 For the price per bit, 3D XPoint is more expensive than NAND, but cheaper than DRAM.Some SSDs, called NVDIMM or "Hyper DIMM" devices, use both DRAM and flash memory.When the power goes down, the SSD copies all the data from its DRAM to flash; when the power comes back up, the SSD copies all the data from its flash to its DRAM.