7.6 KiB
Refresh quotas usage
https://blueprints.launchpad.net/nova/+spec/refresh-quotas-usage
For some reasons1, 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.
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.
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 aforementionedquota_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.
Specifying --user
is optional since some resources are
per-project quotas, like fixed_ips, floating_ips and networks.
Similarly, the nova quota-update
command takes an optional
user.
Example:
$ nova-manage project quota_usage_refresh --project demo --user john_doe
--key cores
$ nova-manage project quota_usage_refresh
--project f85aa788e8ee48fca3da27e0579d3597
--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:
- Check quota
- until_refresh or max_age threshold reached?
- 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:
-
Chuck Carmack <carmack@us.ibm.com>
- Other contributors:
-
Romain Hardouin <romain.hardouin@cloudwatt.com>
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
In-tree unit and functional testing should be sufficient.
Documentation Impact
Document the new nova-manage command.
References
- "nova quota statistics can be incorrect": https://bugs.launchpad.net/nova/+bug/1284424
- "Test job failes with FixedIpLimitExceeded with nova network": https://bugs.launchpad.net/nova/+bug/1353962
- "How to reset incorrect quota count?": https://ask.openstack.org/en/question/494/how-to-reset-incorrect-quota-count/
- "nova 'absolute-limits': [...] (they are wrong)" http://lists.openstack.org/pipermail/openstack/2014-November/010250.html
- "[...] usage now out-of-sync": https://ask.openstack.org/en/question/11943/deleted-vms-still-showing-in-nova-dashboard-usage-now-out-of-sync/
For information, on Horizon side:
- "absolute-limits sometimes returns negative value" :
History
Release Name | Description |
---|---|
Newton | Re-proposed with No changes |
Mitaka | Re-proposed with 2 changes
|
Liberty | Introduced |
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.↩︎