this is a multithreaded disk i/o microbenchmark.  it requires O_DIRECT,
pread/pwrite, and pthreads.

WARNING: this will destroy data if used incorrectly.

usage: ./randomio filename nr_threads write_fraction_of_io fsync_fraction_of_writes io_size nr_seconds_between_samples

filename
        filename or device to read/write.

write_fraction_of_io
        what fraction of i/o should be writes.  for example 0.25 for
        25% write traffic.

fsync_fraction_of_writes
        what fraction of writes should be fsync'd.

io_size
        how many bytes to read/write.  must be a positive multiple
        of 512 bytes

nr_seconds_between_samples
        how many seconds to average samples over.  recommended 10 or
        more.


for example:

% ./randomio /dev/sdb 8 0.5 1 4096 10
 total | read:         latency (ms)       | write:        latency (ms)
  iops |  iops   min    avg    max   sdev |  iops   min    avg    max   sdev
-------+----------------------------------+---------------------------------
  86.9 |  43.2   5.9  134.4  494.4   94.2 |  43.7   0.3   49.0  335.9   53.2
  85.4 |  40.9   7.7  146.8  831.4  103.3 |  44.5   0.3   45.2  243.0   44.8
  84.5 |  43.2   4.2  138.4 1126.4  112.5 |  41.3   0.2   46.8  263.2   50.7