Host Aggregates

Host Aggregates are a way of grouping hosts in an OpenStack cloud. This allows you to create groups of certain types of hosts and then steer certain classes of VM instances to them.

Host Aggregates¹ 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.

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 aggregates. This information can be used in the scheduler to enable advanced scheduling or to define logical groups for migration. In general, Host Aggregates can be thought of as a way to segregate compute resources behind the scenes to control and influence where VM instances will be placed.

Host Aggregates in Nova

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 setting matching flavor extra specifications. The scheduler then endeavors to match user requests for instances of the given flavor to a host aggregate with the same key-value pair in its metadata. Hosts can belong to multiple Host Aggregates, depending on the attributes being used to define the aggregate.

A 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 NVMe 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. Examples include:

• Hosts with GPU compute resources
• Hosts with different local storage capabilities (e.g. SSD vs NVMe)
• Different CPU manufacturers (e.g. AMD vs Intel)
• Different CPU microarchitectures (e.g. Skylake vs Raptor Cove)

Host Aggregates vs. Availability Zones

While Host Aggregates themselves are hidden from OpenStack cloud users, Cloud administrators are able to optionally expose a host aggregate as an Availability Zone. Availability zones differ from host aggregates in that they are explicitly exposed to the user, and hosts membership is exclusive -- hosts can only be in a single availability zone.

Host Aggregates in Genestack

Aggregates and Placement

The Placement service also has a concept of Aggregates. However, these are not the same thing as Host Aggregates in Nova. Placement Aggregates are defined purely as groupings of related resource providers. As compute nodes in Nova are represented in Placement as resource providers, they can be added to a Placement Aggregate as well.

Host Aggregates and Glance

The primary way that Glance can influence placement and work with Host Aggregates is via Metadata.

You can map flavors and images to Host Aggregates by setting metadata on the Host Aggregate, and then set Glance image metadata properties to correlate to the host aggregate metadata. Placement can then use this metadata to schedule instances when the required filters are enabled.