strange i/o and tty lockups/delays on 6.1, strange sync() issue

[Marek Blaszkowski]

Hi,
my config:
thunder k8sr, dual opteron 244, 8GB RAM, FreeBSD 6.1, sata disk


for time to time (every 10 or 20 secs) any process which try write or read any, even small file,
is locked for a couple seconds (about 7 or 9 secs), very iritating when I try save
small files edited in vim (size about 2-3 kbytes), it's looks like this:
#vim /path/to/file
...editing..
:w
and now (afetr w command) vim freeze for 7-10 secs until it displays "file written".

Delays accours on tty, too. In vim I can't move around edited file (up/down in rows),
vim just got hang, strange is that I can switch (durning vim lockup) beetwen virtual screen
consoles.

System is not swapping (4GB ram free), iostat says:

#iostat 1
      tty             ad4              ad6             cpu
 tin tout  KB/t tps  MB/s   KB/t tps  MB/s  us ni sy in id
   1  353 17.94  45  0.78   0.64   0  0.00  11  1  6  1 82
   0  360 16.00  72  1.12   0.00   0  0.00  17  0  2  1 81
   0  120 16.00  77  1.20   0.00   0  0.00   6  0  3  1 90
   0  120 14.66  73  1.04   0.00   0  0.00  41  0 11  1 47
   0  120 16.00  78  1.22   0.00   0  0.00  27  0  1  0 72
   0  120 16.22  74  1.17   0.00   0  0.00   0  0  0  0 100
   0  122 17.22  79  1.33   0.00   0  0.00   8  1  5  0 85
   0  120 16.46  79  1.27   0.00   0  0.00  14  0  3  2 81
   0  120 13.85  48  0.65   0.00   0  0.00   7  0  4  0 88
   0  120 15.13  79  1.17   0.00   0  0.00  12  0  4  1 83
   0  120 11.11  83  0.90   0.00   0  0.00   8  0  3  1 89

as You see, there is no heavy i/o load.

Strange things is I have two identical setup of the same system configuration with the same
apps running (web server mirror) but other motherboard (K8W), on the second system there no such delay/lockup
issue.
It's looks like some other process calls sync() on every 7 or 10 seconds and it couse such i/o delay.
Other question is why sync takes so much time, when i/o disk performance its not bad (tested with dd is about 50MB/sec)
and iostat (durning sync calls) doesn't show any high disk transfers.
Example:
#time sync
real    2m0.853s
user    0m0.000s
sys     0m0.145s

2 mins was the longest time amount I saw durning tests, in most cases sync takies about 10 secs, when sync was called on the
other tty iostat doesn't print any high transfer, never it exceed 1MB/s.
Qestion is: if disks transfers (using cp or dd) are high, why sync waits so long ?

I played with vfs.write_behind=0 but it doesn't change anything.

PS.
I checkced motherboards manuals: K8W and K8SR have the same sata chip (Silicon Image SIL3114), so I think
above problems are not ata driver issues.

regards,
-- 
Marek B.