time to wall-clock
time writes and reads in
programs mkrandfile (section 3.4.3)
and xrandfile (section 4.3.4).
You must ensure that individual writes and reads are
sufficiently large for this to work, because function
time returns time with the accuracy of one second,
which is a very long time in computing. But this may just
about work, if individual writes and reads are huge and take long time.