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-   -   z370 + 8700k + M.2 + PCIe question about PCIe lanes sharing (https://www.overclock.net/forum/6-intel-motherboards/1640221-z370-8700k-m-2-pcie-question-about-pcie-lanes-sharing.html)

ASUSfreak 10-17-2017 01:27 PM

I'm planning to buy the rig in the title but it drives me crazy atm...

I just don't understand.


Can I run 1 card at 16x AND an NVMe drive (M.2) at 4 speed?


Or is it because CPU only has 16 lanes it will cut it to 8x for graphics card and 4 speed for M.2? --> intel.ark shows 16 PCIe lanes for 8700K

Or does the CPU takes care for 16 lanes and the motherboard chipset takes care for additional 4 lanes? --> ASUS site shows 16x cpu and 4x chipset somewhere but can't find it anymore biggrin.gif


I really can't figure that out/understand it...


I just want to build it when nVidia Volta comes out next year... and I want only 1 card at 16x and M.2 also IF it does not "steel" lanes from videocard...

chessmyantidrug 10-17-2017 02:22 PM

Short answer: yes.

Longer answer: Your CPU offers 16 PCI-e 3.0 lanes that would be allocated to x16 slots. If you're only using one slot, it will allocate all 16 lanes to that slot. The M.2 slots are supplied lanes from the Z370 chipset.

Sin0822 10-18-2017 07:19 PM

I don't think any Z370 motherboards have M.2 slots routed to the CPU directly, they are, for the most part routed to the PCH, which has 24 PCi-E lanes. So no, you can even install 3x x4 PCI-E 3.0 M.2 drives and not interfer with your CPU's x16 for GPUs.

ASUSfreak 11-02-2017 09:14 AM

Aah good

Xel_Naga 11-02-2017 09:47 AM

Quote:
Originally Posted by Sin0822 View Post

I don't think any Z370 motherboards have M.2 slots routed to the CPU directly, they are, for the most part routed to the PCH, which has 24 PCi-E lanes. So no, you can even install 3x x4 PCI-E 3.0 M.2 drives and not interfer with your CPU's x16 for GPUs.

This is correct.

once you settle on a board you would like. Reference the manual most companies include a PCI-E layout or explanation for the M.2 drives.

For example on the Gigabyte Z370 Gaming 7 has 3 M.2 slots. Top slot has dedicated PCI-e lanes from the PCH, Middle slot shares lanes with two sata ports and disables said sata ports once a drive is installed, and the bottom slot shares with the bottom most PCI-e card slot.

GBT-MatthewH 11-02-2017 04:08 PM

This may help visualize it. As others have said the PCIe slots will go to the CPU, the rest will go through PCH. Different boards will have switches in different spots, but AFAIK all boards from all manufacturers use the CPU lanes for PCIe slots.


pantsaregood 11-03-2017 05:13 AM

Also please be aware that "the PCH provides 24 lanes" is a bit deceptive, as the PCH is connected to the CPU with a DMI 3.0 link, which has the same bandwidth as a PCI-E 3.0 x4 link.

What this means is that even though each device connected to the PCH can communicate to the PCH at PCI-E x4 speeds, the PCH can only communicate to the CPU at PCI-E x4.

Bluntly, two nVME SSDs going through the PCH would be using a single PCI-E x4 link to try to move the bandwidth a PCI-E x8 link would carry.

HKPolice 11-04-2017 01:09 AM

Quote:
Originally Posted by GBT-MatthewH View Post

This may help visualize it. As others have said the PCIe slots will go to the CPU, the rest will go through PCH. Different boards will have switches in different spots, but AFAIK all boards from all manufacturers use the CPU lanes for PCIe slots.


Which M.2 slots on the board are shared & which one is direct?

pas008 11-04-2017 01:56 AM

Quote:
Originally Posted by pantsaregood View Post

Also please be aware that "the PCH provides 24 lanes" is a bit deceptive, as the PCH is connected to the CPU with a DMI 3.0 link, which has the same bandwidth as a PCI-E 3.0 x4 link.

What this means is that even though each device connected to the PCH can communicate to the PCH at PCI-E x4 speeds, the PCH can only communicate to the CPU at PCI-E x4.

Bluntly, two nVME SSDs going through the PCH would be using a single PCI-E x4 link to try to move the bandwidth a PCI-E x8 link would carry.

dont they have to be tugging at the bandwidth at the same time though

Sin0822 11-04-2017 07:58 PM

Quote:
Originally Posted by HKPolice View Post

Which M.2 slots on the board are shared & which one is direct?

There is the manual, and it says which M.2 slots are shared with SATA or the last x4 slot. There are three m.2 slots, so if you just guess, you have a 1/3 chance of picking the one that isn't being shared! Althought if you don't fill the last x4 slots, you now have a 2/3 chance, and if you use no SATA ports then you have a 100% chance!
Quote:
Originally Posted by pas008 View Post

dont they have to be tugging at the bandwidth at the same time though

This is very true, so far, aside from sequential speeds on three RAIDed high speed NVMe drives through the M.2 slots, nothing really has been bottlenecked.


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