Slow on heavy I/O operations.

[Roman Gorohov.]

Hello list.
There is a server with FreeBSD FreeBSD 4.11-STABLE(can't upgrade fornow) on HP ProLiant DL140.
Disk system: <SEAGATE ST373207LC 0005> at scbus0 target 0 lun0(pass0,da0) on a ASC-29320A.
If I do:
[idle at hst ~]#dd if=/dev/zero of=bigfile bs=64k count=5000
5000+0 records in
5000+0 records out
327680000 bytes transferred in 37.515699 secs (8734477 bytes/sec)
All is hung while dd working.

Its look like that:
[idle at hst ~]#systat -v
skip...
Disks   da0 pass0 pass1   md0         
KB/t  20.33  0.00  0.00  0.00        
tps     812     0     0     0        
MB/s  16.11  0.00  0.00  0.00                      
% busy  100     0     0     0                                                               
skip...

[idle at hst ~]#vmstat -w 5
 procs      memory      page                    disks     faults      cpu
 r b w     avm    fre  flt  re  pi  po  fr  sr da0 md0   in   sy  cs us sy id
 1 0 0 1996756  54208   42  23  25   7  39 336   0   0  531  638 404 21  5 74
 1 6 0 2011488  13272  571   0   0   1 2506   8 129   0 1356 2125 159  3  4 93
 4 27 0 2022852   4228  267   0   0   1 1657   4 272   0 1450  784  96  1  3 96
 1 10 0 2027492  40220  449   5   0  12 3257 9384 574   0 1773 4366 189  8  7 85
 1 10 0 2035064  25604  746   2   1   7 2965 4466 157   0 1380 4486 184  8  7 85
 0 11 0 2030860   4228  515   2   1   3 2098  31 214   0 1412 3077 149 16  5 79
 3 14 0 2042792  10004 1014   4   1   5 2787 4432 267   0 1440 1735 118  2  4 94
 2 16 0 2062664   4228  301   0   1   2 2843  27 564   0 1759 1250 107  5  4 91


[idle at hst ~]#iostat -w 5
      tty             da0            pass0            pass1             cpu
 tin tout  KB/t tps  MB/s   KB/t tps  MB/s   KB/t tps  MB/s  us ni sy in id
   0   25  0.00   0  0.00   0.00   0  0.00   0.00   0  0.00  21  0  4  0 74
   1  109 62.56 242 14.81   0.00   0  0.00   0.00   0  0.00   7  0  9  0 83
   0  101 18.29 433  7.73   0.00   0  0.00   0.00   0  0.00   2  0  2  0 95
   0   76 26.45 281  7.25   0.00   0  0.00   0.00   0  0.00   3  0  4  0 93
   0   94 15.99 357  5.57   0.00   0  0.00   0.00   0  0.00   2  0  2  0 96
   0   15 16.04 667 10.45   0.00   0  0.00   0.00   0  0.00   0  0  3  1 96
   0   93 15.99 558  8.71   0.00   0  0.00   0.00   0  0.00   0  0  2  1 97
   0   80 16.16 252  3.98   0.00   0  0.00   0.00   0  0.00   0  0  1  0 98
   0  117 23.44 240  5.49   0.00   0  0.00   0.00   0  0.00   9  0  6  0 85

Why this happen? What should I do?
And one more question - how do I find process that doing heavy IO operations?
For now I suspecting zope that run on the server.
Thanks in advance.
Roman.