Strange IOStat xk. Moving a mere 40GB data is too slow. Can anyone explain?

Question

iostat -xk 5

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.99    0.00    1.10   11.01    0.00   86.89

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.00     0.00    0.00    1.50     0.00     6.00     8.00     0.00    1.73   0.40   0.06
sdb               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
sdd               0.00   836.00    0.00   37.50     0.00  3506.00   186.99     0.33    8.75   0.28   1.06
sdc               0.00   961.80  137.60   17.80  3221.20  3928.40    92.02     3.36   21.78   6.21  96.53

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.53    0.00    0.69   14.48    0.00   84.30

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.00     0.00    0.00    0.70     0.00     2.80     8.00     0.00    1.43   0.86   0.06
sdb               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
sdd               0.00   775.10    0.00   39.70     0.00  3270.40   164.76     0.52   13.13   0.24   0.94
sdc               0.00   466.60  103.20  170.60  2480.80  2552.80    36.77    72.97  266.53   3.59  98.42

Why it's strange?

1. The sole main operation is moving 40G data from sdc to sdd.
2. So sdd is being written and sdc is being read. Yet sdc, which is being read, has huge write
3. total disk activity is around 4MB/s and hard disk is already at full speed capacity. C'mon..... Only 4 MB/s? I know hard disk is not fast. But my hard disk at home does more than 4MB/s.

Answer 1

You are looking at statistics of the devices since machine boot up.

Quoting from the man page

The first report generated by the iostat command provides statistics concerning the time since the system was booted. Each subsequent report covers the time since the previous report. All statistics are reported each time the iostat command is run. ...

To observe the I/O performance while copying, do the following

cp <a file on sdc> <a path on sdd> &
iostat 1

The above commands will put the cp command in background, then run iostat with 1 second interval. So after the 1st report, all sub-sequence reports will be statistic of the past second.