'Return performance counters associated with the given disk_id on the
given instance.  These are returned as [rd_req, rd_bytes, wr_req,
wr_bytes, errs], where rd indicates read, wr indicates write, req is
the total number of I/O requests made, bytes is the total number of
bytes transferred, and errs is the number of requests held up due to a
full pipeline.
All counters are long integers.
This method is optional.  On some platforms (e.g. XenAPI) performance
statistics can be retrieved directly in aggregate form, without Nova
having to do the aggregation.  On those platforms, this method is
unused.
Note that this function takes an instance ID.'

def block_stats(self, instance, disk_id):

    raise NotImplementedError()
