kernel 2.6.1: erroneous statistics in /proc/diskstats?

[Jean-Luc Fontaine <jfontain@free.fr>]

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I would like to make sure /proc/diskstats is reliable or if I am totally
misunderstanding the significance of the milliseconds spent reading or
writing figures.
Copying a huge file from hdb to hdc, I get:

$ date; egrep 'hdb|hdc' /proc/diskstats
Sun Jan 18 17:52:51 CET 2004
3 64 hdb 12949 88 1335982 352530 8506 8009 132344 257702 1 102525 610631
3 65 hdb1 13034 1335950 16543 132344
22 0 hdc 260 0 2072 15180 3026 87831 727760 1232708 113 25415 1299749
22 1 hdc1 256 2040 90970 727760
$ date; egrep 'hdb|hdc' /proc/diskstats
Sun Jan 18 17:52:57 CET 2004
3 64 hdb 13162 88 1384702 356272 8506 8009 132344 257702 0 106045 614311
3 65 hdb1 13246 1384670 16543 132344
22 0 hdc 260 0 2072 15180 3725 109431 906136 1931270 111 31685 2000878
22 1 hdc1 256 2040 113267 906136

According to the above, hdc has spent writing (field #8):
~  1931270 - 1232708 = 698562 ms = 699 s
doing I/O (field #10):
~  31685 - 25415 = 6270 ms = 6.27 s
in about a 6 s interval (from date command).
It seems field #8 is incorrect.

As for hdb, it has spent reading (field #4):
~  356272 - 352530 = 3742 ms = 3.74 s
doing I/O:
~  106045 - 102525 = 3520 ms = 3.52 s
Looks correct.

Would somebody please enlighten me?

PS: from Documentation/iostats.txt:
Field  8 -- # of milliseconds spent writing
This is the total number of milliseconds spent by all writes (as
measured from __make_request() to end_that_request_last()).
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