16 GB/s? Bah, we can do better. Fill a truck with microSD cards and drive down the road to your neighbor. There, you've got a bandwidth on the order of PB/s. And nobody minds waiting millions of milliseconds to see if they managed to move their cursor to the correct button, right?
Okay, so that's an extreme example, but it demonstrates what happens when you focus only on bandwidth and ignore the other half of data transfer performance: latency.
There's a nice breakdown of the latency of various data accesses over on StackOverflow. The important takeaway is that RAM latency is measured in hundreds of nanoseconds whereas SSD latency is measured in tens of microseconds. So instead of waiting 100 clock cycles when your 1 GHz CPU needs something not in a cache, it would have to wait 10000 or more. That's a lot of time to have to try to fill with other work.
And then there's the fact that M.2 SSDs don't actually have as much bandwidth as you think. The M.2 slot only supports up to x4 PCI-E lanes, which, with the PCI-E 4.0 standard, limits the bandwidth to ~7.9 GB/s and the far future PCI-E 5.0 standard to a likely 15.8 GB/s.
As for virtual memory, yes, similar to a HDD, we can use an SSD for virtual memory, but keep in mind this is as an extension to RAM, not a replacement.
Something else of interest: AMD announced a GPU with a pair of PCI-E x4 M.2 SSDs on board in a RAID-0. This isn't a replacement for the GPU's ram (which would have a bandwidth measured in hundreds of GB/s), but rather as a storage drive (it's presented as such to the OS). This mainly means that the GPU can get data from the drive without any overhead from the motherboard's PCI-E interface. This resulted in a bump from 900 MB/s talking to the system drive to ~4 GB/s talking to the onboard drive, although it's not specified if the system drive was also RAID-0.
No and as most boards share the PCI-E lanes from what I know you usually won't be able to reach that top speed for a single device. – Seth – 2016-10-12T13:02:49.970
From what I know, memory slots on motherboards have a more direct connection to the cpu, which increases access speeds. Furthermore, I believe that ram uses more power than is available on an M.2 connector. I'm afraid you'll need to upgrade your motherboard if you want to use DDR4. – cascer1 – 2016-10-12T13:17:28.430
You just mention the bandwidth but don't take into account the differences in the architecture of the memory being used. I guarantee you that the memory used in M.2 devices are not running at 2133 MHz. "n my case my mobo support only DDR3 up to 2133 MHZ which is slower than my M.2" - Its only slower if the only thing you consider is the communication bandwidth. – Ramhound – 2016-10-12T13:18:44.227
I agree with seth on this one, also using the SSD for a ramdisk or pagefile will put so many reads and writes on the drive that it will burn out quicker. – BigElittles – 2016-10-12T13:19:27.910
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Possible duplicate of Why not use SSD space as RAM?
– Ƭᴇcʜιᴇ007 – 2016-10-12T13:20:31.947@BigElittles "burn out quicker": So it'll only last 20 years instead of 200? ;) This idea that "using your SSD too much will kill it" is not relevant these days, and that same thinking could be equally applied to practically anything -- If you use a HDD a lot, it won't last as long either. – Ƭᴇcʜιᴇ007 – 2016-10-12T13:23:08.203
The M.2 connector carries up to four PCIe 3.0 lanes. That means 32 GT/s or 3940 MB/s (considering the encoding). So it’s neither 16 GB/s nor 16 Gb/s and most certainly not 131072 MB/s or Mb/s or whatever. Even DDR2 memory offers more bandwidth. – Daniel B – 2016-10-12T16:18:05.317