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Speed issue, wondering if it is to be expected, or more likely correctable?

Discussion in 'Storage Drives' started by 3080guy, 23 Feb 2021.

  1. 3080guy

    Gangster

    Joined: 26 Sep 2020

    Posts: 191

    I have this NVME m.2 adapter
    https://smile.amazon.co.uk/gp/product/B07RZZ3TJG/

    Into which I installed a WD_BLACK SN750 1TB High-Performance NVMe SSD (3500mb/s)

    My motherboard has only one spare PCIe slot.
    With a PCIe 2.0 interface on the spare PCIe slot...
    I was told to expect around 2000mb/s from it.

    The WD SSD running through the PCIe 2.0 interface (which should give around 2000mbs/s), and its not a big issue, but Im only getting 1621mb/s read and 1617b/s write speeds, in my best test. Or very close to 1600 or just above 1600 for both read and write, in other tests.
    Which im happy enough with...

    However, Im keen to learn. Why might I be getting only 80% of the 2000mb/s potential the PCIe 2.0 slot offers. Is the 2000mb/s what I ought to be peaking at without too much effort. Or is 2000mb/s an impossibility in real life, and is the 1600mb/s figure to be expected? (Read/Write speeds tested using CrystalDiskMark 8.0.1x64)

    Im not especially knowlegable, and note that the M.2 NVME adapter cost £14 quid.
    Is it worth trying another adapter?

    My other drives perform around 10% slower than the Manufacturer claims of top speed.
    This setup is giving me around 20% less than the claimed speed (if 2000mb/s is the top claimed speed).
    Just curious. I can happily live with what Ive got. But Im equally happy to try another PCIe M.2 adapter.
     
  2. Joxeon

    Soldato

    Joined: 15 Oct 2019

    Posts: 7,482

    Location: Uk

    1600 sounds about right as you never quite get the full speed as there is some overhead, PCIe gen 3.0 is 4000 but you only max out at 3500 even if using as gen 4.0 SSD and it's the same with gen 2.0 not hitting the full 2000.
     
  3. Quartz

    Capodecina

    Joined: 1 Apr 2014

    Posts: 14,110

    Location: Aberdeen

    Are you sure that 2000 Mbps figure is correct? Are you mixing bits and bytes? Given that the sequential read performance of that drive is 3470 MBps I think you may well be.

    A PCIE 2.0 x1 slot has 500 MByte / sec bandwidth or 4 Mbps.
    A PCIE 2.0 x4 slot has 2000 MByte / sec bandwidth or 16 Mbps.

    Those figures are before overheads like handshaking, parity, checksums, and so on.
     
  4. Journey

    Capodecina

    Joined: 18 Oct 2002

    Posts: 10,358

    Location: West Midlands

    Your answer is as above, as I said in the other thread the theoretical maximum before overheads is 2000 MB/s with a 4x PCI-E 2.0 slot, you also have the jump from the slot through the chipset to the CPU to take into account as well, rather than a direct connection to the CPU like the M.2 slot on the motherboard.
     
  5. 3080guy

    Gangster

    Joined: 26 Sep 2020

    Posts: 191

    Thats what I thought as a possibility. I dont understand it, but it seems to be that 'dont hit maximums in real life' scenario.
     
  6. 3080guy

    Gangster

    Joined: 26 Sep 2020

    Posts: 191

    I really dont know. I thought Mbytes and Mbps were the same anyway. I never quite understood what megabits were however.
     
  7. 3080guy

    Gangster

    Joined: 26 Sep 2020

    Posts: 191

    Im not sure if youre saying a different m.2 NVME adapter would make it faster, or that in real terms it probably wouldnt.

    Sounds like if the 2000mb/s is a theoretical maximum, I can just be happy with the 1600mb/s Im getting.
     
  8. Quartz

    Capodecina

    Joined: 1 Apr 2014

    Posts: 14,110

    Location: Aberdeen

    Simply put there are 8 bits in a byte and the abbreviation letter for a bit is b (lower case) and for a byte is B (upper case).
     
  9. Journey

    Capodecina

    Joined: 18 Oct 2002

    Posts: 10,358

    Location: West Midlands

    I'm not saying anything of the sort. Theory is exactly that, just like when you drive a car in theory you could 70 miles in an hour at a fixed 70mph, but due to 'overheads' e.g. acceleration, chaining direction, traffic, lights, and road works.
     
  10. 3080guy

    Gangster

    Joined: 26 Sep 2020

    Posts: 191

    Ah, got it, thank you.