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 - Sunday, April 23, 2017 - link

    It is not expensive because it is new, it is expensive because intel and micron wasted a crapload of money on RDing it and it turned out to be mediocre - significantly weaker than good old and almost forgotten SLC. So now they hype and lie about it and sell it significantly overpriced in hopes they will see some returns of the investment.

    Also, it seems like you are quite ignorant, ignorant enough to not know what "order of magnitude" means. You just heard someone smart using it and decided to imitate, following some brilliant logic that it will make you look smart. Well, it doesn't. It does exactly the opposite. Now either stop using it, or at the very least, look it up, understand and remember what it actually means, so the next time you use it, you don't end up embarrassing yourself.
    Reply
  • factual - Sunday, April 23, 2017 - link

    "significantly weaker than good old and almost forgotten SLC"

    Seriously ?! You must be getting paid to spew this bs! no one can be this ignorant!! can you read numbers ?! what part of 8.9us latency don't you understand, this is at least 10x better than the latest and greatest NVMe SSDs (be it TLC, VNAND or whatever bs marketing terms they feed idiots like you nowadays).

    what part of 95K/108K QD1 IOPS don't you understand ?! This is 3-10x compared to this best SSDs on the market.

    So I repeat again, Xpoint is orders of magnitude better performing than the latest and greatest SSDs (from Samsung or whichever company) on the market. This is a fact.

    You don't even understand basic math, stop embarrassing yourself by posting these idiotic comments!
    Reply
  • ddriver - Monday, April 24, 2017 - link

    LOL, your intellect is apparently equal to that of a parrot. Reply
  • factual - Monday, April 24, 2017 - link

    Well if this fruitless exchange is any evidence my intellect is far superior to yours. So If my intellect is equal to that of a parrot, yours must be equal to that of a maggot ... lol Reply
  • evilpaul666 - Saturday, April 22, 2017 - link

    So where are the 32gb client ones? Reply
  • tomatus89 - Saturday, April 22, 2017 - link

    Who is this ddriver troll? Hahaha you are hillarious. And the worse is that people keep feeding him instead of ignoring him. Reply
  • peevee - Saturday, May 27, 2017 - link

    From your testing, looks like the drive offers real advantages on low QD, i.e. for desktop/small office server use. For these uses a normal SSD is also enough though.
    Given that modern Xeons have up to 28 cores (running 56 threads each) and server motherboards have 2 or more CPU slots, a properly loaded server will offer QD > 64 all day long, and certainly not just 4 active threads - where the Micron 9100 offers even higher performance, and if the performance is good enough there, it certainly good enough on lower QDs where it is even better PER REQUEST.
    And who cares what 99.999% latency is, as long as it is milliseconds and not seconds - network and other latencies on the accesses to these servers will be higher anyway.

    An incredibly good first attempt, but it really does not push the envelope in the market it is priced for - high-performance storage-bottlenecked servers.
    Reply

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