Comment 2 for bug 1160749

I'm not sure that there is a good way of preventing this situation in the light of potential race conditions (allocation of the 16th core at the same time the quota is being set to 15, say). Depending on the quota being changed (default, per-quota-class, per-tenant), we could be having to search for the usage of thousands of tenants, too. Issuing a warning of some sort might be easier in the nova-manage case, but we're trying to deprecate nova-manage, and I know of no easy way of issuing warnings through the nova API response…and I definitely don't want to see novaclient sending a lot of traffic to nova-api just to check whether a quota change results in someone being over quota.