extremely slow disk access (using SCSI, RAID)

Hello,

we are running 7.1.3 on a linux machine. By running simple 'group by'
queries on large tables we are we are experiencing __extremely slow__
disk read rates.
The size of the database is cca. 40GB+. Size of a table cca. 1GB

CONFIGURATION

CPU - Dual Intel PIII 1,133
Mem - 2GB Kingston ECC
Storage - RAID 5, 3 x IBM SCSI 36GB (10,000rpm)
Raid controller - Adaptec 3210

TEST QUERY

select field, count(*) from table where date between '2001-1-1' and
'2002-1-1' group by field order by count;

DISK I/O

argosz(at)it:~$ vmstat -n 1
procs memory swap io system
cpu
r b w swpd free buff cache si so bi bo in cs
us sy id
0 0 0 24744 1050124 47660 835532 0 0 0 0 160 113
1 3 96
0 0 0 24744 1050128 47660 835532 0 0 0 0 227 201
1 3 96
1 0 0 24744 1050032 47660 835632 0 0 64 128 203 169
5 2 93
BEGINNING OF QUERY
0 0 0 24744 1049272 47664 836368 0 0 500 0 210 234
4 4 92
0 0 0 24744 1048768 47672 836800 0 0 296 0 249 277
1 3 96
3 0 0 24744 1048296 47676 837188 0 0 220 0 171 127
1 3 96
2 0 0 24744 1043928 47680 840760 0 0 1744 0 322 425
23 1 76
1 0 0 24744 1040568 47680 844092 0 0 1416 128 333 423
24 9 67
1 0 0 24744 1036880 47680 847776 0 0 1536 0 284 386
36 3 61
1 0 0 24744 1032924 47680 851724 0 0 1640 0 301 417
32 5 63
1 0 0 24744 1029420 47684 854996 0 0 1328 52 334 480
33 6 61
1 0 0 24744 1026172 47684 858344 0 0 1424 0 286 366
28 5 67
1 0 0 24744 1022572 47684 861932 0 0 1424 0 284 403
34 2 64
1 0 0 24744 1019244 47688 865592 0 0 1548 0 308 398
29 3 68
1 0 0 24744 1015216 47688 869608 0 0 1664 0 298 406
27 7 66
2 0 0 24744 1011464 47688 873468 0 0 1536 0 361 450
32 5 63
1 0 0 24744 1007740 47692 877180 0 0 1620 256 309 441
20 5 75
1 0 0 24744 1004948 47692 879968 0 0 1144 0 262 340
21 4 75
1 0 0 24744 1001040 47696 883864 0 0 1628 0 311 446
27 5 68
1 0 0 24744 997192 47700 887700 0 0 1540 0 285 373
30 2 68
1 0 0 24744 992972 47700 891896 0 0 1776 0 319 443
30 5 65
1 0 0 24744 989212 47700 895652 0 0 1584 256 310 409
31 2 67
1 0 0 24744 985440 47704 899408 0 0 1500 0 320 436
38 4 58
1 0 0 24744 982676 47704 902176 0 0 1120 0 248 321
24 4 72
1 0 0 24744 978656 47704 906188 0 0 1664 0 324 439
37 2 61
1 0 0 24744 975008 47708 909828 0 0 1540 0 273 386
35 2 63
1 0 0 24744 971300 47712 913532 0 0 1492 256 330 435
30 5 65
1 0 0 24744 968260 47712 916568 0 0 1304 0 262 334
21 5 74
1 0 1 24744 967072 47712 917612 0 0 256 0 146 117
86 6 8
1 0 0 24744 967016 47712 917612 0 0 0 0 126 62
93 7 0
1 0 0 24744 966908 47712 917760 0 0 96 0 139 101
95 5 0
END OF QUERY
0 0 0 24744 981816 47712 903864 0 0 0 256 161 142
41 5 54
0 0 0 24744 981392 47712 904272 0 0 256 0 161 129
3 1 96

Sometimes the io-bi is cca. 3-5000/sec, but it's still way too low in
my mind.
What is the best rate we can achieve with this RAID 5 configuration
and how?

Also, can you give me a idea what rate we should expect using RAID
0+1?

As a comparison I quoted the output of vmstat when we read a simple
file from the disk. (Still not very fast, but faster)

argosz(at)it:~$ cat /var/log/apache/default-access-log.log > /dev/null

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 24636 238876 49268 1595264 0 0 3656 0 208 399
3 6 91
2 0 0 24636 211156 49296 1622272 0 0 27036 0 587 957
3 12 85
1 0 0 24636 181732 49324 1650952 0 0 28700 0 648 1077
6 11 83
2 0 0 24636 157812 49332 1674268 0 0 23316 0 494 863
37 14 49
2 0 0 24636 140100 49332 1691548 0 0 17280 128 647 834
5 8 87
2 0 0 24636 121072 49332 1710112 0 0 18560 0 705 943
1 10 89
0 0 0 24636 109020 49332 1721948 0 0 11796 0 505 726
3 11 86

Thanks for your help

Peter