Comment 402 for bug 620074

Here some tests:

I do to defaults settings before tuning:
# echo 100 > /proc/sys/vm/vfs_cache_pressure
#
# echo cfq > /sys/block/sda/queue/scheduler
#
# echo 128 > /sys/block/sda/queue/nr_requests

# dd if=/dev/zero of=testfile.1gb bs=1M count=1000
^C
116+0 records in
116+0 records out
121634816 bytes (122 MB) copied, 20.5609 seconds, 5.9 MB/s
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^ (!!!)

During a riunning of 'dd' i do vmstat 1:

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
-
 r b swpd free buff cache si so bi bo in cs us sy id wa st
 0 10 116 760168 503488 1329836 0 0 4 4 0 0 9 3 65 23
0
 0 11 116 756132 502488 1330536 0 0 1332 5648 1744 4909 5 2 2 91
0
 0 12 116 760208 503128 1330856 0 0 1136 4388 1875 3053 4 2 0 94
0
 0 11 116 759832 502668 1331608 0 0 1004 7488 2379 4032 1 2 0 97
0
 0 12 116 758740 503288 1331832 0 0 1280 3252 1818 2402 1 1 0 98
0
 0 10 116 733976 502936 1356780 0 0 1232 4476 1753 4143 1 3 0 96
0
 1 8 116 733596 502368 1357324 0 0 804 5792 1831 2980 20 2 0 79
0
 1 7 116 738388 502920 1357788 0 0 928 6652 1875 2349 17 2 4 77
0

**************************

Now i after this to do:

# echo 50 > /proc/sys/vm/vfs_cache_pressure
#
# echo deadline > /sys/block/sda/queue/scheduler
#
# echo 1024 > /sys/block/sda/queue/nr_requests

# dd if=/dev/zero of=testfile.1gb bs=1M count=1000

^C
638+0 records in
638+0 records out
668991488 bytes (669 MB) copied, 10.463 seconds, 63.9 MB/s
                                     ^^^^^^^^^^^^^^^^ (!!! :-))) )

During 'dd' i do in other terminal:
# vmstat 1

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
-
 r b swpd free buff cache si so bi bo in cs us sy id wa st
 1 7 116 718764 502884 1371484 0 0 4 4 0 0 9 3 65 23
0
 0 9 116 687208 502924 1405624 0 0 8 26664 2708 746 6 4 3 87
0
 1 8 116 668924 502976 1422116 0 0 16 21404 2246 8462 1 4 9 87
0
 0 8 116 654804 501632 1434492 0 0 24 30804 2072 9249 10 4 0 86
0
 0 10 116 613152 501692 1475220 0 0 20 42880 2021 4408 15 5 7 73
0
 2 10 116 559860 499464 1524600 0 0 32 58504 2108 10612 5 6 15 74
 0
 0 11 116 510132 499528 1578340 0 0 36 59400 984 1748 17 5 2 77
0
 0 10 116 399420 499672 1689316 0 0 108 111332 910 957 4 11 2 84
 0
 1 7 116 331556 499756 1750580 0 0 104 62268 1501 5255 11 6 10 74
0

*********************

and noticing:

I have other servers, there other hardware. I cannot repeat this iowait problem with and without this turning (there Fedora release 7 (Moonshine), kernel 2.6.23.17-88.fc7). Now i think that this trouble is not for all HDDs. May be this trouble is hardware dependent.

I am researching now what option will help to resolve iowait problem