[SOLVED] Performance of a PCIe x8 card in a x4 slot

[Bongert]

I was intending to put a raid card in my computer instead of getting a separate NAS and decided on an x8 card. The x16 slot will obviously be populated by a GPU. I looked up a nice motherboard that sports a x16 as well as an x8 slot (electrically) but found out that no AM4 based motherboard will run those two slots at full capacity simultaneously. They either run the 16x at full capacity and turn off the x8 or run both at x8 capacity. though im only talking about the two that would be connected to the CPU and not the chipset.

So my next best guess was to just put the x8 card in a 4x slot. and though i know that that would obviously impact performance i dont know clearly to what extend. To clarify a bit more i was eyeing the MSI MEG x570 Unify, though that doesnt really matter as all x570 boards should have the same amount of PCIe lanes. The raid card i chose is this one and i wanted to hook up 4x2 TB HDDs and run them in raid 5.

Now my question essentially boils down to if the limitation of the x4 slot relative to the x8 card would bottleneck my storage performance. I assume a bunch of hard drives coulndnt max out 8 full PCIe 3.0 lanes, but then again the only x4 raid cards are for PCIe 2.0 and there are only 3 of them.

 

[Barty1884]

Likely theoretical limitations, but in practice x4 3.0 should be sufficient.

I'm curious though, why not just run the x8 card in the x8 slot?

While yes, the primary x16 will drop to x8:

1. There's only a few workloads that'll exhaust PCIe 3.0 x16 - even then, that's only likely possible with Titans, 3090 etc.
2. Are you running a 3000 series CPU? If so, you're looking at PCIe 4.0 - double the bandwidth, so your x4 4.0 slot has the same bandwidth as 3.0 x8 (and 4.0 x8 = 3.0 x16)

 

[kanewolf]

Now my question essentially boils down to if the limitation of the x4 slot relative to the x8 card would bottleneck my storage performance. I assume a bunch of hard drives coulndnt max out 8 full PCIe 3.0 lanes, but then again the only x4 raid cards are for PCIe 2.0 and there are only 3 of them.

HDDs are typically around 150MB/s -- 1.2Gb/s -- So four disks would have a maximum of 5Gb/s -- LESS than a single SATA which generally needs only a single PCIe lane. Your 4 HDDs should not be hindered by an x4 slot.

 

[Bongert]

Likely theoretical limitations, but in practice x4 3.0 should be sufficient.

I'm curious though, why not just run the x8 card in the x8 slot?

While yes, the primary x16 will drop to x8:

1. There's only a few workloads that'll exhaust PCIe 3.0 x16 - even then, that's only likely possible with Titans, 3090 etc.
2. Are you running a 3000 series CPU? If so, you're looking at PCIe 4.0 - double the bandwidth, so your x4 4.0 slot has the same bandwidth as 3.0 x8 (and 4.0 x8 = 3.0 x16)

ive looked a bit into it, and though the article was a bit dated and compared a 1080 in a x16 slot vs x8, there really seems to be virtually no difference though i would be curious how that holds up to current hardware.

I dont know how this works, would a card specified for 3.0 x8 really be able to use the bandwidth of 4.0 lanes? i assumed the 3.0 card would simply be the bottleneck
Ok, i think i found some good info and will probably go with the much cheaper Gaming Plus board