I/O report detailed meanings

Bo Yang
2007-10-08T12:36:16Z

Hi,
I hope this message is going into the right newsgroup. If it is not,
please tell me, thanks.
I have get a recent report from my machine and there is something I
do not understand.
Please take a look at the result first:

tps rd_sec/s wr_sec/s avgrq-sz avgqu-sz await svctm %util
143.93 1534.74 1342.01 19.99 18.49 128.43 3.29 47.41
120.24 1424.39 1054.50 20.62 14.47 120.33 3.41 41.00
102.01 1270.69 976.18 22.03 12.87 126.20 3.80 38.79
81.82 947.90 765.50 20.94 8.94 109.30 4.07 33.27
89.55 660.23 730.50 15.53 7.76 86.61 3.80 34.01
182.51 2088.51 1199.39 18.01 16.24 89.00 3.16 57.61
73.11 1207.13 583.24 24.49 6.22 84.67 4.13 30.16
56.75 866.81 504.46 24.17 4.06 72.02 4.71 26.73
48.33 397.68 439.40 17.32 3.35 69.31 5.05 24.38
46.44 490.52 430.11 19.82 3.43 73.84 4.94 22.94
41.16 330.09 367.70 16.95 2.86 69.38 5.36 22.05
408.19 13068.02 1939.98 36.77 7.25 17.75 2.21 90.23
469.36 5933.15 1763.28 16.40 8.07 17.20 2.13 99.89
463.78 5695.35 1419.88 15.34 5.76 12.42 2.15 99.86
446.36 5535.90 1712.86 16.24 6.81 15.25 2.24 99.83
449.05 5166.26 1424.83 14.68 6.43 14.31 2.22 99.89
460.23 5596.55 1585.21 15.60 6.44 14.00 2.17 99.85
454.00 5361.55 1518.18 15.15 7.34 16.18 2.20 99.88
576.51 7397.60 1870.47 16.08 9.02 15.64 1.73 99.91
562.26 8555.19 2776.98 20.15 9.28 16.50 1.78 99.92
561.50 9001.87 3462.99 22.20 11.94 21.27 1.78 99.95
502.35 9393.50 4081.07 26.82 12.17 24.22 1.99 99.98

I am sorry that the format is still a little confused although I have
make some change to it to be more suitable for news group. Any way, I
think it is now readable.

Above is I/O report from 00:00 to 06:00 or some time in my machine.
There are about two main status in the report, in the first half the
whole load is very normal and the %util is about 40%. Suddenly, the
%util is up to 99% and the whole disk is in high load. I know, in that
time I run some I/O sensive task. But I can not understand that when the
%util is low the await/svctm/avgqu-sz is very high, but when the %util
is high the await/svctm/avgqu-sz is very low. I do not understand why?

I guess that maybe this has something to do with the way Linux deal with
I/O. But I can not figure out the real reason, could you please give me
some detailed explaination? Thanks in advance!

Regards!
Bo