Default allocation ratio configuration

https://blueprints.launchpad.net/nova/+spec/initial-allocation-ratios

Provide separate CONF options for specifying the initial allocation ratio for compute nodes. Change the default values for CONF.xxx_allocation_ratio options to None and change the behaviour of the resource tracker to only override allocation ratios for existing compute nodes if the CONF.xxx_allocation_ratio value is not None.

The primary goal of this feature is to support both the API and config way to pass allocation ratios.

Problem description

Manually set placement allocation ratios are overwritten

There is currently no way for an admin to set the allocation ratio on an individual compute node resource provider’s inventory record in the placement API without the resource tracker eventually overwriting that value the next time it runs the update_available_resources periodic task on the nova-compute service.

The saga of the allocation ratio values on the compute host

The process by which nova determines the allocation ratio for CPU, RAM and disk resources on a hypervisor is confusing and error prone. The compute_nodes table in the nova cell DB contains three fields representing the allocation ratio for CPU, RAM and disk resources on that hypervisor. These fields are populated using different default values depending on the version of nova running on the nova-compute service.

Upon starting up, the resource tracker in the nova-compute service worker checks to see if a record exists in the compute_nodes table of the nova cell DB for itself. If it does not find one, the resource tracker creates a record in the table, setting the associated allocation ratio values in the compute_nodes table to the value it finds in the cpu_allocation_ratio, ram_allocation_ratio and disk_allocation_ratio nova.conf configuration options but only if the config option value is not equal to 0.0.

The default values of the cpu_allocation_ratio, ram_allocation_ratio and disk_allocation_ratio CONF options is currently set to 0.0.

The resource tracker saves these default 0.0 values to the compute_nodes table when the resource tracker calls save() on the compute node object. However, there is code in the ComputeNode._from_db_obj that, upon reading the record back from the database on first save, changes the values from 0.0 to 16.0, 1.5 or 1.0.

The ComputeNode object that was save()’d by the resource tracker has these new values for some period of time while the record in the compute_nodes table continues to have the wrong 0.0 values. When the resource tracker runs its update_available_resource() next perioidic task, the new 16.0/1.5/1.0 values are then saved to the compute nodes table.

There is a fix for bug/1789654, which is to not persist zero allocation ratios in ResourceTracker to avoid initializing placement allocation_ratio with 0.0 (due to the allocation ratio of 0.0 being multiplied by the total amount in inventory, leading to 0 resources shown on the system).

Use Cases

An administrator would like to set allocation ratios for individual resources on a compute node via the placement API without that value being overwritten by the compute node’s resource tracker.

An administrator chooses to only use the configuration file to set allocation ratio overrides on their compute nodes and does not want to use the placement API to set these ratios.

Proposed change

First, we propose to change the default option values of existing CONF.cpu_allocation_ratio, CONF.ram_allocation_ratio and CONF.disk_allocation_ratio options relating to allocation ratios to None from the existing default values of 0.0. The reason we change it is that this value will be change from 0.0 to 16.0, 1.5 or 1.0 later, which is weird and confusing.

We will also change the resource tracker to only overwrite the compute node’s allocation ratios to the value of the cpu_allocation_ratio, ram_allocation_ratio and disk_allocation_ratio CONF options if the value of these options is NOT ``None``.

In other words, if any of these CONF options is set to something other than None, then the CONF option should be considered the complete override value for that resource class’ allocation ratio. Even if an admin manually adjusts the allocation ratio of the resource class in the placement API, the next time the update_available_resource() periodic task runs, it will be overwritten to the value of the CONF option.

Second, we propose to add 3 new nova.conf configuration options:

  • initial_cpu_allocation_ratio

  • initial_ram_allocation_ratio

  • initial_disk_allocation_ratio

That will used to determine how to set the initial allocation ratio of VCPU, MEMORY_MB and DISK_GB resource classes when a compute worker first starts up and creates its compute node record in the nova cell DB and corresponding inventory records in the placement service. The value of these new configuration options will only be used if the compute service’s resource tracker is not able to find a record in the placement service for the compute node the resource tracker is managing.

The default value of each of these CONF options shall be 16.0, 1.5, and 1.0 respectively. This is to match the default values for the original allocation ratio CONF options before they were set to 0.0.

These new initial_xxx_allocation_ratio CONF options shall ONLY be used if the resource tracker detects no existing record in the compute_nodes nova cell DB for that hypervisor.

Finally, we will need also add an online data migration and continue to read the xxx_allocation_ratio or initial_xxx_allocation_ratio config on read from the DB if the values are 0.0 or None. If it’s an existing record with 0.0 values, we’d want to do what the compute does, which is use the configure xxx_allocation_ratio config if it’s not None, and fallback to using the initial_xxx_allocation_ratio otherwise.

And add an online data migration that updates all compute_nodes table records that have 0.0 or None allocation ratios. Then we drop that at some point with a blocker migration and remove the code in the nova.objects.ComputeNode._from_db_obj that adjusts allocation ratios.

We propose to add a nova-status upgrade check to iterate the cells looking for compute_nodes records with 0.0 or None allocation ratios and signal that as a warning that you haven’t done the online data migration. We could also check the conf options to see if they are explicitly set to 0.0 and if so, we should fail the status check.

Alternatives

None

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

None

Other deployer impact

None

Developer impact

None

Upgrade impact

We need an online data migrations for any compute_nodes with existing 0.0 and None allocation ratio. If it’s an existing record with 0.0 values, we will replace it with the configure xxx_allocation_ratio config if it’s not None, and fallback to using the initial_xxx_allocation_ratio otherwise.

Note

Migrating 0.0 allocation ratios from existing compute_nodes table records is necessary because the ComputeNode object based on those table records is what gets used in the scheduler 1, specifically the NUMATopologyFilter and CPUWeigher (the CoreFilter, DiskFilter and RamFilter also use them but those filters are deprecated for removal so they are not a concern here).

And clearly in order to take advantage of the ability to manually set allocation ratios on a compute node, that hypervisor would need to be upgraded. No impact to old compute hosts.

Implementation

Assignee(s)

Primary assignee:

yikun

Work Items

  • Change the default values for CONF.xxx_allocation_ratio options to None.

  • Modify resource tracker to only set allocation ratios on the compute node object when the CONF options are non- None

  • Add new initial_xxx_allocation_ratio CONF options and modify resource tracker’s initial compute node creation to use these values

  • Remove code in the ComputeNode._from_db_obj() that changes allocation ratio values

  • Add a db online migration to process all compute_nodes with existing 0.0 and None allocation ratio.

  • Add a nova-status upgrade check for 0.0 or None allocation ratio.

Dependencies

None

Testing

No extraordinary testing outside normal unit and functional testing

Documentation Impact

A release note explaining the use of the new initial_xxx_allocation_ratio CONF options should be created along with a more detailed doc in the admin guide explaining the following primary scenarios:

  • When the deployer wants to ALWAYS set an override value for a resource on a compute node. This is where the deployer would ensure that the cpu_allocation_ratio, ram_allocation_ratio and disk_allocation_ratio CONF options were set to a non- None value.

  • When the deployer wants to set an INITIAL value for a compute node’s allocation ratio but wants to allow an admin to adjust this afterwards without making any CONF file changes. This scenario uses the new initial_xxx_allocation_ratios for the initial ratio values and then shows the deployer using the osc placement commands to manually set an allocation ratio for a resource class on a resource provider.

  • When the deployer wants to ALWAYS use the placement API to set allocation ratios, then the deployer should ensure that CONF.xxx_allocation_ratio options are all set to None and the deployer should issue Placement REST API calls to PUT /resource_providers/{uuid}/inventories/{resource_class} 2 or PUT /resource_providers/{uuid}/inventories 3 to set the allocation ratios of their resources as needed (or use the related osc-placement plugin commands 4).

History

Revisions

Release Name

Description

Stein

Proposed