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I am testing this board: Gigabyte B550M DS3H > pdf manual

It has two M.2 slots and I want to also use two PCI-e connectors with adapters to add 2 more NVME drives (so 4 in total). It is for a MySQL db server, spreading read/write load.

I am confused about all the PCIe speeds because of the 3/4 version and x1, x2, x4 and so on.

Let's asume the actual NVME drives are fast, PCIe 4 running at 8GB/s.

  • First M.2 slot says PCIe 4.0 x4 so it means 8 GB/s ?
  • Second M.2 slot says PCIe 3.0 x2 so that is only 2GB/s ?
  • 1 x PCI Express x16 slot (PCIEX16) [...] support PCIe 4.0 x16 mode (main GPU connector), how many GB/s is that ?
  • 1 x PCI Express x16 slot (PCIEX4) [...] Supporting PCIe 3.0 x4 mode that is 4GB/s, correct ?

So I am trying to use the two wide PCI connectors to connect NVME drives using 16x PCIe NVME adapters.

For a video card I plan to plug a basic one with x1 connector in the remaining slot in the middle.

What are the maximum speeds for above ports ? And anything else that could go wrong with this ?

A second question ... will 2 very fast NVME (e.g 8GB/s) perform better both connected in same one 4GB pci port than each NVME connected to two separate but slower 2GB/s pci ports ? (By "two in one port" I mean using a NVME adapter with two NVME slots). Can they slow one another if on same motherboard PCI slot ?

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  • A PCIe 4.0 x16 slot would provide a potential maximum 31.508 GB/s transfer rate. In reality, the real limit will be determined by the device which is likely limited to 8 GB/sec. Due to the numerous grammatical errors in your second question, I cannot make heads or tails of it. I have no idea what you are attempting to ask.
    – Ramhound
    Sep 8 at 7:11
  • Thanks, I reformulated the second question, does it make more sense ? :) I mean two NVME both connected in same motherboard PCIe 4GB/s slot vs two NVME connected in separate motherboard PCIe slots but 2GB/s each port.
    – adrianTNT
    Sep 8 at 19:13
  • No; It does not. You only have a single PCIe 3.0 x2 slot. So only one of your devices would run at that speed. However, most if not all motherboards, cannot use all of the PCIe slots if the M.2 slots are used.
    – Ramhound
    Sep 8 at 19:15
  • OK, I need to check with Gigabyte on that one, I came across this before but I don't think it's in their manual.
    – adrianTNT
    Sep 8 at 23:18
  • It absolutely is in the manual
    – Ramhound
    Sep 9 at 0:30
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+50

The Gigabyte B550M DS3H (rev. 1.x) has the information that was asked:

enter image description here

The PCIe 4.0 x16 port can theoretically deliver up to 31.5 GB/s, while the PCIe 3.0 x16 can do up to 15.8 GB/s.

You will find below the theoretical limit of all ports together (again, very theoretically):

enter image description here

While Gigabyte boasts that theoretically this design delivers up to 64 Gb/s data transfer speed, it should be remembered that when launched, this was the cheapest and the lowest-level motherboard in the B550 product stack.

It's also normal for the highest-performance ports to have lane priority over the others, so devices connected to lower-performance ports, when the higher-performance ports are active, may perform at a fraction of their speed (this can be as low as one-half and one-fourth - example).

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  • M.2 slots are always x4 and each M.2 drive can at most use 4 lanes even if by some means connected to a PCIe slot that actually provides 16 lanes. So x16 is not relevant here.
    – Tom Yan
    Sep 11 at 9:03
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    @adrianTNT Yeah like if you would like to connect more than one PCIe 4.0 drive, that could probably help. Note that PCIe bifurcation requires support from the motherboard though. Also, some of the PCIe x16 slots on a motherboard could provide less than 16 lanes each. Ultimately the numbers are bound by the total number of PCIe lanes provided by the CPU and so.
    – Tom Yan
    Sep 12 at 2:41
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    From the manual page 28: "PCIEX16 Bifurcation Allows you to determine how the bandwidth of the PCIEX16 slot is divided. Options: Auto, PCIE 2x8, PCIE 1x8/2x4, PCIE 2x4/1x8, PCIE 4x4. (Default: Auto)". So the motherboard does support up to a 4x4 mode. What is your CPU?
    – harrymc
    Sep 12 at 8:20
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    My analysis: My above link to the motherboard says "20x usable PCIe 4.0 lanes with a supporting AMD Ryzen Processor, Up to 10x usable PCIe 3.0". Your Ryzen CPUs all have 24 lanes, 16 going to the GPU, 4 to a dedicated M.2 drive and 4 to the chipset/motherboard. From there, the chipset separates that into the above 20 chipset lanes, all at PCIe 4.0 speeds, but being shared over 4 CPU lanes. Your four NVMe disks take up only 16 lanes, so you should be good regarding connections, but you will only run 2 nvme drives at full speed.
    – harrymc
    Sep 12 at 10:59
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    If you don't use the 16x for the GPU, this frees 16 lanes that you can use for the 4 NVMe at full speed in parallel, as mentioned in the manual about PCIEX16 Bifurcation. I thought this was out of the question for you, but if you don't need the slot for the GPU, then it might be the best solution. Just don't ask me to recommend hardware.
    – harrymc
    Sep 12 at 17:32

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