Unified Limits Integration in Nova


The spec is about adopting Keystone’s unified-limits. Includes using oslo.limit to enforce the Nova related limits set in Keystone.

This spec proposes having unified limits in parallel with the existing quota system for at least one cycle, to allow for operators to transition from setting quotas via Nova to setting limits relating to the Nova API endpoint via Keystone.

All per user quota support is dropped, in favor of hierarchical enforcement that will be supported by unified limits.

Only server count limits and limits on Resource Class resources requested in the flavor will be supported with unified limits. All other existing quotas will no longer support per project or per user limits.

Given this placement focused approach, we will depend on the work done here: http://specs.openstack.org/openstack/nova-specs/specs/train/approved/count-quota-usage-from-placement.html

Problem description

While much work has been done to simplify how quotas are implemented in Nova, there are still some major usability issues for operators with the current system:

  • We don’t have consistent support for limit/quota hierarchy across OpenStack

  • Requiring operators to set limits individually in each service (i.e. Cinder, Nova, Neutron, etc)

  • Nova’s existing quotas don’t work well with Ironic

  • No support for custom Resource Class quotas (includes “per flavor” quotas)

  • Confusion when API defined quota limits override any changes made to the configuration

  • Some Nova quotas are unrelated to resource consumption, causing confusion

Transitioning to use Keystone’s unified limits, via oslo.limit, will help fix these issues.

For more details on unified limits in keystone see: https://docs.openstack.org/keystone/stein/admin/identity-unified-limits.html

Use Cases

The key use cases driving this work are:

  • API User tries to understand why they got an Over Quota error

  • Operator migrates to unified limits from existing limits

  • Operator sets a default limit for a given endpoint via Keystone. Note there can be different limits for each Region, even with a shared Keystone.

  • Operator sets specific limits for a given project via Keystone

  • Operator defines limits of a set of projects via non-flat enforcement i.e. the feature formally known as hierarchical quotas

We will focus on adding unified limits relating to:

  • total number of servers

  • amounts of each Resource Class requested in the flavor

Note, this includes things like DISK_GB which is not supported today, along with things like custom resource class resources that are requested in extra specs (e.g. Ironic flavors).

We will now look at all quotas exposed via the API and what they map to when using unifed limits: https://docs.openstack.org/api-ref/compute/?expanded=show-a-quota-detail#show-a-quota

The follow existing quota types move to unified limits, allowing for per endpoint defaults and per project overrides (and hierarchical limits) via the unified limits system:

  • cores -> class:VCPU

  • instances -> servers

  • ram -> class:MEMORY_MB

The following existing quota becomes defined only by registered (default) limits in the unified limits system, and we no longer support and per project or user overrides via the API, we just report the existing limit as defined in Keystone.

  • key_pairs (counted per user) -> server_key_pairs

  • server_groups (counted per project)

  • server_group_members (counted per server group)

  • metadata_items (counted per server) -> server_metadata_items

The above are purely protecting database bloat (i.e to stop a denial of service attack that fills up the database). They are similar to the hardcoded limit of the number of tags you can attach to a server.

While deprecated in the API, we will also treat these quotas in the same way as the quotas above, i.e. they will now be set via registered limits with no per project overrides possible:

  • injected_file_content_bytes -> server_injected_file_content_bytes

  • injected_file_path_length -> server_injected_file_path_bytes

  • injected_files -> server_injected_files

Proposed change

There are several parts to this spec:

  • Enforce Unified Limits

  • No per-user limits

  • No uncountable limits

  • Deprecate Nova’s Quota APIs

  • Operator tooling to assist with the migration

Enforcing Unified Limits

We will support the following limits:

  • servers

  • class:<RESOURCE_CLASS>

  • server_group_members

  • server_groups

  • server_injected_files

  • server_injected_file_content_bytes

  • server_injected_file_path_bytes

  • server_key_pairs

  • server_metadata_items

All the resource class usage will be counted using placement, but server count will make use of instance mappings. This only works if the queued for delete data migration has been completed. Due to no user based quotas, we don’t need the user_id migration. If the operator tries to use unified limits before completing the migration, the code will block all new usage until the migration is completed. It is expected a blocking migration will be added before we turn on unified limits by default. For more details on the this data migration see this point in the existing quota code: https://github.com/openstack/nova/blob/0d3aeb0287a0619695c9b9e17c2dec49099876a5/nova/quota.py#L1053

To allow the new system to co-exist with the older quota system, we make use of the existing quota driver sytem. The default will be unchanged, but operators can opt-into the new system in the following way:

  • [quota]driver=nova.quota.UnifiedLimitsDriver

For further details on the transition, please see the update section of this specification. Note the new unified limits code will have a hard dependency on counting usage via placement; as such it will ignore the value of CONF.quota.count_usage_from_placement.

Looking at the existing quotas, instances becomes servers, cores becomes class:VCPU and ram becomes class:MEMORY_MB.

This work will re-use a lot of the new logic to query placement for resource usage, and use the instance mapping table to count servers added in this spec: http://specs.openstack.org/openstack/nova-specs/specs/train/approved/count-quota-usage-from-placement.html

To find out what resources a server will claim, we reuse this code to extract the resources from any given flavor: https://github.com/openstack/nova/blob/2e85453879533af0b4d0e1178797d26f026a9423/nova/scheduler/utils.py#L387

For server build, we use the above function to get the Resource Class resource amounts for the requested flavor. This will then be checked using olso.limit, which ensures the additional usage will not push the associated project over any of its limits. The oslo.limit library is responsible for counting all the current resource usage using a callback we provide that makes use of placement to count the current resource usage.

Once resources are claimed in placement, we optionally recheck the limits to see if we were racing with other server builds to consume the last bits of available quota. The only change is using oslo.limit to do the recheck. That is, we will still respect the config: quota.recheck_quota Note: we do the first check of limits in nova-api, and the recheck in nova-conductor after resource allocation in placement succeeds.

It is a similar story with resize. Except in this case, we check that we can claim resources for both the new flavor and old flavor at the same time. Note that this is quite different to the current quota system, even when counting usage via placement.

For further details on the semantic changes relating to counting with placement see: http://specs.openstack.org/openstack/nova-specs/specs/train/approved/count-quota-usage-from-placement.html

Note baremetal instances no longer claim any VCPU or MEMORY_MB resources. With this method, baremetal instances can be limited using custom resource class resources they request in the flavor.

Should we choose to allow additional custom inventory entries from hypervisor based compute nodes, such as {‘CUSTOM_GPU_V100’:1} we will be also be able to apply quotas on these resources.

The oslo.limits library will likely add additional configuration options. In particular, operators will need to specify the Nova API’s endpoint uuid to oslo.limit, so it knows what unified limits apply to each particular Nova API service.

No per user limits

Nova currently supports “per user” limits. They will no longer be supported when: [quota]driver=nova.quota.UnifiedLimitsDriver

There are no plans for migration tools, however it is expected that users that need a similar model can test out using the unified limits support for hierarchical limits, and report back on what could help others migrate.

Note: Keypairs will still have a max limit enforced, and that max limit will still be enforced per user. However, there will now only be a single default registered limit value in Keystone to set the max number of keypairs each user is allowed.

No uncountable limits

As stated above, the focus for unified limits is the instance count and resource class allocations in placement.

There are limits that are specific to nova-network. These are all ready deprecated. There are no plans to support these with unified limits turned on:

  • fixed_ips

  • floating_ip

  • security_group_rules

  • networks

The remaining limits are all mainly used to protect the database from rogue users using up all available space in the database and/or missuse the API as some sort of storage system. As such, it is not expected that operators need per project overrides for any of these limits.

The following limits will be changed to only be set via registered limits in the unified limits system that applies equally to all projects:

  • server_metadata_items

  • server_injected_files

  • server_injected_file_content_bytes

  • server_injected_file_path_bytes

  • server_key_pairs (counted per user)

  • server_groups (counted per project)

  • server_group_members (counted per server group)

Note that the server_group_members are currently counted per user, but this is frankly very confusing, so above we propose the simpler limit servers in the server group. This seems consistent with removing per user limits for all other project owned resources.

Using registered limits only means:

  • no per project overrides

  • no per user overrides

These are limits on the amount of data that can be stored in various Nova databases. There is no way to display a project’s usage of these limits, which further demonstrates how these are different to the resource limits unified limits has been designed for.

Currently we honor quota.recheck_quota for all of these quotas. This adds significant code complexity, however most users never hit these limits and they are all very soft limits. As such, when we transition to a single default registered limit value for all of these, we also will stop doing any rechecks.

In summary the impact on the configuration options is:

  • quota.recheck_quota will have an updated description, noting what functionality is lost when [quota]driver=nova.quota.UnifiedLimitsDriver

  • quota.floating_ips, quota.fixed_ips, quota.security_groups, security_group_rules: remain deprecated, and will be ignored when [quota]driver=nova.quota.UnifiedLimitsDriver.

  • quota.metadata_items, quota.injected_files, quota.injected_file_content_bytes, quota.injected_file_path_length, quota.server_groups, quota.server_group_members, quota.key_pairs: these will all be kept, but the description will be updated to note if [quota]driver=nova.quota.UnifiedLimitsDriver all updates via the API are ignored.

Enforcement of limits

For consistency, all quota limits will be enforced using the oslo.limit Enforcer.enforce API. In this pattern, an Enforcer object is initialized with a callback that will count the existing usage of the resource being enforced. Some resources, however, are counted only from the Nova API request payload in which they were specified. We call these resource limits “API limits”:

  • server_metadata_items

  • server_injected_files

  • server_injected_file_content_bytes

  • server_injected_file_path_bytes

For example, when a server create request is received in the Nova API and metadata items are included in the request, we will compare the number of metadata items requested with the quota limit for server_metadata_items and we will not count anything in the database or placement as usage. An oslo.limit quota limit enforcement involves a limit, a delta, and the current usage. For API limits, we have only a limit and a delta. The current usage in these cases will always be zero. As such, we initialize the Enforcer object for API limits with a callback that always returns zero usage. This way, oslo.limit can do the enforcement properly using only the quota limit and the supplied delta.


With unified limits, the behavior of VCPU and PCPU quota usage will change to count VCPU resources independently from PCPU resources, consistent with how they are represented in the placement service.

Legacy quota behavior counts PCPU as VCPU and returns the sum of VCPU + PCPU usage as the usage count for VCPU. A config option [workarounds]unified_limits_count_pcpu_as_vcpu will be provided for operators who require the legacy quota usage behavior where VCPU = VCPU + PCPU. Note that if class:PCPU is specified in the flavor explicitly, it will be expected to have its own unified limit registered and PCPU usage will not be merged into VCPU usage even when this option is set to True.

Deprecate Nova’s Quota APIs

To query and set limits, Keystones APIs should be used. To query a user’s usage, the Placement API should be used, assuming placement is happy changing the default policy to allow users to query their usage.

The one exception is server count can’t currently be checked via Placement. When placement implements consumer records, or similar, then all usage could be queried via Placement. To avoid using a proxy API, users can do a server list API and count the number of servers returned.

When [quota]driver=nova.quota.UnifiedLimitsDriver a best effort will be made to keep the older micro-versions working by proxing API calls to Keystone and Placement as needed. No quota related DB tables will be accessed when [quota]driver=nova.quota.UnifiedLimitsDriver.

This includes the follow API resources:

  • /limits

  • /os-quota-sets

  • /os-quota-class-sets

Existing tooling to set quotas should continue to operate, as long as it only changes quotas relating to instances, cores and ram. Requests to change any other quotas will be silently ignored. As one example, this should allow Horizon to function as normal during the transition.

There are some trade-offs with this approach:

  • Proxy APIs suck, but horizon must keep working as such all current operator tooling around these existing APIs.

  • We don’t need a micro version to enable/disable this proxy of the quota APIs, as it doesn’t really change the API.

  • In a future release when unifed limits becomes the default, we should deprecate the APIs /os-quota-sets and /os-quota-class-sets and tell users to talk to the Keystone API instead. API based discovery of when Nova is enforcing the limits set in Keystone is left for a future spec.

  • It is expected the above API deprecation will follow the pattern used by nova-network proxy APIs, i.e. the APIs return 404 in new microversions but continue to work in older microversions. Its possible in the more distant future the APIs could be removed by returning 410 error.

  • Rejecting updates to quotas that we were previously able to set would be a breaking change in behaviour, and require a microversion. Adding a new API microversion that returns BadRequest for unsupported quotas would be a nice addition if we were not planning on deprecating the API in favor of calling Keystone instead.

  • Ideally we would also deprecate /limits in favor of a cross project agreed direction that is aware of both flat and hierarchical limit enforcement. Howerver we do not yet have consenus on what direction we take. For this spec, we leave /limits in its current form, even though it does not report on all the types of resource usage we now support have limits on, and even though it lists limits that can now only be changed via registered unified limits in Keystone.

  • When hierarchical limits are added, the per project usage information in /limits does not mention anything about parent limits. As such quota APIs may claim resources are available, but you will be unable to build any new resources. It is not clear what action the user can make to be able to build those new resources. Operators can avoid this confusion by not over allocating quota. We could extext the API to include a boolean to say if the limit has been exceeded in the parent project, and as such the user is unable to consume more resources even though their own usage is not over their own limits. We could consider extending the API to include the usage of the full tree

Migration to Unified Limits

The migration of all users to unified limits is happening in three phases:

  • enable unified limits as an option, with migration path from existing quotas

  • make unified limits the default, deprecate existing quota system

  • remove existing quota system

To help with the transition we need operator tooling to:

  • Set registered limits in Keystone for each Nova endpoint in Keystone, based on current limits in DB and/or configuration

  • Copy per-project quotas set in Nova into Keystone unified-limits

  • Operator confirms unified limits works for them

  • Drop all quota info from the DB to signal operator has completed transition

  • Upgrade status check to check there is no data left in quota DB tables

Note the setting of project limits and registered limits in keystone will happen via files that are generated and passed to keystone-manage. This allows fast-forward upgrades where no API are available during the migration of limits from Nova to Keystone.

There will be a new tool to setup the registered limits in keystone. It will read from the Nova DB and configuration and generate a file. That file can be by used with keystone-manage to register the current endpoint defaults in keystone.:

nova-manage limits generate_registered_limits --endpoint <endpoint-uuid>

The following tool will generate the unified limits overrides (if any) that needs to be added into Keystone for each project. Again this too produces a file that is handed to keystone-manage which will update keystone:

nova-manage limits generate_project_limits [--project_id <project_id>]

Once the operator sets [quota]driver=nova.quota.UnifiedLimitsDriver, the Nova DB is ignored, and limits are accessed from Keystone only.

To complete the migration, there is an operation to remove all the DB entries relating to the quota overrides. The tool only works when [quota]driver=nova.quota.UnifiedLimitsDriver. It also removes all any per user limits associated with each project.:

nova-manage limits remove_db_quota_entries [--project_id project_id]

Note the last two tools allow operators to iterate per project, to limit the load on the running system. If these tools are used on a running system, it is recommended that operators don’t change quotas via the API during the transition.

The nova status command will warn users that have failed to remove all the quota information from the DB. This will become an error in the release when [quota]driver defaults to nova.quota.UnifiedLimitsDriver.

It is worth noting that the Nova database may contain entries for projects that have been deleted in keystone. As such, it is advisable to get a list of active projects from keystone, and only generate_project_limits for those particular projects.

This transition leaves several configuration options redundant, in particular the following will all be deprecated once unified limits is on by default:

  • quota.instances, quota.cores, quota.ram: deprecate all these as the limit now comes from keystone for unified limits, which will default to unlimited if there is no limit in keystone.

The setting quota.recheck_quota will be kept, and will be used in the same way with unified limits to avoid races when multiple instances are built at the same time.


Ideally we would not add any more proxy APIs, however, operators pushed back at the Train Forum session, requesting that their tooling continue to work across the transition. No operators reported using limits other than the instances, cores and ram limits.

We could implement hierarchical quotas in isolation, and not adopt unified limits.

We could limit the types of resources we limit, but it will be hard to transition to supporting different kinds of resource limits in a clear and interoperable way.

Data model impact

See upgrades, no changes in Victora due to having old and new quota systems side by side. Once we remove the old quota system, we could drop all the quota related DB tables.

REST API impact

When [quota]driver=nova.quota.UnifiedLimitsDriver Nova will proxy the requests to Keystone’s unified limits API, where possible. The aim will be to keep horizon functioning correctly during the transition.

Once using unified limits, operators should move to using Keystone’s unified limit APIs to set and query limits. Usage information should be queried via Placement and the Servers API.

Security impact

The removal of quota rechecks for some limits slightly reduces the protection provided, but really it encourages the proper implementation of API rate limiting as replacement protection.

Notifications impact


Other end user impact

Quota errors with unified limits will use the standard and consistent error messages from oslo.limit after this change.

Performance Impact

It is possible to have more complicated quota counts with hierarchical quotas, but the implementation of that is delegated to oslo.limit.

Other deployer impact

There are several tools to help ease the transition to unified limits noted above. Although it is expected that use of the feature will help inform the end direction.

Developer impact

There will now be two limit system to maintain for a few cycles during the transition. But this avoids the long term need to maintain complicated hierarchical limit code, which still getting the advantages, such as being able to tidy up API policy.

Upgrade impact

To get the best experience, operators need to start using the unified limits API via Keystone. Users should start querying usage from Placement.

The transition between the existing quota system and unified limits is detailed in the proposed solution section.

It is expected that oslo.limit will limit versions of Keystone that can be used to Queens and newer, which is not expected to affect most users.



Primary assignee:


Other contributors:


Feature Liaison

Feature liaison:


Work Items

  • Add calls to oslo_limits, guarded by config to enable it

  • Move quota APIs to proxy to Keystone when unified limit quotas enabled

  • Add tools to migrate default and tenant limits from Nova into Keystone

  • Upgrade checks to ensure above tooling is used



Grenade test that runs the migration of quota settings (after adding some quotas).

Functional tests to ensure quotas are enforced based on unified limits correctly.

Documentation Impact

Building on the work to document quota usage from placement, we should describe how the new system operates. The admin guide needs to detail how to smoothly migrate to unified limits.





Release Name