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Re: Seriously though: New Mac or carry on with maxed out 5.1 Tower

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Patrick Sheppard
Re: Seriously though: New Mac or carry on with maxed out 5.1 Tower
on Apr 24, 2020 at 6:10:30 pm

Terry,

To your note about the OWC drive -- which I assume is the Accelsior 4M2 based on the 6,000 MB/s spec -- there may be a few things going on there that are affecting the read/write speed.

First, your Mac Pro model uses PCIe 2.0, which is about half the speed of PCIe 3.0. The 4M2 is PCIe 3.0 and therefore achieves the advertised 6,000 MB/s speeds with PCIe 3.0, even though it is backwards compatible with PCIe 2.0.

Second, the 4M2 is an x8 (aka 8-lane) card, so even if you have it installed into one of your bottom two PCIe slots, which are x16 (aka 16-lane), you would only see a theoretical maximum speed of 4,000 MB/s (and in reality it would be less than that due to system overhead).

But based on your stated read/write speeds, it sounds like maybe you have the 4M2 installed into one of the top two PCIe slots? If so, that would explain the 1,300 MB/s speeds you reported. The top two PCIe slots are x4 (aka 4-lane), for which the theoretical top speed is 2,000 MB/s, and in reality is more like 1,500 MB/s -- which is very close to the 1,300 MB/s that you're seeing.

To sum up, it sounds like what's happening is that the 4M2 card is being speed-crippled three times over:

-- first, by the speed difference between PCIe 2.0 and 3.0;

-- second, by the fact that the 4M2 card is only x8 and therefore would only get up to x8 speeds even when installed into one of the Mac Pro's x16 slots; and

-- third, if I'm right about the 4M2 actually being installed into one of the top two PCIe slots, then you'd only have an effective speed consistent with PCIe 2.0 x4.

If the third factor is true, then you should see a significant speed increase by moving the 4M2 card to one of the Mac Pro's bottom two PCIe slots. Or if it is already installed into one of those slots, then there's something else affecting the read/write speed.


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