Out-of-sync quotas in OpenStack Nova

From time-to-time user quotas in OpenStack become out of sync, i.e usage will show X number of instances in use and the reality, Y, is a different value altogether. Until now the way to fix this has been to manually update the nova database and amend the quota_usages table and then trigger a refresh by launching a new machine, i.e:

update quota_usages set in_use = 0 where id = 890 and project_id = '509d6ae853114ea9aaaac02804d3a4dd' limit 1;

Or potentially updating en masse - if you're feeling brave - with something like:

update quota_usages, (select usage_id, sum(delta) as sum from reservations where project_id='86d829ffc0ff4efb943131f7d2a18d52' and deleted!=id group by usage_id) as r set quota_usages.in_use = r.sum where quota_usages.id = r.usage_id;
update quota_usages, (select distinct(usage_id) as usage_id from reservations where project_id='ad3e3ee7a08d45df965908704f29b873' and deleted=id ) as r set quota_usages.in_use = 0 where quota_usages.id = r.usage_id;

However, the CERN chaps have just released a handy little tool that checks and then updates where necessary if there's any mismatches. They've blogged a bit about it here and the script itself is over here.