logical abstraction for partitioning a cloud without knowing the physical infrastructure. That abstraction doesn’t come up in Nova with an actual database model since the availability zone is actually a specific metadata information attached to an aggregate. Adding that specific metadata to an aggregate makes the aggregate visible from an end-user perspective and consequently allows to schedule upon a specific set of hosts (the ones belonging to the aggregate).

That said, there are a few rules to know that diverge from an API perspective between aggregates and availability zones:

  • one host can be in multiple aggregates, but it can only be in one availability zone
  • by default a host is part of a default availability zone even if it doesn’t belong to an aggregate (the configuration option is named default_availability_zone)


That last rule can be very error-prone. Since the user can see the list of availability zones, they have no way to know whether the default availability zone name (currently nova) is provided because an host belongs to an aggregate whose AZ metadata key is set to nova, or because there are at least one host belonging to no aggregate. Consequently, it is highly recommended for users to never ever ask for booting an instance by specifying an explicit AZ named nova and for operators to never set the AZ metadata for an aggregate to nova. That leads to some problems due to the fact that the instance AZ information is explicitly attached to nova which could break further move operations when either the host is moved to another aggregate or when the user would like to migrate the instance.

Xen Pool Host Aggregates

Originally all aggregates were Xen resource pools, now an aggregate can be set up as a resource pool by giving the aggregate the correct key-value pair.

You can use aggregates for XenServer resource pools when you have multiple compute nodes installed (only XenServer/XCP via xenapi driver is currently supported), and you want to leverage the capabilities of the underlying hypervisor resource pools. For example, you want to enable VM live migration (i.e. VM migration within the pool) or enable host maintenance with zero-downtime for guest instances. Please, note that VM migration across pools (i.e. storage migration) is not yet supported in XenServer/XCP, but will be added when available. Bear in mind that the two migration techniques are not mutually exclusive and can be used in combination for a higher level of flexibility in your cloud management.


The OSAPI Admin API is extended to support the following operations:

  • Aggregates
    • list aggregates: returns a list of all the host-aggregates (optionally filtered by availability zone)
    • create aggregate: creates an aggregate, takes a friendly name, etc. returns an id
    • show aggregate: shows the details of an aggregate (id, name, availability_zone, hosts and metadata)
    • update aggregate: updates the name and availability zone of an aggregate
    • set metadata: sets the metadata on an aggregate to the values supplied
    • delete aggregate: deletes an aggregate, it fails if the aggregate is not empty
    • add host: adds a host to the aggregate
    • remove host: removes a host from the aggregate
  • Hosts
    • start host maintenance (or evacuate-host): disallow a host to serve API requests and migrate instances to other hosts of the aggregate
    • stop host maintenance: (or rebalance-host): put the host back into operational mode, migrating instances back onto that host

Using the Nova CLI

Using the nova command you can create, delete and manage aggregates. The following section outlines the list of available commands.


* aggregate-list                                                    Print a list of all aggregates.
* aggregate-create         <name> <availability_zone>               Create a new aggregate with the specified details.
* aggregate-delete         <id>                                     Delete the aggregate by its id.
* aggregate-details        <id>                                     Show details of the specified aggregate.
* aggregate-add-host       <id> <host>                              Add the host to the specified aggregate.
* aggregate-remove-host    <id> <host>                              Remove the specified host from the specified aggregate.
* aggregate-set-metadata   <id> <key=value> [<key=value> ...]       Update the metadata associated with the aggregate.
* aggregate-update         <id> <name> [<availability_zone>]        Update the aggregate's name and optionally availability zone.

* host-list                                                         List all hosts by service
* host-update              --maintenance [enable | disable]         Put/resume host into/from maintenance.