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slayme1974

Pci Expess 2.0 and 3.0 together

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Hi I have a question.

Ive noticed many motherboards have pci-e 3.0 x4 nvme support. Many motherboards these days have only pci-e 3.0 slots. Will a m2 ssd still run at pci-e 3.0 if I plug in a pci-e 2.0 card?

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PCIe devices and hosts should be backward compatible, e.g. PCIe 1.0 card works in 3.0 slot and 3.0 card works in 2.0 slot.

Mix/match situations where slower cards are put in faster slots shouldn't drag the others down.

The usual limitation that comes into play is that only so many PCIe lanes are available and can be reassigned. The usual case is that x16 slots can lose lanes if more than a certain number of slots of lesser capability are populated. Also PCIe lanes can be taken by USB and Sata devices, sometimes Bios config allows prioritorizing which type gets to run faster.

 

It varies depending on board, chipset and CPU. Generally the server type socket types e.g. LGA 1366 and 2011 will have more lanes available by default and suffer less resource loss as described with lots of slots populated.

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Yes, it will just run at 2.0 speeds, PCIE slots are backward compatible, well they kinda have to be,

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Yes, it will just run at 2.0 speeds, PCIE slots are backward compatible, well they kinda have to be,

I know a pci-e slot will never work faster that the generation it was configured for and they are backward compatible but that's not what I was asking.

What I mean was, say a had a pci-e 3.0 nvme ssd such as the samsung 950pro and I installed, say a pci-e 2.0 raid card, will the pci-e 3.0 nvme ssd still run at pci-e 3.0 speed?

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Nup. I assume you mean the drive is hanging off the Raid card? There'll be slowdown to match the slowest point in the chain. Actually, probably a bit slower again due to overhead.

 

As an example, my machine has a mix of PCIe 1.0 and 2.0 slots and only Sata2 onboard. To get better performance from my SSD I hang it off a Sata3 Raid card that's on a PCIe 2.0 x1 card (drive is just single, not Raid).

So, the PCIe 2.0 port gives a 500 Meg/second IO rate. The Sata3 ports on the card are good for 600 Meg/sec. But the best-case burst rate I get is 500 Meg/sec.

In reality though I benched it @ 375 Meg/sec which would be around what's expected of that model of SSD.

Edited by Rybags

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Nup. I assume you mean the drive is hanging off the Raid card? There'll be slowdown to match the slowest point in the chain. Actually, probably a bit slower again due to overhead..

No what I meant was if the drive was install in a pci-e 3.0 x4 slot and I put say, a pci-e 2.0 raid controller with 2 hard drives in raid 0 into a pci-e 3.0 x1 slot, will the pci-e x4 drive still operate at pci 3.0 speed and get about 2GB/s?

Edited by slayme1974

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This is getting confusing.

 

First up, the PCIe version determines what speed each set of lanes can operate at. e.g. PCIe 1.0 is 250 Meg/sec, 2.0 is 500 Meg/sec, 3.0 is 985 Meg/sec. Backward compataiblity means newer card works in older slot, newer slot accepts older card etc.

Next up, the x1, x16, whatever, decribes how many lanes are available. This is something that can vary depending on resource allocation and in fact can be less than you'd think from the start as some boards have extra x16 slots that are incapable of operating that many lanes and are usually only x4 or x8.

 

But the bottom line is, the best case speed you achieve will be determined by the "slowest" bottleneck in the system. Whether that be the fact that a card only is x1 and uses just that one pair of lanes even in an x16 slot, or the fact that you have a nice fast PCIe 3.0 card or device but it's only in a PCIe 2.0 slot.

(in saying that, I don't mean that everything will slow down to PCIe 1.0 speeds because a particular motherboard has both PCIe 1.0 and 2.0 slots, one shouldn't handicap the other).

 

When playing around with physical configurations and trying to get the best out of a drive, I'd recommend grabbing the AS SSD benchmark. For a quick test, uncheck all except the "Seq" tick box then run it to get an idea of your best-case read and write speeds.

Edited by Rybags

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you still haven't answer my question. If I have only 3.0 slots (some x1,some x4, some x16 etc) will plugging in a pci-e 2.0 card, say a raid controler with 2x ssds in a raid 0 array into a pci-e 3.0 slot will all the pci-e 3.0 will that slow all the pci-e 3.0 slots (some x1, some x4 some x16) down to 500 MB/s per lane eg. 8GB/s for x16 or will the pci-e 3.0 bus operate at the full 985MB eg just under 16GB/s for x16?

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OK... no, the other slots shouldn't be affected. Generally with PCIe stuff, any slowdown a system experiences will be due to resource reallocation, as in the example where the extra x16 slot/s for graphics cards can lose lanes when lots of other slots are populated.

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Can pci-e 3.0 cards plugged into pci-e 3.0 slots work at 985MB/s even if I have some PCI-e 2.0 cards installed in other PCI-e 3.0 slots? I know that the Pci-e 2.0 slots wil work at 500MB/s (that's basic it)

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The slower cards shouldn't affect the other slots. The big advantage of PCIe is that the lanes are dedicated and independant so you don't get slow devices or data hogs killing everything else like you could with the old PCI.

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I think PCIE is electrically determines the speed by what is plugged and doesn't have contend with bus contention issues which affect USB and other tech since it's direct to the motheboard.

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There's pins dedicated to informing the host what size card is present. Lanes/speed are negotiated at initilisation time (driver startup?) Unlike older PCI, each lane is a dedicated 2 way synchronous serial link between the host and peripheral so no bus mastering, no contention other than the fact that lanes can be reassigned depending on physical config and Bios/UEFI settings.

Edited by Rybags

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