3D XPoint Refresher

Intel's 3D XPoint memory technology is fundamentally very different from NAND flash. Intel has not clarified any more low-level details since their initial joint announcement with Micron of this technology, so our analysis from 2015 is still largely relevant. The industry consensus is that 3D XPoint is something along the lines of a phase change memory or conductive bridging resistive RAM, but we won't know for sure until third parties put 3D XPoint memory under an electron microscope.

Even without knowing the precise details, the high-level structure of 3D XPoint confers some significant advantages and disadvantages relative to NAND flash or DRAM. 3D XPoint can be read or written at the bit or word level, which greatly simplifies random access and wear leveling as compared to the multi-kB pages that NAND flash uses for read or program operations and the multi-MB blocks used for erase operations. Where DRAM requires a transistor for each memory cell, 3D XPoint isolates cells from each other by stacking them each in series with a diode-like selector. This frees up 3D XPoint to use a multi-layer structure, though not one that is as easy to manufacture as 3D NAND flash. This initial iteration of 3D XPoint uses just two layers and provides a per-die capacity of 128Gb, a step or two behind NAND flash but far ahead of the density of DRAM. 3D XPoint is currently storing just one bit per memory cell while today's NAND flash is mostly storing two or three bits per cell. Intel has indicated that the technology they are using, with sufficient R&D, can support more bits per cell to help raise density.

The general idea of a resistive memory cell paired with a selector and built at the intersections of word and bit lines is not unique to 3D XPoint memory. The term "crosspoint" has been used to describe several memory technologies with similar high-level architectures but different implementation details. As one Intel employee has explained, it is relatively easy to discover a material that exhibits hysteresis and thus has the potential to be used as a memory cell. The hard part is desiging a memory cell and selector that are fast, durable, and manufacturable at scale. The greatest value in Intel's 3D XPoint technology is not the high-level design but the specific materials and manufacturing methods that make it a practical invention. It has been noted by some analysts that the turning point for technologies such as 3D XPoint may very well be in the development in the selector itself, which is believed to be a Schottky diode or an ovonic selector.

In addition to the advantages that any resistive memory built on a crosspoint array can expect, Intel's 3D XPoint memory is supposed to offer substantially higher write endurance than NAND flash, and much lower read and write times. Intel has only quantified the low-level performance of 3D XPoint memory with rough order of magnitude comparisons against DRAM and NAND flash in general, so this test of the Optane SSD DC P4800X is the first chance to get some precise data. Unfortunately, we're only indirectly observing the capabilities of 3D XPoint, because the Optane SSD is still a PCIe SSD with a controller translating the block-oriented NVMe protocol and providing wear leveling.

The only other Optane product Intel has announced so far is another PCIe SSD, but on an entirely different scale: the Optane Memory product for consumers uses just one or two 3D XPoint chips and is intended to serve as a 32GB cache device accelerating access to a mechanical hard drive or slower SATA SSD. Next year Intel will start talking about putting 3D XPoint on DIMMs, and by then if not sooner we should have more low-level information about 3D XPoint technology.

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  • ddriver - Friday, April 21, 2017 - link

    So if you populate the dimm slots with hypetane, where does the dram go? Reply
  • kfishy - Friday, April 21, 2017 - link

    You can have a hybrid memory subsystem, the current topology of CACHE-DRAM-SSD/HDD is not the only way to go. Reply
  • tuxRoller - Friday, April 21, 2017 - link

    Why are you mentioning dimms?
    Are you just posting random responses?
    Neither of your posts in this thread actually addressed anything that the posters were discussing.
    Reply
  • Kakti - Saturday, April 22, 2017 - link

    Have you been living in a cave the past five years? SATA 3.0 has been the limiting factor for SSDs for a while now - all max out around 450MB/sec.

    Now there are plenty of SSD that connect via PCIe instead of SATA and are able to pull several gigabytes/sec. Examples include Samsung 960 Pro/Evo, 950 Pro, OCZ RD400, etc. SATA has ben the bottleneck for a while and now that we have NVMe, we're seeing what NAND can really do with m.2 or pci-e connections
    Reply
  • cfenton - Thursday, April 27, 2017 - link

    That speed is only for high queue depth workloads. Even the 960 Pro only does about 137mb/s average in random reads over QD1, QD2, and QD4. The QD1 numbers are something like 34mb/s. Those numbers are far below the SATA spec. Almost all consumer tasks are low queue depth.

    With this drive, you get about 400mb/s even at QD1, and something like 1.3gb/s at QD4.
    Reply
  • CajunArson - Thursday, April 20, 2017 - link

    A very very sweet piece of technology assuming you have the right workloads to take advantage of what it can offer. Obviously it's not going to do much for a consumer grade desktop, at least not in this form factor & price.

    It's pretty clear that in at least some of those tests the PCIe interface is doing some bottlenecking too. It will be interesting to see Optane integrated into memory DIMMs where that is no longer an issue.
    Reply
  • tarqsharq - Thursday, April 20, 2017 - link

    I can imagine this on a heavily trafficked database server would be insanely effective. Reply
  • ddriver - Friday, April 21, 2017 - link

    Not anywhere nearly as fast as an in-memory database. Reply
  • Chaitanya - Thursday, April 20, 2017 - link

    Like most recent Intel products: overpriced, and overhyped. Reply
  • vortmax2 - Thursday, April 20, 2017 - link

    I don't agree. For Gen1, I'd say it's about right on. It seems that consumer storage advancements are accelerating (SSD, NAND, now this inside a decade). I for one am happy to see a part of Intel (albeit a joint partnership) pressing ahead and releasing revolutionary tech - soon to me enjoyed by consumers. Reply

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