Help understanding iostat output

Question

I am watching iostat while doing a remote copy using WinSCP. WinSCP is reporting a ~8MB/s transfer rate.

iostat reports a 2.37MB/s read rate.

How does that make sense?

Every 13.0s: iostat -m /dev/sdb1                        Fri Apr  1 19:22:38 2011

Linux 2.6.35-27-generic (moabdemo)      04/01/2011      _x86_64_        (1 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           3.26    0.00   11.25   45.07    0.00   40.42

Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sdb1             66.74         2.37         0.01       3050         18

And every minute or so the MB/s rate has been increasing from an initial .04 to it's current level.

Answer 1

Run once-off as you are doing it, iostat reports the average statistics since the system was last booted, not real-time usage. If you use the interval argument (eg. iostat -m /dev/sdb1 5), then only the first report will be averages since the last boot; all the subsequent reports will show averages since the previous report.

Personally, I prefer the dstat command for real-time system monitoring, as it gives me a much broader view of the system and in a more compact format. Try eg. dstat -clmndst