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nova-specs
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Amend unified limits spec to explain "API limit" enforcement 

During code review of the unified limits implementation, reviewers
requested detail around the enforcement of "API limits" (limits that
are enforced only from the content of an API request). This adds a
paragraph to explain how they are enforced using the oslo.limit APIs.

A note about the [workaround]unified_limits_count_pcpu_as_vcpu config
option is also added to explain how operators can opt in to legacy
VCPU + PCPU quota usage counting behavior.

Related to bp unified-limits-nova

Change-Id: I41e7b8552763850a94374a6f31f1be72e7bf1626
This commit is contained in:
melanie witt 2022-02-15 21:52:36 +00:00
parent d345f4dbd4
commit 385d6d25a2
1 changed files with 44 additions and 4 deletions
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48
specs/yoga/implemented/unified-limits-nova.rst
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@ -220,8 +220,7 @@ No uncountable limits
---------------------
As stated above, the focus for unified limits is the instance count and
resource class allocations in placement. No other limits will be moved to
unified limits, as agreed with operators in the Train Forum session.
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:
@ -239,7 +238,7 @@ 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_metadata_items``
* ``server_injected_files``
* ``server_injected_file_content_bytes``
* ``server_injected_file_path_bytes``
@ -276,11 +275,52 @@ In summary the impact on the configuration options is:
``[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_groups_members``,
``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 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.
.. note::
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.
.. |oslo.limit API| replace:: oslo.limit ``Enforcer.enforce`` API
.. _oslo.limit API: https://docs.openstack.org/oslo.limit/latest/user/usage.html#enforce-a-limit
Deprecate Nova's Quota APIs
---------------------------