[openstack-dev] [nova][quotas] quota accounting for failed resource creation

Eoghan Glynn eglynn at redhat.com
Fri Dec 14 12:24:17 UTC 2012


I'd like to get some clarity on how exactly we intend to do quota
accounting wehn a resource creation fails *outside* the API layer.

It seems that there is/was at least an aspiration to try to
ensure that the resource has been sucessfully created before
committing the reservation.

However, following through on the code paths for several
different resource types (instances in nova, volumes in cinder)
seems that this failure detection is usually limited to the API
layer. For example if the attempt to create the DB record for the
new resource fails, then we rollback the reservation, whereas if
say the compute node fails to spin up the instance, then the
quota headroom will already have been consumed even though the
resource never springs into life.

Interestingly, nova *used to* ensure that the instance could at
least be scheduled before committing the reservation. However
IIUC this was changed prior to the Folsom release in:


I'm wondering whether this change in policy was deliberate in the
above commit, or just an unintended side-effect?

Whereas in another case, we do attempt to follow through the
resource allocation path to ensure success before commiting - the
quota accounting associated with an instance resize is contigent
on successfully reaching the FINISH_RESIZE state.

So seems at the very least we have some asymmetry going on

Another related issue is that the healing/sync logic does not
seem to take the resource state into account (it excludes deleted
instances but not those in ERROR). So even if we successfully
avoid over-counting quota on a still-born instance, the sync logic
may kick in later and overwrite this intention.

So the main point here is whether we really want to ensure that
quota isn't consumed for failed resource creation attempts?

If so, seems we have a bit of work to do in nova and cinder to
ensure this is the case.


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