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  1. #1
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    Crosshair VII: M.2 drives steal PCIE bandwidth

    If the user installs an M.2 drive in the upper slot closest to the CPU, the graphics card runs at 8x instead of 16x.

    This seems like poor design, given that the heatsink is installed by default on that M.2 slot. It would lead the average person to assume it is the primary M.2 slot, but it seems to be the lower slot.

    Will running a dual M.2 setup lower the performance of my graphics card?

    I've seen users complain of stuttering in games until they moved their M.2 to the lower slot.

  2. #2
    ROG Guru: Orange Belt Array brkkab123 PC Specs
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    The top m.2 slot that has the heat-sink on it does use PCIe lanes.
    If you use it your x16 PCIe slot will only use x8 speeds.
    m.2 slot 2 on the bottom doesn't use PCIe.
    If you have a single m.2 ssd use the lower m.2 slot for it.

  3. #3
    ROG Enthusiast Array schweigert PC Specs
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    Quote Originally Posted by brkkab123 View Post
    The top m.2 slot that has the heat-sink on it does use PCIe lanes.
    If you use it your x16 PCIe slot will only use x8 speeds.
    m.2 slot 2 on the bottom doesn't use PCIe.
    If you have a single m.2 ssd use the lower m.2 slot for it.
    wow..... thanks for this info. I was going nuts, figuring out why my card was only at 8x


    now i wonder whats the net best config --- m.2 using pcie lanes w/ card at 8x. or m.2 using the bottom mount non pcie but card at 16x?

  4. #4
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    Quote Originally Posted by schweigert View Post
    wow..... thanks for this info. I was going nuts, figuring out why my card was only at 8x


    now i wonder whats the net best config --- m.2 using pcie lanes w/ card at 8x. or m.2 using the bottom mount non pcie but card at 16x?
    Use the bottom M.2 slot (M.2_1). It's PCIe X4 too, directly to the CPU. https://youtu.be/S0mR4IoNWkQ?t=3m20s

    This is one nice and unique feature of the C7H. As far as I know, it's the only board that allows 2 M.2 operating at PCIe X4. I think all other motherboards have any second slot sharing lanes with other I/O through the chipset.

  5. #5
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    From the Motherboard manual for clarification

    Hey guys, I know this thread is probably extremely dead, but posting here for anyone else who stumbles upon this;

    From the Motherboard manual .pdf on ASUS' website; https://www.asus.com/us/Motherboards...lpDesk_Manual/

    From page ix of the specifications summary, it says devices placed in M.2_2 will steal PCIE lanes from PCIE_SLOT_x8/x4_2.

    Diagram of the .pdf's silkscreen, with the corresponding slots w/ respective colors.

    You shouldn't see reduced performance as long as you're not running SLI/Crossfire on the board, or putting a graphics card in the x8 slot and running a M.2 SATA/NVME at the same time, for any people aspiring to run a dual NVME set-up like I will be. Honestly, this was a good move from ASUS, using the bandwidth second slot that most experienced PC builders wouldn't be using anyway, since it's an x8 lane to begin with. I'd say if you wanna go full cordless, use this lane for another PCIE storage device, or leave it empty, which is what I would so since it's too close to the x16 PCIE for adequate airflow for your GPU.

  6. #6
    ROG Member Array sdamico PC Specs
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    Quote Originally Posted by TheRoosterneeb View Post
    Hey guys, I know this thread is probably extremely dead, but posting here for anyone else who stumbles upon this;

    From the Motherboard manual .pdf on ASUS' website; https://www.asus.com/us/Motherboards...lpDesk_Manual/

    From page ix of the specifications summary, it says devices placed in M.2_2 will steal PCIE lanes from PCIE_SLOT_x8/x4_2.

    Diagram of the .pdf's silkscreen, with the corresponding slots w/ respective colors.

    You shouldn't see reduced performance as long as you're not running SLI/Crossfire on the board, or putting a graphics card in the x8 slot and running a M.2 SATA/NVME at the same time, for any people aspiring to run a dual NVME set-up like I will be. Honestly, this was a good move from ASUS, using the bandwidth second slot that most experienced PC builders wouldn't be using anyway, since it's an x8 lane to begin with. I'd say if you wanna go full cordless, use this lane for another PCIE storage device, or leave it empty, which is what I would so since it's too close to the x16 PCIE for adequate airflow for your GPU.
    Yes, I've been confused by reviews and posts stating the M.2_2 was sharing lanes with the PCIE_1, which is not what the manual states. Hopefully the manual is correct, but either way I'm running dual M.2 Samsung EVO 970s.

  7. #7
    ROG Guru: Blue Belt Array gupsterg PC Specs
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    The video gorpo linked is correct, also the information posted by TheRoosterneeb is correct. Below is block diagram for ASUS ROG Crosshair VII Hero.

    Click image for larger version. 

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  8. #8
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    Quote Originally Posted by gupsterg View Post
    The video gorpo linked is correct, also the information posted by TheRoosterneeb is correct. Below is block diagram for ASUS ROG Crosshair VII Hero.

    Click image for larger version. 

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    Yes, that's how the PCI-E Lanes are diagramed. But row 3 in the chart on page 1-8 of the manual illustrates how the PCI lanes are managed depending on what devices you have installed.
    Basically putting a NVME ssd in the M2_2 is using part of the bandwidth from PCIEX8/X4_2, basically like you putting a second video card. Thus lowering to x8/x8 or in this case the motherboard will split the lanes down to x8/x4/x4.
    So basically if you have 1 ssd and it's NVME put it in the M.2_1 slot, the lower one. If you put it on the upper M.2_2 your GPU on PICEX16 will work x8.
    Click image for larger version. 

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    Last edited by Kamui004; 12-01-2018 at 02:20 PM.

  9. #9
    ROG Guru: Blue Belt Array gupsterg PC Specs
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    Quote Originally Posted by Kamui004 View Post
    Yes, that's how the PCI-E Lanes are diagramed. But row 3 in the chart on page 1-8 of the manual illustrates how the PCI lanes are managed depending on what devices you have installed.
    Basically putting a NVME ssd in the M2_2 is using part of the bandwidth from PCIEX8/X4_2, basically like you putting a second video card. Thus lowering to x8/x8 or in this case the motherboard will split the lanes down to x8/x4/x4.
    So basically if you have 1 ssd and it's NVME put it in the M.2_1 slot, the lower one. If you put it on the upper M.2_2 your GPU on PICEX16 will work x8.
    Click image for larger version. 

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    I never disputed placing a NVMe in M2_2 does not take PCI-E lanes, nor did gorpo's info or TheRoosterneeb's.
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  10. #10
    New ROGer Array Tex-Rec PC Specs
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    Question

    Quote Originally Posted by brkkab123 View Post
    The top m.2 slot that has the heat-sink on it does use PCIe lanes.
    If you use it your x16 PCIe slot will only use x8 speeds.
    m.2 slot 2 on the bottom doesn't use PCIe.
    If you have a single m.2 ssd use the lower m.2 slot for it.
    I just noticed today , when walking a little bit trought the BIOS that my EVGA GTX 1080 FTW2 runs at 8x instead of 16 and im using a single m.2 on the bottom slot since i builded this rig. Now i wonder why , as i known this before and thought i run it like it should runn , but it sems something wrong. Bios is on the latest version 2008 and im on the ASUS RGO Crosshair Hero 7.

    is there a solution to give the GPU back 16x , btw the GPU is pluged in the First slot ( right below the rams )

    EDIT: Reseated the GPU seems to solve my problem , a little dust evil landed on a pin , cleaned with 100% Alkohol all the pins , pluged in and 16x was again active.
    Last edited by Tex-Rec; 03-29-2019 at 06:43 PM.

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