How to pass through a GPU to a VM with a "link width" of 16X ?

[anonlio]

Hi,

I have been enjoying my first Unraid setup since a few months
...and started playing around with gaming vms.

The motherboard I use (Asus x390) has 3 PCIe-16X and 2 PCIe-1X slots, but apparently can only use a video card at 16X if there is only one of the 3 PCIe-16X slot that is populated. Otherwise, it downgrades to 8X. Unfortunately, I do not have a integrated video card on my CPU so I can't "default" Unraid to use this and have a single PCIe-16X card pass through a vm.

 

Is there a popular work around this situation that is available to allow the 16X card to be pass through at full 16X to a vm?

I have been tinkering with:
- Replacing my motherboard with one that can offer 16X on more than one slot (if that's possible/available).

- Replacing my CPU for one with a integrated video card.
- Using a USB video card as the main/default Unraid video output to have only one PCIe-16X populated.
- Using a PCIe-1X (the much shorter slots) video card as the main or default Unraid video output to have only one PCIe-16X populated.
- Getting Unraid to not need a main video card ? I'm not sure if that is doable ?

many thanks for any help 🙂

Edited April 25, 2020 by anonlio
typos

[cpshoemake]

Honestly, I wouldn't worry about it. You're not going to notice the difference. With a 2080Ti, you're looking at a 2-3% performance hit. With anything else, it's pretty much nil. See https://www.techpowerup.com/review/nvidia-geforce-rtx-2080-ti-pci-express-scaling/

 

3 hours ago, anonlio said:

I have been tinkering with:
- Replacing my motherboard with one that can offer 16X on more than one slot (if that's possible/available).

Nope. Ryzen chips have 24 lanes: 16 for PCIe slots, 4 for NVME, & 4 for chipset. Those 16 PCIe lanes are going to be x16, x8x8, x8x4x4, or x4x4x4x4 depending on how you fill the slots. You'd have to move to a HEDT or server platform (Threadripper, Xeon, EPYC) to get multiple x16 slots.

 

3 hours ago, anonlio said:

- Replacing my CPU for one with a integrated video card.

This would work, but it's more money for not much gain.

 

3 hours ago, anonlio said:

- Using a USB video card as the main/default Unraid video output to have only one PCIe-16X populated.
- Using a PCIe-1X (the much shorter slots) video card as the main or default Unraid video output to have only one PCIe-16X populated.
- Getting Unraid to not need a main video card ? I'm not sure if that is doable ?

These are technically possible, but I've never seen a board that supports these configs.

 

 

[anonlio]

thank you @cpshoemake,

I am reassured.

My OC side can now fully enjoy the ride 🙂 !