Poweredge 800 multi-disk SATA slowdown?

[Brendan Conoboy]

Hello,

Does anybody else have a Poweredge 800 with multiple SATA drives?  I'm 
running Fedora Core 4 and seeing some unexpected bottlenecks when 
accessing multiple drives at one time.

There are 4 drives in my system.  If I do a raw read for 10G of any 
single drive, the sustained transfer rate is about 55MB/s.  Very nice.

The odd thing is, if I do a raw read of any two drives, 10G each, the 
sustained transfer rate of each drive is only about 27MB/s.  Likewise, 
reading 4 drives in parallel always tops out at around 13MB/s per drive. 
Would you expect this?

I know this isn't an all-inclusive benchmark like Bonnie++ or IOZone, 
but it's akin to the workload this particular system does.

This is with a 2.6.12 kernel and the ata_piix driver using the 4 port 
onboard controller.

Any advice or insight appreciated.

-Brendan (blc at redhat.com)