PCI on PCIe motherboards; separate bus?

[erwos]

Odd question, but does each PCI slot on a PCIe motherboard have its own bus (eg, full 133mB/s bandwidth each), or do they still share a common bus (and therefore, bandwidth)?

I'm just trying to determine whether I'm going to run into horrible bottlenecking if I slap more than one 4 port SATA PCI card on modern motherboards... I understand there are PCIe cards of this sort, but they're also double the price, and I'm trying to stay in-budget...

 

[Heidfirst]

PCI is a common bus so it's still going to share bandwidth with any other PCI traffic.

 

[erwos]

Originally posted by: Heidfirst
PCI is a common bus so it's still going to share bandwidth with any other PCI traffic.

Except that's not true, to my understanding. Server motherboards have had multiple PCI busses for forever. I was under the impression that PCIe motherboards didn't actually have native PCI - that they simulated PCI with a PCIe lane. The question, then, is whether they're using multiple PCIe lanes for multiple slots, or just one lane.

Then again, how bad is the bus contention really going to be for six SATA drives?

 

[tcsenter]

Except that's not true, to my understanding. Server motherboards have had multiple PCI busses for forever.

PCI is in fact a common bus. Motherboards with more than one PCI bus have more than one PCI bus (and separate PCI controller bridge chip).

As you imply, this is found on server class chipsets only. Desktop chipsets don't support multiple PCI conventional busses, at least most of them don't (none that I know of).

The question, then, is whether they're using multiple PCIe lanes for multiple slots, or just one lane.

The number of lanes is determined by the maximum operation or mode of the slot. If its a PCI x1 slot, it uses 1 PCI Express lane; PCI x4 = 4 lanes, etc.

Each PCI Express slot has dedicated bandwidth to and from the chipset.

 

[Madwand1]

Originally posted by: erwos
[Then again, how bad is the bus contention really going to be for six SATA drives?

Two modern drives can easily saturate a PCI bus, and before the year's over, one SATA drive might be able to by itself. This can easily be seen in performance tests of RAID arrays on PCI controllers, which flat-line at around 100 MB/s whereas on-board, PCIe and PCI-X ones don't.