Remove resource information from os-hypervisors API

The os-hypervisors API is around since the early days of nova. Back in those halcyon days, it provided a nice way to get a quick summary of the resource usage of individual compute nodes in your deployment. Today, however, it’s a shell of itself. The more detailed information it returns is hypervisor-specific and frequently wrong, especially with advanced features like CPU pinning or file-based memory. With the elevation of placement to its rightful place as lord of (almost) all things resource’y, the os-hypervisor API needs to slim down significantly.

Problem description

The majority of this spec is focused on changes to the /os-hypervisors/detail API. Consider the output of a typical GET /os-hypervisors/detail call, as taken from the API ref 1:

    "hypervisors": [
            "cpu_info": {
                "arch": "x86_64",
                "model": "Nehalem",
                "vendor": "Intel",
                "features": [
                "topology": {
                    "cores": 1,
                    "threads": 1,
                    "sockets": 4
            "current_workload": 0,
            "status": "enabled",
            "state": "up",
            "disk_available_least": 0,
            "host_ip": "",
            "free_disk_gb": 1028,
            "free_ram_mb": 7680,
            "hypervisor_hostname": "host1",
            "hypervisor_type": "fake",
            "hypervisor_version": 1000,
            "id": 2,
            "local_gb": 1028,
            "local_gb_used": 0,
            "memory_mb": 8192,
            "memory_mb_used": 512,
            "running_vms": 0,
            "service": {
                "host": "host1",
                "id": 6,
                "disabled_reason": null
            "vcpus": 2,
            "vcpus_used": 0
    "hypervisors_links": [
            "href": "",
            "rel": "next"

The fields here broadly fall into three categories: useful but duplicated in the summary (non-detailed) view, useful and unique to the detailed view, and not useful. First, the useful but duplicated fields. These should remain:

  • id

  • status

  • state

  • hypervisor_hostname

Next, the useful fields unique to the detailed view. These should also remain:

  • host_ip

  • hypervisor_type

  • hypervisor_version

  • service

Finally, the useless fields. There are varied reasons their uselessness, described below, but all should be removed:

  • current_workload

    This tracks “the number of tasks the hypervisor is responsible for” and it “will be equal or greater than the number of active VMs on the system (it can be greater when VMs are being deleted and the hypervisor is still cleaning up)” 2. This information is easily calculated by listing active and deleted instances.

  • cpu_info

    Useful at face value but the only thing relevant for scheduling purposes are the CPU architecture and CPU features, all of which are already handled by placement trait requests. The topology field is an oddity that should likely never have been added. It’s not usable in scheduling and is possibly wrong, given it doesn’t reflect offline CPUs or those not available to nova due to configuration, and it doesn’t handle non-uniform CPU topologies where there are e.g. more cores on one socket than another. If the operator wants this information, they can simply inspect the host like they would have to do to identify e.g. the specifics of PCI devices or storage devices.

  • free_disk_gb, local_gb, local_gb_used

    💩 Almost always wrong if shared storage is in use and doesn’t take overcommit into account. Use placement.

  • disk_available_least

    Reflects the estimated available disk space on the hypervisor if all instances on the host were to use all their allocated disk. This can go negative if disk overcommit is enabled or if an instance is force migrated to a host, bypassing the scheduler. This value is hard to use and frequently misunderstood by end-users.

  • free_ram_mb, memory_mb, memory_mb_used

    Doesn’t take overcommit or non-default pagesizes into account. Use placement.

  • vcpus, vcpus_used

    Doesn’t take overcommit or PCPU inventory into account. Use placement.

  • running_vms

    Easily figured out by filtering running instances by host (admin-only, like this API).

In addition to the changes to the /os-hypervisors/detail API, there are also two other APIs that appear to have outlived their usefulness: /os-hypervisors/statistics, which provides summary information of the above, and /os-hypervisors/{hypervisor_id}/uptime, which provides an entire API to a single figure. The former can be removed entirely in favour of placement, while the latter can be replaced by a new uptime field on the /os-hypervisors/{hypervisor_id} API.



Use Cases

As a user, I don’t want to see misleading information reported from my API.

Proposed change

Remove the resource-related fields from the output of the /os-hypervisors/detail API and remove the /os-hypervisors/statistics API in its entirety.


We could document the incorrect nature of these APIs. This is less desirable since people don’t read documentation.

Data model impact


REST API impact

Starting from the new API microversion, the /os-hypervisors/detail API will no longer include the following fields in its response: cpu_info, free_disk_gb, local_gb, local_gb_used, disk_available_least, free_ram_mb, memory_mb, memory_mb_used, vcpus, vcpus_used, and running_vms.

The /os-hypervisors/statistics API contains summary information of the above fields and will be removed entirely, returning a HTTP 404 (Not Found) on the new API microversion.

The uptime information shown by the /os-hypervisors/{hypervisor_id}/uptime API doesn’t warrant its own API and will also return a HTTP 404 (Not Found) on the new API microversion. This information will be accessible via a new uptime field on responses from the /os-hypervisors/{hypervisor_id} API.

Security impact


Notifications impact


Other end user impact

The clients will need to be updated. Documentation referencing these APIs will need to be updated with recommendations to look at placement or other APIs instead. Regarding the changes to the /os-hypervisors/detail API:

  • The free_disk_gb, local_gb, local_gb_used, free_ram_mb, memory_mb, memory_mb_used, vcpus and vcpus_used values can be identified using a combination of openstack resource provider inventory list and openstack resource provider usage show. For example:

    $ openstack hypervisor show devstack-1 \
        -c local_gb -c local_gb_used -c free_disk_gb \
        -c memory_mb -c memory_mb_used -c free_ram_mb \
        -c vcpus -c vcpus_used
    | Field          | Value |
    | local_gb       | 18    |
    | local_gb_used  | 1     |
    | free_disk_gb   | 19    |
    | memory_mb      | 16035 |
    | memory_mb_used | 1024  |
    | free_ram_mb    | 15011 |
    | vcpus          | 12    |
    | vcpus_used     | 1     |
    $ openstack resource provider inventory list bde27f9d-1249-446f-ae14-45f6ff3e63d5
    | resource_class | allocation_ratio | min_unit | max_unit | reserved | step_size | total |
    | VCPU           |             16.0 |        1 |       12 |        0 |         1 |    12 |
    | MEMORY_MB      |              1.5 |        1 |    16035 |      512 |         1 | 16035 |
    | DISK_GB        |              1.0 |        1 |       19 |        0 |         1 |    19 |
    $ openstack resource provider usage show bde27f9d-1249-446f-ae14-45f6ff3e63d5
    | resource_class | usage |
    | VCPU           |     1 |
    | MEMORY_MB      |   512 |
    | DISK_GB        |     1 |
  • The running_vms value can be identified using by filter instances by host using openstack server list --host <HOST>. For example:

    $ openstack hypervisor show devstack-1 -c running_vms
    | Field       | Value |
    | running_vms | 1     |
    $ openstack server list --host devstack-1 -c ID -f yaml | wc -l


    This is not a 1:1 replacement since the running_vms setting will track all VMs running on the hypervisor, including those not managed by nova. However, having VMs not managed by nova on a hypervisor is considered a misconfiguration and is irrelevant for scheduling purposes.

  • The cpu_info.arch and cpu_info.features values are published as traits and can be inspected using openstack resource provider trait list. For example:

    $ openstack hypervisor show devstack-1 -f yaml -c cpu_info
    cpu_info: '{"arch": "x86_64", "model": "IvyBridge-IBRS", "vendor": "Intel", "topology":
      {"cells": 2, "sockets": 1, "cores": 3, "threads": 2}, "features": ["xsaveopt", "erms",
      "ssbd", "arch-capabilities", "nx", "cx16", "ht", "mca", "tsc-deadline", "amd-ssbd",
      "pcid", "pse", "ss", "syscall", "md-clear", "tsc_adjust", "mmx", "rdtscp", "f16c",
      "fxsr", "lahf_lm", "spec-ctrl", "smep", "pse36", "vme", "de", "sse", "xsave", "clflush",
      "cmov", "msr", "pat", "aes", "hypervisor", "mtrr", "sep", "fsgsbase", "tsc", "sse2",
      "apic", "pdpe1gb", "cx8", "umip", "vmx", "pae", "skip-l1dfl-vmentry", "popcnt",
      "ssse3", "avx", "pclmuldq", "x2apic", "lm", "stibp", "fpu", "ibpb", "rdrand", "sse4.1",
      "pni", "pge", "sse4.2", "pschange-mc-no", "mce", "arat"]}'
    $ openstack --os-placement-api-version 1.8 \
        resource provider trait list bde27f9d-1249-446f-ae14-45f6ff3e63d5 | grep CPU
    | HW_CPU_X86_AMD_SVM                    |
    | HW_CPU_X86_SSE2                       |
    | HW_CPU_X86_SSE                        |
    | HW_CPU_X86_SVM                        |
    | HW_CPU_HYPERTHREADING                 |
    | HW_CPU_X86_MMX                        |
  • The disk_available_least, cpu_info.model, cpu_info.vendor and cpu_info.topology values are not relevant for scheduling and therefore have no direct replacement in placement or another API. They can, however, be identified through inspection of the host.

Horizon will need to be updated to talk to placement or use this API with an older microversion. Similarly, any users of the /os-hypervisors/{hypervisor_id}/uptime API will need to use the /os-hypervisors/{hypervisor_id} API instead.

Performance Impact


Other deployer impact


Developer impact


Upgrade impact




Primary assignee:


Other contributors:


Feature Liaison


Work Items

  • Update the APIs in a new microversion.

  • Update the documentation to remove references to these deprecated APIs.

  • Update the clients to reflect the deprecations.




Unit and functional tests. Tempest tests will need to be updated to cap against the latest microversion to support these APIs.

Documentation Impact

References to the APIs will need to be removed or updated.





Release Name





Updated to remove references to policy changes, which were deferred.