Nova

Services

• nova-api
• nova-scheduler
• nova-compute
• nova-conductor
• Hypervisor

Where the services run

Generally, the api, scheduler, conductor are running on Control Node. compute, Hypervisor are running on Compute Node.

General flow

1. User (End user or other services) sends requests to nova-api to create a VM instance.
2. nova-api sends message to Message Queue to ask nova-scheduler to schedule a Compute Node to create the VM.
3. nova-scheduler gets the message and pick a Compute Node via a filter algorithm, then sends message to let that Compute Node (A) to create VM.
4. Compute Node (A) gets the message, uses the Hypervisor to boot the VM.
5. Any update to the DB will be executed by messaging the nova-conductor.

Config hypervisor driver

In /etc/nova/nova.conf, set compute_driver to libvirt.libvirtDriver.

Scheduler

User specifies the Flavor to create the VM. nova-scheduler will choose the Compute Node.

Flavor defines VCPU, RAM, Disk and Metadata.

In /etc/nova/nova.conf, scheduler_driver, scheduler_available_filters and scheduler_default_filter settings affect the behavior of scheduler.

Filter scheduler

The default scheduler. Two steps:

1. Filter to select the Compute Node meeting the requirements.
2. Weight the Compute Node of step 1.

scheduler_driver = nova.scheduler.filter_scheduler.FilterScheduler

# Available filters
scheduler_available_filters = nova.scheduler.filters.all_filters

# Filte in below order
scheduler_default_filters = RetryFilter, AvailabilityZoneFilter,
                            RamFilter, DiskFilter, ComputeFilter,
                            ComputeCapabilitiesFilter, ImagePropertiesFilter,
                            ServerGroupAntiAffinityFilter, ServerGroupAffinityFilter