SSD - Which is faster - PCIe vs M.2

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mjperk

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Jan 18, 2014
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Let's say you have an M.2 SSD like the Samsung Pro 950. This means that you are running gen 3.0 x4 for maximum performance.

Let's then say you have two ways to connect this SSD to your ASRock Z97 OC Formula. You can use the M.2 PCIe slot that is right on your motherboard, but there is bad news--it is gen 2.0 x2, meaning that the SSD will be "overkill" since it can use more lanes than you have.

Let's then also say that you have an empty PCIe slot on your motherboard that is 2.0 x16 in x4 mode. You can get M.2 to PCIe cards and you could plug the very same SSD into. So you essentially have 4 channels of gen 2.0 to work with. You're still not maxing out the SSD since it is gen 2.0.

Which setup will perform better?
If I'm not mistaken, the PCIe 2.0 interface is theoretically 500 MB/s, per lane. So with 4 lanes working this would be a maximum of 2000 MB/s.

If I'm interpreting correctly, an M.2 2.0 x2 connection is capable of only 800 MB/s at most.

I know that PCIe can fully duplex. I'm not sure about M.2.... Then there is SATAIII which has a theoretical max of 600 MB/s and does not fully duplex..

Am I interpreting correctly that using a given M.2 SSD with x4 capability will offer better performance if plugged into the PCIe 2.0x16 port rather than the dedicated M.2 connector on the motherboard?

 
Solution
atomicWAR
this is an easy one IMO. All the interfaces will potentially bottleneck the 950 pro. A purchase i myself have been researching. The PCIe 2.0 connection at 4x easily beats the 2x connection of the m.2 slot. It's basic math. Now even in the PCI 2.0 4x slot you will not have access to the last 500MB/s read but you will have close to 2000MB/s (1600MB/s minus overhead). The m.2 slot will only give you 1000MB/s (800MB/s with overhead).
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blazorthon

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There are two major factors here and they are the bandwidth of the interface and the performance of the drive. If the drive can't move data fast enough to saturate a higher-bandwidth interface, then the higher bandwidth interface won't improve real-world performance.
 
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mjperk

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Also, I'm talking about a given SSD (950 pro in this case). Theoretical specs put it at 2500 MB/s read and 1500 MB/s write. Even if (when) it doesn't actually hit these numbers due to overhead, it is probably saturating or coming pretty close to all of the options I listed above. And then there is the duplexing. What if I am not just copying massive amount of files, but am doing some operation in conjunction and want to be able to still use the drive? I can't speak to the M.2, but isn't logic correct in thinking that the PCIe interface is superior?

I have an 850 Evo that will get up to 400-450 MB/s on my SATAIII right now. If/when I copy large files there are still times where the computer is pretty well useless until it finishes that operation.

 
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mjperk

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I guess I don't understand that then. Take the 950 Pro, for example again. 2500 MB/s read and 1500 MB/s write, but only using 3.0x4. If I only have 2.0x2 in the M.2 slot, am I not effectively cutting those figures in half or even worse? It's got half the lanes on an older interface, so I can't imagine that it will get anywhere near 2500/1500 even though they are well below the 10GB/s figure.
 
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atomicWAR

atomicWAR

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this is an easy one IMO. All the interfaces will potentially bottleneck the 950 pro. A purchase i myself have been researching. The PCIe 2.0 connection at 4x easily beats the 2x connection of the m.2 slot. It's basic math. Now even in the PCI 2.0 4x slot you will not have access to the last 500MB/s read but you will have close to 2000MB/s (1600MB/s minus overhead). The m.2 slot will only give you 1000MB/s (800MB/s with overhead).
 
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The Original Ralph

The Original Ralph

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Sep 14, 2011
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what atomicWAR said X2

when i put my xp941 into the M.2 slot on my Z97 mobo, as it was only x2, i saw it bottlenecked to 770 MB/s & 570 MB/s read and write. When i moved it to a PCIe expansion card in a PCI 2.0 x4 slot, speeds jumped to full spec, of 1180 MB/s & 880 MB/s

I've since purchased a 950 PRO, and it's residing in a PCI 3.0 x4 slot. Whether PCI 2.0 or 3.0, having the full 4x or 4 lanes is important
 
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