Chapter 1. Compute service (nova) functionality

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You use the Compute (nova) service to create, provision, and manage virtual machine instances and bare metal servers in a Red Hat OpenStack Platform (RHOSP) environment. The Compute service abstracts the underlying hardware that it runs on, rather than exposing specifics about the underlying host platforms. For example, rather than exposing the types and topologies of CPUs running on hosts, the Compute service exposes a number of virtual CPUs (vCPUs) and allows for overcommitting of these vCPUs.

The Compute service uses the KVM hypervisor to execute Compute service workloads. The libvirt driver interacts with QEMU to handle all interactions with KVM, and enables the creation of virtual machine instances. To create and provision instances, the Compute service interacts with the following RHOSP services:

  • Identity (keystone) service for authentication.
  • Placement service for resource inventory tracking and selection.
  • Image Service (glance) for disk and instance images.
  • Networking (neutron) service for provisioning the virtual or physical networks that instances connect to on boot.

The Compute service consists of daemon processes and services, named nova-*. The following are the core Compute services:

Compute service (nova-compute)
This service creates, manages and terminates instances by using the libvirt for KVM or QEMU hypervisor APIs, and updates the database with instance states.
Compute conductor (nova-conductor)
This service mediates interactions between the Compute service and the database, which insulates Compute nodes from direct database access. Do not deploy this service on nodes where the nova-compute service runs.
Compute scheduler (nova-scheduler)
This service takes an instance request from the queue and determines on which Compute node to host the instance.
Compute API (nova-api)
This service provides the external REST API to users.
API database
This database tracks instance location information, and provides a temporary location for instances that are built but not scheduled. In multi-cell deployments, this database also contains cell mappings that specify the database connection for each cell.
Cell database
This database contains most of the information about instances. It is used by the API database, the conductor, and the Compute services.
Message queue
This messaging service is used by all services to communicate with each other within the cell and with the global services.
Compute metadata
This service stores data specific to instances. Instances access the metadata service at or over IPv6 at the link-local address fe80::a9fe:a9fe. The Networking (neutron) service is responsible for forwarding requests to the metadata API server. You must use the NeutronMetadataProxySharedSecret parameter to set a secret keyword in the configuration of both the Networking service and the Compute service to allow the services to communicate. The Compute metadata service can be run globally, as part of the Compute API, or in each cell.

You can deploy more than one Compute node. The hypervisor that operates instances runs on each Compute node. Each Compute node requires a minimum of two network interfaces. The Compute node also runs a Networking service agent that connects instances to virtual networks and provides firewalling services to instances through security groups.

By default, director installs the overcloud with a single cell for all Compute nodes. This cell contains all the Compute services and databases that control and manage the virtual machine instances, and all the instances and instance metadata. For larger deployments, you can deploy the overcloud with multiple cells to accommodate a larger number of Compute nodes. You can add cells to your environment when you install a new overcloud or at any time afterwards. For more information, see Scaling Deployments with Compute Cells.

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