Comment 2 for bug 1796981

Looking at what fails:

def _notify_volume_usage_detach(self, context, instance, bdm):
        if CONF.volume_usage_poll_interval <= 0:
            return

        vol_stats = []
        mp = bdm.device_name
        # Handle bootable volumes which will not contain /dev/
        if '/dev/' in mp:
            mp = mp[5:]
        try:
            vol_stats = self.driver.block_stats(instance, mp)
        except NotImplementedError:
            return

        LOG.debug("Updating volume usage cache with totals", instance=instance)
rd_req, rd_bytes, wr_req, wr_bytes, flush_ops = vol_stats

Clearly volume_usage_poll_interval has to be configured, which it's not by default. Which compute driver are you using that returns None but doesn't raise NotImplementedError?