I am having trouble reconciling this Samsung RAM page and the idea that RAM is supposed to be far faster than any SSD or HDD.
The page says that DDR4 RAM can transfer data at 2666Mbps, which is far slower than SSDs are supposed to be able to transfer data.
What am I misunderstanding?
The 2666Mbps is per data line, not the overall data speed of a fully populated memory DIMM. you are looking at a memory chip, not a DIMM.
A DIMM module consisting of these 2666Mbps chips will have a peak transfer rate of
2666 (data line transfer rate) * 64 (DIMM Memory transfer width)
That gives a peak transfer rate of 170,624 megabits bits per second.
If you divide that by 8 that will give you 21,328 megabytes per second which is 21.328 Gigabytes per second.
That 23.328 Gigabytes per second corresponds to a DDR4-2666 module per Wikipedia.
The reason your Samsung page is listing it in Mbps is because you are looking at a memory chip, not a memory DIMM module. PCB designers, who may use more exotic memory layouts, may only use 2 or 4 chips connected directly to a CPU and so they might not have a "standard" 64-bit wide memory data bus. They might have only a 16-bit or 32-bit bus, in which case they use the Mbps figure and multiply by their bus width to obtain their expected data bandwidth using the calculation I did above.
DDR4 DIMMS typically list an equivalent figure of 2666 MT/s which is "Mega Transfers per second". It is essentially the same as a per data line Mbps figure, but assumes you know what your memory transfer width is. For DDR4 DIMMs that transfer is 64 bits per transfer.
