Refresh quotas usage

Refresh quotas usage

https://blueprints.launchpad.net/nova/+spec/refresh-quotas-usage

For some reasons *, the quotas usage can be out of sync. When a quota is wrongfully reached, a user cannot launch new VMs anymore. This “refresh” feature allows operators to quickly unblock users without manually running queries against the database or temporarily increase the quota.

*

It seems that there are several root causes and there is no procedure to reproduce bugs. Although these root causes will eventually be identified, we cannot guarantee that some bugs will not occur again.

Problem description

Quotas usage can be out of sync in quota_usages table. The number of used resources may not reflect the actual use. For instance we can see 7 cores used while the actual use is only 4.

Use Cases

An end user can be blocked if a quota is wrongfully reached for a resource type.

If a refresh quotas usage feature is implemented in Nova an operator can correct the usage without running a SQL query directly on the database.

Project Priority

None

Proposed change

Currently, the “refresh quotas usage” feature is hidden inside SQLAlchemy implementation. As described in “Alternatives” paragraph, the function nova.db.sqlalchemy.api.quota_reserve() can refresh the database under some circumstances.

The change consists to:

  • Create a function in the DB API to refresh quotas usage: nova.db.api.quota_usage_refresh()

  • Do an “extract function” refactoring on nova.db.sqlalchemy.api.quota_reserve() to implement the aforementioned quota_usage_refresh() DB API. That is: nova.db.sqlalchemy.api.quota_usage_refresh().

  • Expose the nova.db.api.quota_usage_refresh() feature through a nova-manage command.

The nova-manage command would look like:

$ nova-manage project quota_usage_refresh --project <Project name>
    --user <User name> [--key <Quota key>]

If --key is omitted, all quota resources are refreshed.

Example:

$ nova-manage project quota_usage_refresh --project demo --user john_doe
    --key cores

$ nova-manage project quota_usage_refresh
    --project f85aa788e8ee48fca3da27e0579d3597
    --user 62d8d1ca423d41d3970b9ec53a340678
    --key cores

Alternatives

Another nova-manage command

Another nova-manage command can use the already implemented nova.quota.usage_reset() function.

Here is the docstring of this function, note the “for a particular user”:

Reset the usage records for a particular user on a list of
resources.  This will force that user's usage records to be
refreshed the next time a reservation is made.

Note: this does not affect the currently outstanding
reservations the user has; those reservations must be
committed or rolled back (or expired).

:param context: The request context, for access checks.
:param resources: A list of the resource names for which the
                  usage must be reset.

“Reset” means “set to -1 in database”.

The resulting command would be:

$ nova-manage project quota_usage_reset --project <Project name>
    --user <User name> [--key <Quota key>]

If --key is omitted, all quota resources are reset.

Pros

Only nova/cmd/manage.py is modified. Unlike quota_usage_refresh, no changes are required at the database API level.

Cons

The main difference with quota_usage_refresh is that the user won’t see actual quota usages until the next reservation.

Credits: This command is proposed by Joe Gordon (cf. patch set 2)

Configure Nova

We can enable quotas “auto refresh” in nova.conf thanks to:

  • until_refresh (IntOpt) Count of reservations until usage is refreshed

  • max_age (IntOpt) Number of seconds between subsequent usage refreshes

These two settings (disabled by default) allow to refresh quotas usage but only during a quota reservation. The algorithm is:

  1. Check quota

  2. until_refresh or max_age threshold reached?

  3. If yes: refresh

Let’s take an example: a quota of 10 instances is set on a tenant.

The quota is wrongfully reached:

  • nova absolute-limits shows totalInstancesUsed = 10

  • nova quota-show shows instances = 10

The actual instances number is 9.

When a user runs a nova boot he will get an error: “Quota exceeded”. Many users will stop here and contact their support. Actually, a second nova boot might succeed if the first one has refreshed the quotas usage (depending on until_refresh or max_age threshold). We would need to improve this behavior but it’s off topic here.

Note that on Horizon a user will not able to spawn an instance (corresponding to the first nova boot) because the button is disabled when a quota is reached.

To conclude:

  • until_refresh or max_age need to be enabled but a cloud operator may not want to enable them if only few tenants encounter a bug on quotas usage.

  • Even with these two settings enabled, we can’t force a refresh.

Data model impact

None

REST API impact

None

Policy changes

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

The feature hits the table quota_usages the same way nova.db.sqlalchemy.api.quota_reserve() does when triggering a refresh.

Other deployer impact

None

Developer impact

Other implementations of nova.db.api should implement nova.db.api.quota_usage_refresh().

Handle nested projects? https://blueprints.launchpad.net/nova/+spec/nested-quota-driver-api

Implementation

Assignee(s)

Primary assignee:

romain-hardouin

Other contributors:

<launchpad-id or None>

Work Items

Not a big change, this BP can be submitted as a whole.

Two subtasks:

  • Change the DB API

  • Implement the nova-manage command

Dependencies

None

Testing

To be defined depending spec feedbacks.

Documentation Impact

Document the new nova-manage command.

References

For information, on Horizon side:

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