What does the X4 mean in an M.2 PCIe Gen 3.0 X4 NVME drive? Solved

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  1.    #1

    What does the X4 mean in an M.2 PCIe Gen 3.0 X4 NVME drive?


    I need to buy an adapter card to make use of an M.2 PCIe Gen 3.0 X4 NVME drive. The question I have is what does the X4 mean? The adapter card I am looking at can be used in a X4, X8 or X16 PCIe slot and they say I will get up to 32Gbps when it is plugged into a PCIe X16 slot. Is the X4 in the description of the NVME drive a minimum requirement and it will work better plugged into an X8 or X16 slot?

    Ak
      My ComputerSystem Spec

  2.    #2

    Found a nice chart that compares bandwidth for PCIe channels:

    Click image for larger version. 

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    See this article for more information: hardware - Whats the bandwidth and form factor for PCIe x1 x4 x8 and x16? - Server Fault.
    --Ed--
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  3.    #3

    Normally it also the length of the drive as well
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  4. bobkn's Avatar
    Posts : 2,150
    Win 10 X64 Pro 1809 17763.55
       #4

    The X4 is the number of PCI-E lanes used by the M.2 card. Older M.2 cards might only use 2 lanes.

    I doubt that an M.2 drive in an adapter card could perform better when plugged into an X16 slot. I have no deep knowledge of the architecture, but I'm using an adapter card with an old (non-NVME) M.2 drive. The card physically has an X4 interface, so it could be plugged into an X4, X8, or X16 slot. It can still physically only access 4 lanes (else there'd be no point in having all those extra contacts in an X16 slot).

    Why are you using an adapter? Note that the PC's BIOS must support NVME in order to boot form such a drive.
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  5.    #5

    Just imagine an x16 NVME!
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  6.    #6

    EdTittel said: View Post
    Found a nice chart that compares bandwidth for PCIe channels:

    Click image for larger version. 

Name:	pci-bandwidth.gif 
Views:	38 
Size:	20.4 KB 
ID:	236330
    --Ed--
    Thanks for digging up that chart. Right now I have a Samsung M.2 950 Pro 512 GB SSD on an adapter plugged into a Gen 3.0 X8 slot. I just tested it and I got 1,948 MB/s read and 1,480 MB/s write so I am happy about that.

    I am looking at getting a new Samsung M.2 1TB SSD and they say it does up to 3,500 MB/s read and 3,300 MB/sec write. To me it looks like to get those speeds it must be plugged in an X16 slot? I can't do that with my current motherboard unless I remove my graphics card.

    Ak
      My ComputerSystem Spec


  7.    #7

    bobkn said: View Post
    The X4 is the number of PCI-E lanes used by the M.2 card. Older M.2 cards might only use 2 lanes.

    I doubt that an M.2 drive in an adapter card could perform better when plugged into an X16 slot. I have no deep knowledge of the architecture, but I'm using an adapter card with an old (non-NVME) M.2 drive. The card physically has an X4 interface, so it could be plugged into an X4, X8, or X16 slot. It can still physically only access 4 lanes (else there'd be no point in having all those extra contacts in an X16 slot).

    Why are you using an adapter? Note that the PC's BIOS must support NVME in order to boot form such a drive.
    That is the part I don't understand, the adapter card I am looking at has all the contacts for an X16 slot so the M.2 X4 drive should be able to take advantage of that.

    Right now I am using an adapter because my old motherboard has no port for an M.2 NVME drive. My system boots fine from this M.2 drive on the adapter card. I am looking at replacing my current M.2 drive with something faster. It looks like I will have to wait for an X570 motherboard and I then won't need an adapter.
    Ak
      My ComputerSystem Spec

  8.    #8

    swarfega said: View Post
    Just imagine an x16 NVME!
    Isn't that called a ram drive?

    Ak
      My ComputerSystem Spec

  9.    #9

    I'm not aware of any NVMe SSDs that are currently above the M.2 PCIe Gen 3.0 X4 standard. I'm not sure about PCIe adapters, but my motherboard is capable of running M.2 PCIe Gen 3.0 X4 drives in one of its two M.2 slots. I lose two of its six SATA slots when I do this though.
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  10. bobkn's Avatar
    Posts : 2,150
    Win 10 X64 Pro 1809 17763.55
       #10

    alkaufmann said: View Post
    That is the part I don't understand, the adapter card I am looking at has all the contacts for an X16 slot so the M.2 X4 drive should be able to take advantage of that.

    (snip)

    Ak
    What adapter are you looking at? I'm not that familiar with them.

    I very much doubt that you'd get more than X4 performance, regardless of the number of lanes the adapter supports. Just like I get no advantage plugging my old Intel 750 AIC NVME AIC (add in card, not an M.2) into an X16 slot.

    That chart in post 2 is nice, but it's inconsistent with the text in the link, and with other references. The bandwidth of a PCI-E 3.0 bus is supposed to be about 1GBps per lane (unidirectional), so a PCI-E 3.0 X4 bus (4 GBps) can keep up with the data rates that the Samsung drive that interests you is spec'd at.
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