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Question Ryzen based motherboard and PCH lanes

Jul 11, 2020
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I did not yet found an answer, so I try here.

Say I have a R5 2600 CPU and a B450 based motherboard. I understand how work the CPU PCIe 3.0 usable lanes, which are shared between the GPU (1x16 or in some case with crossfire 2x8), the NVMe (1x4) and the chipset (1x4).

So if I add, say, a Radeon RX 5700 at 1x16 PCIe 3.0 lanes and the NVMe at 1x4 PCIe 3.0 lanes, it remains the 4 PCIe 3.0 lanes for the PCH.

So we have 4 PCH PCIe 3.0 lanes that come from the CPU (R5 2600). Then the chipset (B450) can add some more lanes on top of them.

Now, here my questions :
  1. How many PCH PCIe 2.0 lanes a SATA 3.0, 6.0 Gb/s disk takes? Only 1 PCIe 3.0 lane (1.97 0.985 GB/s) should be more than enough as 6 Gbps = 750 MB/s (or 0.75 GB/s) and PCIe 2.0 x4 = 2 GB/s. So I should be able to use up to 4 disks without any bottleneck.
    But, according to here, each disk takes 2 PCIe 3.0 lanes, so 8 PCIe 2.0 lanes??? Seems wastfull? Why?
  2. If I add only 2 SATA disks in a B450 motherboard it should be ok, even if I split the OS between these disks. Now how can I avoid any bottleneck if I want to add more than 2 disks, say:
    • a total of 4 disks (4 SSD or 2 SSD and 2 HDD) even for RAID 1 or RAID 10?
    • a total of 3/4 disks where the OS is "splitted" (say one disk for the OS, one disk for storage, one disk for work, etc…)
  3. B450 chipet can add up to 6 PCIe 2.0 lanes, but how it deals with bottleneck? Indeed, those 6 more PCIe 2.0 lanes exceed the 4 PCIe 3.0 lanes available as these latter can only be use as 8 PCIe 2.0 lanes, but here, if I am not mistaken, we should get something like 14 total PCIe 2.0 lanes…

    According to AMD, there are 36 PCIe 3.0 Total Lanes but only 28 Usable. So the 1x16 PCIe 3.0 lanes + the NVMe at 1x4 PCIe 3.0 lanes + the 1x4 PCIe 3.0 for the PCH. We get 24 lanes. What does AMD mean? 28 PCIe 3.0 Usable lanes with bottleneck as 4 lanes are added?

  4. How to calculate if there is any bottleneck without benshmarking?
  5. Should I buy a X570 motherboard with a R5 3600 to avoid any bottleneck?
 
Last edited:

rgd1101

Titan
Moderator
got the pcie 3.0 lane 1x speed worng. go to the wiki page

https://www.amd.com/en/products/chipsets-am4#paragraph-300951
‘Usable PCIe Lanes’ defined as the total concurrent Graphics, NVMe, and GPP PCIe® lanes from the chipset and processor that can be used concurrently with a Ryzen processor.

depend on the motherboard, most B450 have m.2 and 4 sata

get https://www.tweaktown.com/news/73721/amd-ryzen-threadripper-pro-3995wx-64c-128t-128-pcie-lanes-and-more/index.html
128 PCIe lanes
 
Jul 11, 2020
4
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10
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got the pcie 3.0 lane 1x speed worng. go to the wiki page
Thanks, corrected.

https://www.amd.com/en/products/chipsets-am4#paragraph-300951
‘Usable PCIe Lanes’ defined as the total concurrent Graphics, NVMe, and GPP PCIe® lanes from the chipset and processor that can be used concurrently with a Ryzen processor.
Ok, "total concurrent", but not without potential bottleneck, especially with the OS divided between disks.

depend on the motherboard, most B450 have m.2 and 4 sata
Ok, so as the PCH lanes in B450 are PCIe 2.0 lanes, and PCIe 2.0 running at 2x equals 1.0 GB/s matching the PCIe 3.0 x1 (985MB/s), I should be able to use 4 SATA disks, right? + 1 m.2 disk on the CPU or say 1 m.2 disk on the PCH (if available) + 3 other SATA disks?

While 128 PCIe lanes are pretty cool, a X570 motherboard with a R5 3600 will do, in my case, the job, at a lower price.
 

kanewolf

Titan
Moderator
For example, with this B450 motherboard
I should be able, without any bottleneck, to use up to:
  1. a Radeon RX 5700 at 1x16 PCIe 3.0 ;
  2. a NVMe at 1x4 PCIe 3.0 lanes, on the m.2 slot ;
  3. 4 SATA Disks even at RAID 1.
And if I want 4 SATA disks at RAID 0 or RAID 10, I need at least a X570 based motherboard and a R5 3600 CPU.

Is it correct?
That B450 says RAID 0, 1, 10 with 4 SATA ports if you look at the "details" page.
 

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