创建备份的时候，修改配额使用量：
cinder.backup.api.API#create：
from cinder import quota
from cinder import quota_utils
def create(self, context, name, description, volume_id,
container, incremental=False, availability_zone=None,
force=False, snapshot_id=None):
"""Make the RPC call to create a volume backup."""
<!--省略业务代码-->
# Reserve a quota before setting volume status and backup status
try:
# 创建配额预留
reserve_opts = {'backups': 1,
'backup_gigabytes': volume['size']}
reservations = QUOTAS.reserve(context, **reserve_opts)
except exception.OverQuota as e:
quota_utils.process_reserve_over_quota(
context, e,
resource='backups',
size=volume.size)
try:
<!--省略业务代码-->
# 提交配额预留，修改quota_usage里的使用量
QUOTAS.commit(context, reservations)
except Exception:
# 延迟抛出异常，先把创建了一半的backup先销毁了，并回退配额，然后再抛出异常
with excutils.save_and_reraise_exception():
try:
if backup and 'id' in backup:
backup.destroy()
finally:
# 回退预留
QUOTAS.rollback(context, reservations)
# TODO(DuncanT): In future, when we have a generic local attach,
# this can go via the scheduler, which enables
# better load balancing and isolation of services
self.backup_rpcapi.create_backup(context, backup)
return backup
