XenAPI: Support Nova services independently from hypervisor


The ability to run Nova compute services independently from the XenServer host which is running the virtual machines would enable more flexible deployment and development models, and would significantly lower the barrier to entry for XenAPI developers.

This spec makes the necessary changes to enable this independent mode of deployment, where the Nova compute services could be running on an entirely different machine

Problem description

XenAPI’s driver currently assumes that it is running on the hypervisor which will also be housing the VMs. While this is a very valid deployment method, the requirement to run on the same hypervisor is restricted to a few specific methods and the ability to run Nova compute independently would be highly beneficial.

Use Cases

  • Developer wishes to test the XenAPI driver in their existing Linux environment, by running the nova compute services using DevStack but connecting to a XenServer hypervisor running as a virtual machine inside VirtualBox to deploy the tenant VMs.

  • Deployer wishes to consolidate the Nova compute services to enable specialised Nova compute environments with individual VMs running the services independently of the hypervisor (1:1 relationship between nova compute process and hypervisor)

  • Deployer wishes to consolidate the Nova compute services into docker containers running on a physical host, independently of the XenServer hypervisors while maintaining a 1:1 relationship between the nova compute process and the hypervisors they are controlling.

Proposed change

The enforced link between XenAPI and the hypervisor all stem from a single function which assumes that we can get a VM uuid which is running the compute service. In this case, this is vm_utils.get_this_vm_uuid. Some code inspection has been undertaken to identify all callers (including indirect callers) of this function and propose a specific fix for each of the callers.

This spec is not proposing to make all functionality work when running nova independently of the XenServer host, therefore if get_this_vm_uuid detects that we are not able to identify a VM we are running on (so are running independently) it will check for configuration options that cannot be supported and fail at startup if any are present. To simplify the changes, a variable “independent_compute” will be added to XenAPI’s driver and set at startup.

Through code inspection, the following were identified as depending on the get_this_vm_uuid function and therefore need a resolution as part of this spec.

Function name

Proposed resolution


Fix callers:vm_utils.get_this_vm_ref pool._create_slave_info vm_utils.ensure_correct_host


Fix callers:vm_utils.vdi_attached_here vm_utils.cleanup_attached_vdis


Fix callers:vm_utils.auto_configure_disk vm_utils._generate_disk vm_utils.generate_configdrive vm_utils._fetch_disk_image vm_utils.preconfigure_instance vm_utils._copy_partition


Prevent joining an XS host aggregate if independent_compute[1]


Startup config check: check_host must be disabled if independent_compute



Skip cleanup if independent_compute as no VDIs can be attached


Move to new partition_utils XAPI plugin


Fix callers : vm_utils.resize_disk


Fix callers : create_copy_vdi_and_resize


Only used during resize down, which raises in unsupported modes already, so raise exception if resize down attempted when using independent compute[2]


Move to new partition_utils XAPI plugin


Build config drive as raw drive in the compute then use XAPI’s vdi-import API to import as a disk


Fix callers:vm_utils._fetch_image vm_utils._create_kernel_image


Only support downloading of VHDs if independent_compute. Separate kernel / initrd will raise an exception[3]


Only support downloading of VHDs if independent_compute. Separate kernel / initrd will raise an exception[3]



CONF.flat_injected must be disabled if independent_compute[4]

[1] Pool host aggregates currently depend on restarting the VM running the nova compute services during pool join to restart the nova compute services, therefore it is tricky to make the two features work in parallel. The majority of deployments for the XenAPI driver do not use pool host aggregates and therefore supporting the combination of pool host aggregates with independent compute nodes may be covered by a possible future blueprint.

[2] Resize down could have functionality moved to the new ‘partition_utils’ XAPI plugin, however there are efforts to reduce the use of resize_down

[3] Supporting split images isn’t believed necessary, and would either need changes in the glance plugin (currently expecting to download VHDs) or to download to Nova and then upload to XenServer. Either option could be added to a future spec if it were deemed necessary.

[4] This could also be implemented as a XenAPI plugin, but is not believed to be a commonly set option, so will be deferred to a possible future blueprint.


Support for separate kernel / initrd images would be possible with the xenapi-image-streaming blueprint, however as this blueprint is not yet approved, this implementation will not add support for kernel / initrd images in the independent compute case.

Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact

Performance of the code will only have a minor impact, with an additional call through to XenAPI to perform some partition-related tasks.

Code to be moved to partition_utils is IO-bound, rather than CPU-bound. As the IO operations are routed through tapdisk in the same way as when attached to a guest, there is no additional load as a result of it running in dom0.

Other deployer impact

None (Already mentioned): Additional deployment options may be available (see Use Cases above)

Developer impact




Primary assignee: bob-ball <bob.ball@citrix.com>

Work Items

  • ‘partition_utils’ plugin: Add plugin to move disk-focused functionality from DomU to Dom0

  • Import config drive: Build config drive in DomU as a RAW disk then use API to import the raw into a VDI

  • Complete support: Add check for independent compute in get_this_vm_uuid, add startup checks for incompatible options.




The existing Nova Network CI will test the current Compute-as-a-VM deployment model. Citrix are developing a Neutron CI, which will be modified to test this alternative independent compute deployment model.

Documentation Impact






Release Name