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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  • melgross - Tuesday, April 25, 2017 - link

    You obviously have some ax to grind. You do seem bitter about something.

    The first SSDs weren't much better than many HHD's in random R/W. Give it a break!
    Reply
  • XabanakFanatik - Thursday, April 20, 2017 - link

    I know that this drive isn't targeted for consumers at all, but I'm really interested in how it performs in consumer level workloads as an example of what a full Optane SSD is capable of. Any chance we can get a part 2 with the consumer drive tests and have it compared to the fastest consumer NVM-e drives? Even just a partial test suite for a sampler of how it compares would be great. Reply
  • Drumsticks - Thursday, April 20, 2017 - link

    I imagine it will be insane - the drive saturates its throughput at <QD6, meaning most consumer workloads. It'll obviously be a while before its affordable from a consumer perspective, but I suspect the consumer prices will be a lot lower without the enterprise class requirements thrown in.

    This drive looks incredibly good. 2-4x more than enterprise SSDs for pretty similar sequential throughput - BUT at insanely lower queue depths, which is a big benefit. At those QDs, it's easily justifying its price in throughput. Throw on top of that a 99.999th% latency that is often better than their 99th% latency, and 3D Xpoint has a very bright future ahead of it. It might be gen 1 tech, but it's already justified its existence for an entire class of workloads.
    Reply
  • superkev72 - Thursday, April 20, 2017 - link

    Those are some very impressive numbers for a gen1 storage device. Basically better than an SSD in almost every way except of course price. I'm interested in seeing what Micron does with QuantX as it should have the same characteristics but potentially more accessible. Reply
  • DrunkenDonkey - Thursday, April 20, 2017 - link

    Well finally! I was waiting for this test ever since I heard about the technology. This is enterprise drive, yeah, but it is the showcase for the technology and it shows what we can expect for consumer drive - 8-10x current SSD speeds for desktop usage (that is 98% 4-8k RR, QD=1). That blows out of the water everything in the market. Actually this technology shines exactly at radon joe's PC, while SSDs shine only in enterprise market (QD=16+). Can't wait! Reply
  • Meteor2 - Thursday, April 20, 2017 - link

    But don't we say SATA3 is good enough and we don't really need (for consumer use) NVMe? So what's the real benefit of something faster? Reply
  • DrunkenDonkey - Thursday, April 20, 2017 - link

    All you want (from desktop user perspective) is low latency at low queue depth (1). NVME helps with that regard, tho not by a lot. Equal drives, one on sata, one on nvme will make the nvme a bit more agile resulting in more performance for you. So far no current ssd is ever close to saturate the sata3 bus in desktop use, this one, however, is scratching it. Sure, it will be years till we get affordable consumer drives from that tech, but it is pretty much the same step forward than going from hdd to ssd - first ssds were in the range of 20ish mb per second, while hdds - about 1.5 in these circumstances. Here we are talking a jump from 50 to close to 400+. Moar power! :) Reply
  • serendip - Thursday, April 20, 2017 - link

    Imagine having long battery life and instant hibernation - at 400 mbps, waking up from hibernation and reloading memory contents would take a few seconds. Then again, constantly writing a huge page file to XPoint wouldn't be good for longevity and hibernation doesn't allow for background processes to run while asleep. I'm thinking of potential usage for XPoint on phones and tablets, can't seem to find any. Reply
  • ddriver - Friday, April 21, 2017 - link

    Yeah, also imagine your system working 10 times slower, because it uses hypetane instead of ram.
    And not only that, but you also have to replace that memory every 6 months or so, because working memory is much more write intensive, and this thing's endurance is barely twice that of MLC flash.

    It is well worth the benefit of instant resume, because if enterprise systems are known for something, that is frequently hibernating and resuming.
    Reply
  • tuxRoller - Friday, April 21, 2017 - link

    They didn't say replace the ram with xpoint.
    It's a really good idea since xpoint has faster media access times so even when it's a smaller amount it should still be quite a bit faster than nand.
    Reply

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