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Host aggregates are a mechanism for partitioning hosts in an OpenStack cloud, or a region of an OpenStack cloud, based on arbitrary characteristics. Examples where an administrator may want to do this include where a group of hosts have additional hardware or performance characteristics. Host aggregates are not explicitly exposed to users. Instead administrators map flavors to host aggregates. Administrators do this by setting metadata on a host aggregate, and matching flavor extra specifications. The scheduler then endeavors to match user requests for instance of the given flavor to a host aggregate with the same key-value pair in its metadata. Compute nodes can be in more than one host aggregate. Administrators are able to optionally expose a host aggregate as an availability zone. Availability zones are different from host aggregates in that they are explicitly exposed to the user, and hosts can only be in a single availability zone. Administrators can configure a default availability zone where instances will be scheduled when the user fails to specify one. The nova command-line tool supports the following aggregate-related commands. nova aggregate-list Print a list of all aggregates. nova aggregate-create <name> [availability-zone] Create a new aggregate named <name>, and optionally in availability zone [availability-zone] if specified. The command returns the ID of the newly created aggregate. Hosts can be made available to multiple host aggregates. Be careful when adding a host to an additional host aggregate when the host is also in an availability zone. Pay attention when using the aggregate-set-metadata and aggregate-update commands to avoid user confusion when they boot instances in different availability zones. An error occurs if you cannot add a particular host to an aggregate zone for which it is not intended. nova aggregate-delete <id> Delete an aggregate with id <id>. nova aggregate-details <id> Show details of the aggregate with id <id>. nova aggregate-add-host <id> <host> Add host with name <host> to aggregate with id <id>. nova aggregate-remove-host <id> <host> Remove the host with name <host> from the aggregate with id <id>. nova aggregate-set-metadata <id> <key=value> [<key=value> ...] Add or update metadata (key-value pairs) associated with the aggregate with id <id>. nova aggregate-update <id> <name> [<availability_zone>] Update the name and availability zone (optional) for the aggregate. nova host-list List all hosts by service. nova host-update --maintenance [enable | disable] Put/resume host into/from maintenance. Only administrators can access these commands. If you try to use these commands and the user name and tenant that you use to access the Compute service do not have the admin role or the appropriate privileges, these errors occur: ERROR: Policy doesn't allow compute_extension:aggregates to be performed. (HTTP 403) (Request-ID: req-299fbff6-6729-4cef-93b2-e7e1f96b4864) ERROR: Policy doesn't allow compute_extension:hosts to be performed. (HTTP 403) (Request-ID: req-ef2400f6-6776-4ea3-b6f1-7704085c27d1) One common use case for host aggregates is when you want to support scheduling instances to a subset of compute hosts because they have a specific capability. For example, you may want to allow users to request compute hosts that have SSD drives if they need access to faster disk I/O, or access to compute hosts that have GPU cards to take advantage of GPU-accelerated code. To configure the scheduler to support host aggregates, the scheduler_default_filters configuration option must contain the AggregateInstanceExtraSpecsFilter in addition to the other filters used by the scheduler. Add the following line to /etc/nova/nova.conf on the host that runs the nova-scheduler service to enable host aggregates filtering, as well as the other filters that are typically enabled:scheduler_default_filters=AggregateInstanceExtraSpecsFilter,RetryFilter,AvailabilityZoneFilter,RamFilter,ComputeFilter,ComputeCapabilitiesFilter,ImagePropertiesFilter,ServerGroupAntiAffinityFilter,ServerGroupAffinityFilter This example configures the Compute service to enable users to request nodes that have solid-state drives (SSDs). You create a fast-io host aggregate in the nova availability zone and you add the ssd=true key-value pair to the aggregate. Then, you add the node1, and node2 compute nodes to it. $ nova aggregate-create fast-io nova +----+---------+-------------------+-------+----------+ | Id | Name | Availability Zone | Hosts | Metadata | +----+---------+-------------------+-------+----------+ | 1 | fast-io | nova | | | +----+---------+-------------------+-------+----------+ $ nova aggregate-set-metadata 1 ssd=true +----+---------+-------------------+-------+-------------------+ | Id | Name | Availability Zone | Hosts | Metadata | +----+---------+-------------------+-------+-------------------+ | 1 | fast-io | nova | [] | {u'ssd': u'true'} | +----+---------+-------------------+-------+-------------------+ $ nova aggregate-add-host 1 node1 +----+---------+-------------------+-----------+-------------------+ | Id | Name | Availability Zone | Hosts | Metadata | +----+---------+-------------------+------------+-------------------+ | 1 | fast-io | nova | [u'node1'] | {u'ssd': u'true'} | +----+---------+-------------------+------------+-------------------+ $ nova aggregate-add-host 1 node2 +----+---------+-------------------+---------------------+-------------------+ | Id | Name | Availability Zone | Hosts | Metadata | +----+---------+-------------------+----------------------+-------------------+ | 1 | fast-io | nova | [u'node1', u'node2'] | {u'ssd': u'true'} | +----+---------+-------------------+----------------------+-------------------+ Use the nova flavor-create command to create the ssd.large flavor called with an ID of 6, 8 GB of RAM, 80 GB root disk, and four vCPUs. $ nova flavor-create ssd.large 6 8192 80 4 +----+-----------+-----------+------+-----------+------+-------+-------------+-----------+ | ID | Name | Memory_MB | Disk | Ephemeral | Swap | VCPUs | RXTX_Factor | Is_Public | +----+-----------+-----------+------+-----------+------+-------+-------------+-----------+ | 6 | ssd.large | 8192 | 80 | 0 | | 4 | 1.0 | True | +----+-----------+-----------+------+-----------+------+-------+-------------+-----------+ Once the flavor is created, specify one or more key-value pairs that match the key-value pairs on the host aggregates with scope aggregate_instance_extra_specs. In this case, that is the aggregate_instance_extra_specs:ssd=true key-value pair. Setting a key-value pair on a flavor is done using the nova flavor-key command. $ nova flavor-key ssd.large set aggregate_instance_extra_specs:ssd=true Once it is set, you should see the extra_specs property of the ssd.large flavor populated with a key of ssd and a corresponding value of true. $ nova flavor-show ssd.large +----------------------------+--------------------------------------------------+ | Property | Value | +----------------------------+--------------------------------------------------+ | OS-FLV-DISABLED:disabled | False | | OS-FLV-EXT-DATA:ephemeral | 0 | | disk | 80 | | extra_specs | {u'aggregate_instance_extra_specs:ssd': u'true'} | | id | 6 | | name | ssd.large | | os-flavor-access:is_public | True | | ram | 8192 | | rxtx_factor | 1.0 | | swap | | | vcpus | 4 | +----------------------------+--------------------------------------------------+ Now, when a user requests an instance with the ssd.large flavor, the scheduler only considers hosts with the ssd=true key-value pair. In this example, these are node1 and node2. When using the XenAPI-based hypervisor, the Compute service uses host aggregates to manage XenServer Resource pools, which are used in supporting live migration.