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Exploring OpenStack Placement

Placement API is a separate REST API stack and data model used to track resource provider inventories and usages, along with different classes of resources.

Concepts

Resource Providers

Resource Classes

The types of resources consumed are tracked as classes.

  • VCPU
  • MEMORY_MB
  • DISK_GB
  • …

The nova-compute service code will begin reporting resource provider inventory and usage information as soon as the placement API service is in place and can respond to requests via the endpoint registered in the service catalog.

Workflows

The scheduler gets the allocation candidates from the placement API and uses those to get the compute nodes, which come from the cell(s). The compute nodes are passed through the enabled scheduler filters and weighers. The scheduler then iterates over this filtered and weighed list of hosts and attempts to claim resources in the placement API for each instance in the request. Claiming resources involves finding an allocation candidate that contains an allocation against the selected host’s UUID and asking the placement API to allocate the requested instance resources. We continue performing this claim request until success or we run out of allocation candidates, resulting in a NoValidHost error.

Workflow of Nova Boot Instance

Caveats

The nova.scheduler.filter_scheduler.FilterScheduler in Ocata will fallback to not using the Placement service as long as there are older nova-compute services running in the deployment. This allows for rolling upgrades of the computes to not affect scheduling for the FilterScheduler. However, the fallback mechanism will be removed in the 16.0.0 Pike release such that the scheduler will make decisions based on the Placement service and the resource providers (compute nodes) registered there, This means if the computes are not reporting into Placement by Pike, build requests will fail with NoValidHost errors.

Command Examples

$ openstack --os-placement-api-version=1.17 \
    resource provider list
+--------------------------------------+-----------------+------------+--------------------------------------+----------------------+
| uuid                                 | name            | generation | root_provider_uuid                   | parent_provider_uuid |
+--------------------------------------+-----------------+------------+--------------------------------------+----------------------+
| a294054e-cfc3-4393-af2e-b5b1ad6022e4 | nova-compute-vm |          2 | a294054e-cfc3-4393-af2e-b5b1ad6022e4 | None                 |
+--------------------------------------+-----------------+------------+--------------------------------------+----------------------+
$ openstack --os-placement-api-version=1.17 resource class list
+-------------------+
| name              |
+-------------------+
| VCPU              |
| MEMORY_MB         |
| DISK_GB           |
| PCI_DEVICE        |
| SRIOV_NET_VF      |
| NUMA_SOCKET       |
| NUMA_CORE         |
| NUMA_THREAD       |
| NUMA_MEMORY_MB    |
| IPV4_ADDRESS      |
| VGPU              |
| VGPU_DISPLAY_HEAD |
+-------------------+
$ openstack --os-placement-api-version=1.17 \
    resource provider inventory list a294054e-cfc3-4393-af2e-b5b1ad6022e4
+----------------+------------------+----------+----------+-----------+----------+--------+
| resource_class | allocation_ratio | max_unit | reserved | step_size | min_unit |  total |
+----------------+------------------+----------+----------+-----------+----------+--------+
| VCPU           |              2.0 |     1000 |        0 |         1 |        1 |   1000 |
| MEMORY_MB      |              1.0 |   800000 |      512 |         1 |        1 | 800000 |
| DISK_GB        |              1.0 |   500000 |        0 |         1 |        1 | 500000 |
+----------------+------------------+----------+----------+-----------+----------+--------+
$ openstack --os-placement-api-version=1.17 \
    resource provider inventory show a294054e-cfc3-4393-af2e-b5b1ad6022e4 VCPU
+------------------+-------+
| Field            | Value |
+------------------+-------+
| allocation_ratio | 2.0   |
| max_unit         | 1000  |
| reserved         | 0     |
| step_size        | 1     |
| min_unit         | 1     |
| total            | 1000  |
+------------------+-------+
$ openstack --os-placement-api-version=1.17 \
    resource provider usage show a294054e-cfc3-4393-af2e-b5b1ad6022e4
+----------------+-------+
| resource_class | usage |
+----------------+-------+
| VCPU           |     0 |
| MEMORY_MB      |     0 |
| DISK_GB        |     0 |
+----------------+-------+
$ openstack --os-placement-api-version=1.17 \
    allocation candidate list --resource VCPU=1
+---+------------+--------------------------------------+-------------------------+--------+
| # | allocation | resource provider                    | inventory used/capacity | traits |
+---+------------+--------------------------------------+-------------------------+--------+
| 1 | VCPU=1     | a294054e-cfc3-4393-af2e-b5b1ad6022e4 | VCPU=0/2000             |        |
+---+------------+--------------------------------------+-------------------------+--------+

References