Adaptec vs Areca vs HighPoint

[IanB]

Yes but PCIe x1 has a 250MB/s ceiling. If you got a PCIe 4x or 8x software card, wouldn't this raise the ceiling to 1GB/s or 2GB/s respectively?

Edit: I just read Steve's post. I guess what I'm trying to say is that, 1. if you have a software RAID card that is say 8x PCIe in a slot that is full bandwidth 8x, shouldn't you get close to full transfer speed assuming you have enough drives to bring it there?
If not, then was IS the bottleneck?

I'm assuming that you implementation Serra, of 3-4 drives at 69MB/s max on a software raid card, 69 * 4 drives = 271MB/s If you had a PCIe x1 card then yeah, you're over the limit.

ETlight

But unfortunately you don't get software-only cards with x8 interfaces. Or ones with x4 interfaces that have more than 4 ports.

Look at the Adaptec 1430SA - a good little software card for RAID 1/0. PCIe x4 interface, but restricted to 4 ports. The interface could handle up to 8 100MB/s SATA drives really in terms of raw throughput, but you're not going to get the chance to test your scaling theory as no-one is releasing an 8-port card in that form factor that'll allow software-only RAID.

I'm going to assume that releasing such a card would kill sales of their overpriced and therefore profitable hardware solutions. Consider that you buy a motherboard with seriously larger amounts of real estate, many more components and a much more complex manufacturing process as a result, for around £75 to £200 max, or $150 to $400. How on earth can the price of a hardware RAID card be justified when the starting point is the same as a motherboard with the latest Intel chipset on it? FYI the "simple" Adaptec I mentioned above is around £70 in the UK, the same price as a mainboard.

EDIT: but since I've never written a low-level hardware driver, I'm not entirely sure what the implications are there. As I suggested in a post above, software RAID has to handle each port/drive individually, so there may be a limitation on the number of ports that is practical - if every drive needs to communicate to the driver, then there will be one system interrupt for each drive on the card to say "My data is now ready, come and get it". Hardware RAID is reducing this overall system overhead significantly, because that is all handled on-card and the card only needs to present the system with a single interrupt for every data block requested. That's got to be a valuable trade-off, but I can't really quantify the interrupt effect on the system overall.