ELI5 How dual x4 M.2 slots via PCH (DMI 3.0) work

Discussion in 'Processors and Motherboards' started by IamSpartacus, Mar 26, 2020 at 6:15 PM.

  1. IamSpartacus

    IamSpartacus Well-Known Member

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    I have this board (well still waiting for it to ship) and I'm curious how the dual PCIe 3.0 x4 M.2 slots will work in practice being that they are connected to the PCH which has a single PCIe 3.0 x4 upstream connection. I assume I'm limited to only the bandwidth of that x4 upstream connection.

    My question is, how will this work if practice is data is being transferred to both m.2 slots at the same time (such as two NVMe drives in a RAID0 or RAID1 mirror).
     
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  2. PigLover

    PigLover Moderator

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    I’m confused by this. According to the block diagram on page 18 of the manual the M.2 devices are each connected directly to they CPU PCIe slots, each on its own X4. It is the two x8 PCIe slots that are connected to the PCH (which is in turn connected to the CPU via an X4 link).

    I think having the two PCIe slots connected this way might give me pause if running 10/25 gb nics. At least more then one of them.

    BTW - do you know if this board will allow CPUs with on-chip GPU to work with quicksync (for Plex or Blue Iris)? The HDMI/DP is not exposed off the board as it uses the aspeed BMC for video.
     
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    Last edited: Mar 26, 2020 at 8:22 PM
  3. IamSpartacus

    IamSpartacus Well-Known Member

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    I'm not seeing what you're seeing. I see the PCI slots going right into the CPU and the m.2 slots going to the chipset.

    [​IMG]

    As for the iGPU and Plex I hope so. If not I'll be returning it. I cant find any confirmation as this board appears too new.
     
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  4. PigLover

    PigLover Moderator

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    Ah, you are correct. I mis-read (or didn’t read) the labels on the boxes and mixed up the cpu and PCH.

    Sorry for the confusion.
     
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  5. IamSpartacus

    IamSpartacus Well-Known Member

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  6. edge

    edge New Member

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    The c246 supports 24 pcie lanes. If you look at the block diagram, you will see that M.2-P_1 uses lanes 5 through 8 and M.2-P_2 uses lanes 9 thru 12. Each m.2 slot has it's own 4 dedicated pcie lanes. Where do you get the idea of "only a single pcie 3.0 or upstream"?
     
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  7. IamSpartacus

    IamSpartacus Well-Known Member

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    This is where I got that idea.

    [​IMG]
     
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    vanfawx likes this.
  8. i386

    i386 Well-Known Member

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    Half* the bandwidth for ssd 1 and half of the bandwidth for ssd 2.

    *this is just for illustration, there are a bunch of other devices connected to the chipset that transfer data from/to the cpu and decrease the throughput for the ssds.
     
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  9. edge

    edge New Member

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    That is 4x Direct Media Interface 3.0. A single DMI interface supports 3.93 GB/sec which is roughly 4 pcie lanes. The 4x DMI is roughly equivalent to a pcie 3.0 x16. Yes, there is over subscription of that interconnect considering the 24 pcie lanes plus the sata and usb ports. However, 2 pcie 3.0 x4 lanes can consume only around half the upstream link.
     
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  10. i386

    i386 Well-Known Member

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    My interpretation is that dmi3 equals 4x pcie 3.0...
    I'm seriously confused right now :D
     
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  11. edge

    edge New Member

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    1 DMI 3 =~ 4x pcie 3.

    There are 4x DMI 3.0 links between the c246 and the cpu, so think

    4x (4x pcie 3). Does that help?
     
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  12. IamSpartacus

    IamSpartacus Well-Known Member

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    Pretty sure this is incorrect. Everywhere I've read about DMI 3.0 it's says the total bandwidth is roughly 4GB/s.

     
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  13. edge

    edge New Member

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    I went back and re-read the specs. You are correct, the pch only supports 3.93GB/sec to CPU total (8GT).

    That is pretty anemic. It will definitely bottleneck nvme reads. I hate to think of a couple of nvme drives with the sata lanes loaded with an ssd raid. My initial thoughts were based on it being inconceivable to me that anyone would design a chipset with such a high level of over subscription.

    If you aren't using the cpu connected pcie slots, I would look to use them for the nvme drives.
     
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  14. IamSpartacus

    IamSpartacus Well-Known Member

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    Unfortunately I am. And unfortunately the DMI 3.0 limitation is not limited to this chipset, it's the case with most Intel chipsets.
     
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  15. IamSpartacus

    IamSpartacus Well-Known Member

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    I may just need to rethink using an HBA in this build and instead putting the HBA with all my disks into my other server. Not an issue really since my HBA is connected to external DAS units. I just really was hoping to not have all my bulk storage be non-local to Plex as that causes Plex to not pickup changes automatically.
     
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