Problem with concurrent SATA-Writes

[Tobias Reinhard <tracer@robotech.de>]

I have a problem with two SATA-Discs. I have an onboard SIL3114 with
four SATA-Ports an onboard NVIDIA with two SATA-Ports - both controllers
are disable via BIOS (and are not detected by Linux)(only for this test
of course - normally I have other HDD on this ports). The only
controller that is found is the add-on controller in a PCI-Slot
(SIL3112). And that is the one I have trouble with.

If I read or write  (via dd) from the first one -> no problems. Same
when read or write from the second or when I read (only read!) from both
at the same time. Data-Transfer-Rate is around 45MB/s for one HDD.

The problem occures when I try to write on both discs at the same time.
For example I write /dev/zero to the first one and then start to write
/dev/zero to the second one. The System-Load goes up to 4 with nearly
100% io-wait and nearly no write-access to the drives.

Any hints?

- no errormessages in syslog
- happens with Kernel 2.6.11.7 and with 2.6.12-rc5
- HDDs are Samsung Spinpoint 200GB
- (I use the SCSI-SATA-Drivers)

Tobias