Thanks for the input, so that’s now four brands of motherboards all affected by usage of the 850 on PCH M2 slots, with the exact same throttled write speed problem.
I’ve updated the thread title to reflect.
Well, here’s WD Supports final response, which is interesting for all sorts of reasons…
Unfortunately we do not have such a list to provide to you.
With that being said, we are working with our engineering teams to investigate and resolve this issue swiftly.
We will get back to you as soon as we have an update on the matter
So lets just be clear on this. They cannot provide me with a list of supported x570 chipset motherboards for usage with the SN850 in PCH M2 slots, even though prior comms were blaming me for essentially using the SN850 in a manner (PCH slot) it is not intended to be used in.
This is also after I went back and confirmed that we’re now seeing the same problem across several brands of x570 motherboards and the problem is 100% consistently reproducible.
Well, I just received my SN850 and was eager to check it out, and I quickly saw the issue… 6300mbps-R / 3200~-W. Also x570 Asus Dark Hero and 5900x, plugged a 980 Pro and the problem wasn’t there. After reading all of this thread and some other forums I will have to return this drive and get something else, WD doesn’t seem to be taking this issue seriously so bye bye drive.
same problem with write speed on MSI X570 Unify. Also sometimes whole speed going down to 3.0 even if I switch PCH lanes to Gen4. Will try to refund it.
Dear All,
Root cause is SN850 needs 256bit pcie lane to get high write speeds.
Disable any devices on same pcie bus which are 128bit only and you will see significant increase in write speeds.
I just disabled my Intel WIFI Controller and Titan Ridge thunderbolt controller (which are on same bus and forcing the bus to 128bit) and I am now seeing well over 5GB/s on the writes as the pcie bus is now running at 256bit.
Sadly that’s not correct, or at least not as clear cut.
I have all my motherboard onboard devices disabled and it makes no difference. Will check again though, just to make sure.
Confirmed, all onboard devices disabled and even went through Device Manager and disabled additional stuff like Bluetooth and such. No joy, still capped to 3200.
Might well still have some hard relation to the bus capacity mind. WD have said they’re investigating the issue now, so we’ll see.
Hi,
Please check in aida64, post screenshot of PCIe bus where your SSD is located.
In the screenshot attached half way down you can see the Maximum payload size on this bus for my motherboard is 128bit and I get low write speeds (3.2Gbps), when I disable Thunderbolt 3 and onboard Wi-Fi (these devices are on same bus as SN850) this max payload then rises to 256bit and I get full write speed (5.3Gbps) on the SSD. This is definitely the cause, so post screenshot of your pcie bus. Hope this helps 
Interesting. How do I pull that report up in Aida?
Sorry, made a mistake, info shows in HWInfo64
Thanks but same question, sorry. How do I get that report up?
Expand your pcie lanes on the hw profiler
I realise given my build this may make me sound a bit stupid, but how specifically?
When I run HWINFO, I get the option of Summary or Sensors. Neither gives the ability to expand PCIE lane, with the Summary having seemingly no interaction and the Sensors not having anything to expand.
Could you drop me just a few bulletpoints on what actions to take at each point?
I have no idea what that view is, I’m on the same version of HWINFO, v7.04-4480 and all I have is either a Sensor Status view or a Summary View, as per below.
Do you have a paid version or something? Pro maybe?
Just uncheck boxes on startup and press « run »
Well, that’s about as intuitive as something not very intuitive! But it works!
Right, so what I can see is that one of my PCI Buses has a Max Payload of 128bytes but all the devices on that bus that I can disable, such as onboard audio, has been disabled. The rest, like USB hubs just aren’t an option to disable.
SN850 on M2_3 (CPU)
SN850 on M2_1 (PCH)
So, yes, this appears to bear out that the PCH M2 slots, sharing devices on the chipset, is limiting bandwidth and that being the case, no wonder WD have no comment to make. Because this is a fundamental hardware incompatibility to PCH M2 slots, if the core design requires a Max Payload of double that which is being set.
Which device on the bus is making the whole bus run at 128byte?
Looks like there are several on there. USB controllers, Audio, the LAN controllers, SATA controller. Quite the collection of onboard devices on that PCI Bus on the MSI Meg x570 it seems.
I have a Samsung Pro 1TB NVME on M2_2, also PCH, and that’s quite happily chugging away at 256 bytes. However, interestingly, I previously had a Samsung Pro 500GB on M2_1, just like my current SN850 is on M2_1 and that was running at the full write speeds, with no other change to my system around devices being disabled/changed, indicating that the SN850 appears, like you say, to need less other devices on the bus in order to be effective.
I do have a WD SN750 and it has the exact same problem with MSI B550i Gaming motherboard.
I am using the drive in the second M2 slot (PCH) and it shows only write speed of only 1100 mbps vs read speed of 3400. The MPS/Maximum Payload is only 128.
When I follow your suggestion and disable the built-in WIFI and Bluetooth, the MPS increases to 256 and with it the write speed of the SN750 increases to 3400. Too bad I need the WIFI and Bluetooth.
I hope WD is aware of this, and the same firmware fix they are providing now for the SN850 will also be provided for the SN750.
