Support preemptible instances with Blazar

This spec proposes to add support for preemptible instances with Blazar. It would allow to increase utilization by filling gaps between reservations, as well as provide an implementation of preemptible instances that does not require modifications to the Nova compute service.

Problem description

To ensure resources allocated for reservations are not in use when reservations reach their start time, Blazar relies on exclusive management of dedicated pools of resources. In the case of compute resources, compute hosts managed by Blazar are initially moved to a host aggregate named freepool. Regular instances which are not launched inside a reservation are not allowed to run on nodes in the freepool. This can lead to low utilization if reservations are not making full use of the resources managed by Blazar. This spec proposes to add support for preemptible instances, i.e. instances that can be terminated at any time when required by the infrastructure, to increase utilization by filling gaps between reservations.

This feature would also provide a minimally intrusive implementation of preemptible instances for OpenStack: an alternative implementation based on adding a pending state to Nova has proved difficult to be accepted.

Use Cases

This feature primarily targets deployers who operate Blazar and want to increase the utilization of their infrastructure by filling gaps between user reservations. It would also benefit end users who can run preemptible workloads by providing them with extra resources.

Proposed change

Due to Blazar’s design, preemptible instances can be supported with few modifications to the code. The following mechanisms are required:

  1. A way to identify different types of instance creation requests and of running instances: regular (non-reserved) instances, reserved instances, and preemptible instances.

  2. A modification of the custom Nova scheduler filter, BlazarFilter, to allow instances considered as preemptibles to be launched on hosts located in the freepool and only in the freepool: we do not want preemptibles running on hosts not managed by Blazar or on hosts part of a reservation.

  3. Before a lease becomes active, the Blazar compute plugins implementing instance reservation and host reservation would terminate any preemptible instance running on allocated hosts, identifying them using the method previously mentioned.

These mechanisms are described in more details below.

Identification of preemptible instance creation requests

To identify an instance creation request as one for a preemptible instance, some possible methods are:

  1. Operators would configure via Blazar static configuration or API call a list of project IDs describes projects for which any instance creation request is treated as one for preemptibles.

    • Pros: can restrict preemptibles to specific projects

    • Cons: cannot allow preemptibles for all projects, unless a special globing pattern is used; may also need a method for projects to requests non-preemptible instances; may be difficult to identify running preemptibles.

  2. Via scheduler hints: an instance creation request is identified as being for a preemptible instance if the scheduler hint preemptible=true is passed.

    • Pros: can be used by any project without configuration by operator

    • Cons: can be used by any project without configuration by operator; may be difficult to identify running preemptibles.

  3. Via flavor properties: a flavor property is used to identify preemptible instances. For example, an existing flavor could be configured to launch preemptible instances by using: openstack flavor set FLAVOR-NAME --property blazar:preemptible=true

    • Pros: flavor can be made public or shared only with specific projects; easy to identify running preemptibles.

    • Cons: duplicates the number of flavors if all flavors can potentially be preemptible

  4. Via image properties: an image property is used to identify preemptible instances. For example, an existing image could be configured to launch preemptible instances by using: openstack image set --property blazar:preemptible=true IMAGE_UUID

    • Pros: easy to identify running preemptibles

    • Cons: any project could use preemptibles, unless policy prevents setting image properties; would need to cover volumes as well; could lead to image duplication.

Identification of running preemptible instances

For compute hosts allocated to host reservations, running preemptible instances are simply instances that are executing on allocated hosts outside of any reservation.

For compute hosts allocated to instance reservation, instances launched as part of a reservation are identified by the flavor they use, which is encoded as flavor_id in the reservation information. Blazar flavors also follow a specific naming scheme: reservation:<reservation_id>. Instances running on allocated hosts that don’t use such flavors would be identified as preemptibles.

Termination of preemptible instances

Preemtible instances need to be terminated before a lease becomes active. The Blazar compute plugins implementing instance reservation and host reservation would terminate any preemptible instance running on allocated hosts, identifying them using the previously mentioned method.

To allow preemptible instances to terminate their workload cleanly, a soft shutdown operation could first be triggered, followed by a hard shutdown (instance deletion). In order to keep the lease start operation quick, we may want to implement a before_start_lease event, similar to the before_end_lease event, which would terminate any preemptible instance prior to triggering the start_lease event.

However, we would also want to prevent any new preemptible from launching after having sent instance termination requests. If using the before_start_lease approach, this could be difficult to implement from BlazarFilter which does not have access to the Blazar database, and thus cannot efficiently check the status of leases and events. API calls to Blazar may be too slow to process large number of hosts going through the filter.

Interaction with quotas

Preemptible instances would count as regular instances for Nova quotas of number of instances, vCPUs, memory, and disk. This means that projects having reached their quota by running preemptible instances would not be able to launch any new regular instance: they would first need to terminate preemptible instances to reduce their usage below the maximum allowed by their quota.

This could be prevented by the operator by configuring dedicated projects to exclusively run preemptible instances, i.e. not mix preemptible and regular instances within the same project.


A native implementation within Nova would be more flexible, as it would allow preemptible instances to run on reserved hosts until they become actively used by regular instances. However, a proposed implementation based on introducing a pending state has proved difficult to be accepted by the community.

Data model impact


REST API impact


Security impact

Race conditions between the Nova scheduler and the Blazar manager would have to be handled correctly to prevent preemptible instances from launching while a reservation is started. Failure to terminate all preemptible instances would result in reservation owners not being able to fully use their reservation.

Notifications impact


Other end user impact

Nova end users would use one of the mechanisms described earlier to request preemptible instances.

Performance Impact

Terminating preemptible instances during on_start() would make leases longer to start, particularly if a soft shutdown signal is sent to instances. A solution based on introducing a new event is described earlier in this spec, but it has shortcomings.

Other deployer impact

The following configuration options would be added to blazar.conf:

  • [preemptible]/graceful_shutdown_time: time to wait between a soft shutdown request and a terminate request

Developer impact


Upgrade impact




Primary assignee:


Work Items

  1. Proof of concept with host reservation plugin only

  2. Support for instance reservation

  3. General improvements (e.g. soft shutdown signal)

  4. Documentation and Tempest scenario testing




A Tempest scenario verifying that preemptible instances can be launched and terminated when a reservation starts must be implemented. This requires a full OpenStack environment. The current DevStack can be used.

Documentation Impact

Operators and users launching preemptible instances are most affected by this change. The admin and user guides should be updated.




Release Name