You got the point there, without noticing
Those "onboard IDE raid" mainboards use standard dual channel PCI IDE chips,
and then put (CPU driven) firmware and drivers on that do the mirroring and striping.
If you want RAID 5 for true redundancy et al, you need to have a separate RAID processor somewhere that handles
the drives transparently, so that the CPU sees a single mass storage unit. That's what these RAID SCSI cards do, they have
a number of standard SCSI chips on, but these aren't visible as SCSI controllers to the CPU, they're hidden behind a RAID
entity that distributes and reassembles the data that then go over the PCI bus.
There's also a massive difference in PCI load. Do mirroring with the former, cheap CPU driven solutions, and enjoy the
doubled PCI bus load. Not something you want when you're after performance. With real got-my-own-brain RAID solutions,
just the assembled data go over the PCI bus.
Besides, what do you think onboard controllers connect to? Thin Air Warp Speed bus? No, it's just the same PCI bus that runs
to the slots as well. Whether you have it on a card or on the board, it's going to eat into the same PCI bus bandwidth.
(The exception being chipset-integrated components, and coming soon onboard native HyperTransport controllers.)
regards, Peter