Intel and Micron just announced a new solid-state memory/storage technology that looks to replace NAND flash, and probably a fair amount of DRAM as well. When we talk about new technologies most of the focus is on what it can do for users and applications. What’s remarkable about 3D XPoint (pronounced “cross point”) is what it doesn’t do.
NAND flash memory cells store data by trapping electrons on the ‘floating gate’ mechanism of a transistor, which maintains a charge without requiring power, making it a non-volatile storage device. The binary value of each bit is read by determining the charge level on this floating gate. Instead of using one or more transistors to store each bit of data, 3D XPoint changes the electrical phase of a ‘column’ of substrate, each of which is much smaller than a transistor. Eliminating transistors as the primary mechanism for storing data greatly increases density, meaning more data can be packed into a smaller physical space on the chip. In fact, 3D XPoint storage devices have and 8-10x greater data density than DRAM, according to the Intel and Micron. But eliminating transistors also means eliminating the high-voltage erase process that’s used with NAND flash.
In NAND flash, erasure is accomplished by subjecting or “flashing” this floating gate with a large electric charge, hence the name “flash memory”. This high-voltage erasure process slowly damages the floating gates, causing NAND flash cells to degrade over time and driving up uncorrected bit-error rates. 3D XPoint doesn’t use a flash process to erase cells, eliminating degradation, meaning this technology won’t wear out like NAND flash does. In fact, 3D XPoint promises to deliver 1000x better endurance than NAND flash. One of the things flash controllers do to maintain acceptable endurance levels is run increasingly sophisticated bit-error correction processes. The overhead and latency these processes create is also gone in 3D XPoint devices.
While NAND flash is bit-accessible and written in pages, it can only be erased in multi-page blocks. Flash erasure requires a large dose of electricity, which can’t be applied at the bit level. This means that the flash controller is constantly moving data pages around to consolidate blocks and facilitate the erasure process. Called “garbage collection”, this process adds significant latency to flash write performance, after the device has been filled for the first time. 3D XPoint can change the value of each memory cell independently, so it doesn’t have to erase in large blocks, eliminating the overhead and latency associated with block erasure. According to Micron and Intel this technology is 1000x faster than NAND flash and 3D XPoint devices can write data as fast as they read it, something flash can’t do. Intel and Micron plan to ship 3D XPoint devices in early 2016.
This is a potential ‘game changer’ for the solid-state memory/storage industry. Much of the development in flash technology has been in the sophisticated controller processes required to compensate for NAND flash’s inherent degradation and inefficient block-level erase process. 3D XPoint eliminates the need for these processes altogether, and the latency and complexity they bring.
By virtue of what it doesn’t do, 3D XPoint does provide some compelling potential benefits. Three orders of magnitude better speed and endurance makes it an obvious replacement for NAND flash. It’s better density, plus the ability to change at the bit-level and avoid substrate degradation, makes 3D XPoint a potential alternative for DRAM as well.
The Flash Memory Summit 2015 in Santa Clara, CA, Aug 11-13 is a great place to get more information on this remarkable new technology, as well as tutorials, technical sessions and keynotes on every area of solid-state memory and storage.
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