inferior SCSI performance

I have been experimenting with a new Seagate Cheetah 10k-RPM SCSI to
compare with a cheaper Seagate Barracuda 7200-RPM IDE (each in a
single-drive configuration). The Cheetah definately dominates the generic
IO tests such as bonnie++, but fares poorly with pgbench (and other
postgresql operations).

I don't understand why switching to a SCSI drive in an otherwise identical
setup would so seriously degrade performance. I would have expected the
opposite. vmstat does not reveal (to me) any bottlenecks in the SCSI
configuration.

The only difference between the two test scenarios is that I stopped the
postmaster, copied the data dir to the other drive and put a symlink to
point to the new path. I ran the tests several times, so these are not
flukes.

Can anyone explain why this might be happening and how to better leverage
this 10k drive?

thanks,
Mike Adler

System info:

Box is a Dell 600SC with Adaptec 39160 SCSI controller.
Linux 2.4.18-bf2.4
CPU: Intel(R) Celeron(R) CPU 2.00GHz stepping 09
Memory: 512684k/524224k available (1783k kernel code, 11156k reserved,
549k data, 280k init, 0k highmem)

postgresql.conf settings:
shared_buffers = 10000
random_page_cost = 0.3
sort_mem = 4096

##################################################
TEST 1:

IDE Seagate Baracuda
hde: ST340014A, ATA DISK drive
hde: 78165360 sectors (40021 MB) w/2048KiB Cache, CHS=77545/16/63

bonnie++ -f:
Version 1.02b ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
dellmar 1G 27001 10 11863 4 20867 3 161.7 0

sample vmstat 1 output during bonnie++:
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
1 0 0 9332 4456 5056 467728 0 0 20864 0 429 698 0 5 95
0 1 1 9332 4380 5056 467728 0 0 5248 27056 361 207 1 4 95
0 1 1 9332 4376 5056 467728 0 0 384 26936 338 55 0 0 100
0 1 0 9332 4416 5064 468368 0 0 10112 9764 385 350 0 4 96
1 0 0 9332 4408 5056 468120 0 0 20608 0 427 684 1 7 92
1 0 0 9332 4392 5056 467864 0 0 20992 0 431 692 0 5 95

pgbench:
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 2
number of transactions per client: 400
number of transactions actually processed: 800/800
tps = 110.213013(including connections establishing)
tps = 110.563927(excluding connections establishing)

sample "vmstat 1" output during pgbench:
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
2 0 0 160 4348 50032 419320 0 0 240 3432 514 3849 34 7 59
0 2 0 160 4392 50764 418544 0 0 224 3348 500 3701 33 6 61
2 0 0 160 4364 51652 417688 0 0 240 3908 573 4411 43 8 50
2 0 0 160 4364 52508 416832 0 0 160 3708 548 4273 44 8 49
1 1 1 160 4420 53332 415944 0 0 160 3604 541 4174 40 13 48
0 1 1 160 4420 54160 415120 0 0 104 3552 526 4048 42 14 45
1 0 0 160 4964 54720 414576 0 0 128 4328 645 5819 69 7 24

########################################################
TEST 2:

SCSI Drive Seagate Cheetah 10k.6
Vendor: SEAGATE Model: ST336607LW Rev: DS08

bonnie++ -f:
Version 1.02b ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
dellmar 1G 40249 14 21177 7 46620 7 365.8 0

sample vmstat 1 output during bonnie++:
0 1 1 8916 4400 1844 467216 0 0 384 42348 475 80 0 0 100
0 1 1 8916 4392 1844 467216 0 0 512 46420 472 103 0 2 98
1 0 0 8916 4364 1852 469392 0 0 7168 26552 507 268 0 3 97
1 0 0 8916 4452 1868 469392 0 0 28544 12312 658 947 1 15 84
1 0 0 8916 4416 1860 468888 0 0 47744 4 850 1534 0 18 82
1 0 0 8916 4436 1796 468312 0 0 48384 0 859 1555 0 19 81
1 0 0 8916 4452 1744 467724 0 0 48640 0 863 1569 2 20 78

pgbench (sounds thrashy):
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
number of clients: 2
number of transactions per client: 400
number of transactions actually processed: 800/800
tps = 33.274922(including connections establishing)
tps = 33.307125(excluding connections establishing)

sample "vmstat 1" output during pgbench:
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 1 1 160 4356 36568 432772 0 0 0 1120 232 1325 12 2 86
0 1 1 160 4452 36592 432652 0 0 0 1108 229 1295 14 2 84
0 1 1 160 4428 36616 432652 0 0 0 1168 234 1370 9 4 87
0 1 1 160 4392 36636 432668 0 0 0 1120 231 1303 12 3 85
0 1 0 160 4364 36664 432668 0 0 0 1084 230 1361 16 5 79
0 1 0 160 4456 36696 432548 0 0 0 1196 234 1300 13 2 85

Mike Adler