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RAM is generally much faster than NAND flash storage. It is also much more expensive, not nearly as scalable, and volatile.
RAM tends to cost an order of magnitude more per GB than NAND. RAM DIMM DDR4 boards typically scale to no more than 64GB; although in some cases it scales as high as 128GB. NAND DDR4 boards scale twice as much to 256GB. Making RAM nonvolatile requires either batteries or super capacitors that add significant cost to a DIMM. NAND is, by definition, nonvolatile.
It is that huge differential in cost and scale that has led to NAND flash storage being architected to run directly on the memory channel. Keep in mind that for NAND to run in the memory channel requires some minor changes to the bios. These are the same factors driving the development of other, faster nonvolatile memory technologies such as 3D Xpoint, RRAM, MRAM, memristor, ReRAM, NRAM, PCM and several others.
For now, the key benefit of DDR4 DIMM-based NAND flash storage is extremely low latency tier 0 storage -- not direct memory -- that delivers more latency consistency than any other flash storage including NVMe PCIe flash storage.
That DDR4 DIMM-based NAND flash storage is also heavily overprovisioned to handle the high levels of writes it will have to endure. The manufacturer warranties their DIMM drives at over 35 drive writes per day, every day, for 5 years. That means a 256 GB DDR4 DIMM-based NAND drive would have to be written to approximately 8.96 TB per day every day or 16.352 PB over five years to wear it out. This makes it well-suited for high performance computing and other memory-intensive applications.
Currently, there is only one vendor, Diablo Technologies, of DIMM-based NAND flash storage and it is for OEMs only. Expect DIMM-based flash to start being an option from several server vendors.
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