MP 7,1 PCIe NVME configuration

[Eneco]

Hi,

I have a general question about the use of multiple NVME drives with PCIe cards in a Mac Pro.

When I want to add 2 separate NVME drives (no RAID configuration) to the system, what is the best approach?

a) Every NVME drive on its own PCIe card in its own PCIe slot.
b) Both NVME drives on the same PCIe card using the same PCIe slot.

From my non-technical point of view I would say, that a) gives more performance per drive, as every drive can use the full bandwidth of its own PCIe slot, whereas with b) both blades would need to share the same bandwidth of the same PCIe slot. But I don't know if NVME drives can even max out a PCIe slot's bandwidth.

So the question is: Where is the bottleneck here - the NVME drive's performance or the PCIe slot's bandwidth?

 

[tsialex]

You are forgetting that M.2 NVMe blades are x4 PCIe devices, you will never use the full bandwidth of a Mac Pro PCIe slot with one M.2 blade.

 

[joevt]

MacPro3,1 supports PCIe gen 2 x16 for slot 1 and slot 2 and only gen 1 x4 for slot 3 and slot 4.
MacPro4,1 and MacPro5,1 supports PCIe gen 2 x16 for slot 1 and slot 2 and only gen 2 x4 for slot 3 and slot 4.
MacPro7,1 supports PCIe gen 3 x8 or x16.

NVMe devices are gen 3, gen 4, or gen 5 with usually 4 lanes.

If you want to connect a gen 3 x4 NVMe then you can connect it directly to MacPro7,1 (with adapter). For earlier Macs, use a gen 3 PCIe card with at least 8 lanes.

If you want to connect a gen 4 x4 NVMe then it should be on a gen 4 PCIe card with at least 8 lanes for MacPro7,1 and 16 lanes for earlier Macs.

If you want to connect a gen 5 x4 NVMe then it should be on a gen 5 PCIe card with 16 lanes for MacPro7,1.

The PCIe cards need to have a PCIe switch chip since Macs don't support bifurcation. PCIe switch chips are expensive.
https://forums.macrumors.com/threads/pcie-ssds-nvme-ahci.2146725/

 

[Eneco]

Thank you very much for this detailed explanation.