2.4.21 opteron 32 Gig RAM has 10k block writes/sec on running posgresql 7.4.6 disk i/o intensive app

[Paramveer Singh]

we are using a RedHat AS3 U3 box (2.4.21-4.ELsmp) to run a very
intensive database app which does a _huge_ number of inserts durnig
the data generation phase. However, we are noticing a unexpected
performance drops with user cpu utilization numbers falling to near 0.

most of the cpu is used up in iowait. 
CPU states:  cpu    user    nice  system    irq  softirq  iowait    idle
          total    2.0%    0.0%    7.6%   0.0%     0.0%  362.4%   27.2%

/proc/slabinfo showed huge numbers in buffer_head. The line was:
buffer_head       4908452 4962249    200 260854 261171    1 : 16000 4000

some lines from vmstat 1
procs                      memory      swap          io     system         cpu
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy wa id
 4  2  10104 8585908 109820 19090508    0    0    37  1160  282  1421 
8  4 67 22
 1  4  10104 8585908 109824 19090684    0    0     0  4940 1359  4762 
4  1 42 52
 0  3  10104 8585908 109824 19091084    0    0     0  5296 1442 17178
13  4 40 43
 8  1  10104 8585908 109824 19092068    0    0     0  2856 1437 34481
33  6 30 32
 1  3  10104 8585908 109832 19093620    0    0     0  9012 1538 28554
35  7 34 25
 0  3  10104 8585908 109832 19094004    0    0     0  6120 1552  3059 
6  2 47 45
 0  3  10104 8585908 109836 19094012    0    0     0  6188 1502   332 
0  0 70 30
 0  3  10104 8585908 109836 19094020    0    0     0  6436 1548   865 
0  1 67 32
 2  3  10104 8585908 109836 19094332    0    0     0  5980 1404  9322
10  3 52 36
 0  3  10104 8585908 109840 19094332    0    0     0  5252 1492   190 
0  0 74 25

Interestingly, the swap is not being used at all
free output:
Swap:      2040212      10104    2030108


the box is 
Quad AMD Opteron 1.8Ghz 
Tyan Mother board S4882UG2NR 
with Rack Chassis, 12 X 1 Hot swap bays, power supply +  RPS Support
32Gb DDr sdram @ 266
8 X 80Gb SATA Hot Swap Hard Drive, 7200 RPM
Adaptec  SATA RAID controller

I tried to google but found some kswapd related bugs in AS3 which were
fixed in U3. We upgraded even though this does not seem to be a kswapd
error. I am gussing I have used some inappropriate numbers while
setting up the box.

Would someone be kind to point me in a direction where I can find more
information?

thanks a lot!
ps
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