The current mainstream memory technologies, namely DRAM (quick memory accessed by the processor) and NAND (solid-state storage), have been around for decades. While the cell designs have evolved over the years to allow scaling to 20nm and below, the fundamental physics behind DRAM and NAND operation haven't changed a bit and both technologies have their unique technological limitations. DRAM offers nanosecond-level latency and unlimited endurance, but this comes at the cost of large cell size, cell volatility, and power consumption. Since DRAM cells need to be constantly refreshed, the cells don't retain data in an off state, requiring quite a bit of power and making DRAM unsuitable for permanent storage. NAND, on the other hand, has much higher latency (especially write operations) and has a limited number of write cycles, but the cells are non-volatile and the structure is much more efficient, enabling low cost and suitability for storage.

Combining DRAM and NAND at the system-level architecture provides the best of both worlds, which is why modern computers use DRAM as a memory/cache and NAND for storage. However, there's still a latency and capacity gap between DRAM and NAND, so the question arises: what if you were to combine the best of DRAM and NAND at the silicon level? The mission of next generation memory technology across the industry has been to develop a new type of memory that provides low latency and high endurance while offering a small and scalable cell size.

We have seen numerous startups, such as Crossbar and Nantero, discuss and demonstrate their next generation memory technologies, but we have yet to see the established DRAM and NAND vendors come out with their solutions. Intel and Micron are here to change that with the announcement of their new 3D XPoint (Cross Point) non-volatile memory technology this week.

First and foremost, Intel and Micron are making it clear that they are not positioning 3D XPoint as a replacement technology for either NAND or DRAM, and in that scale it has been talked about more in its applications nearer NAND than DRAM. It's supposed to complement both and provide a technology that sits in between the two by filling the latency and cost gap exists between DRAM and NAND. Basically, 3D XPoint is a new tier in the computer architecture because it can be used as either slower, non-volitile memory or much faster storage.

  DRAM 3D XPoint NAND
Endurance (P/E Cycles) 10^15 10^7 10^3
Read Latency Nanoseconds 10s of Nanoseconds ~100 Microseconds

Intel and Micron are claiming that 3D XPoint provides up to a thousand times higher endurance than NAND. Assuming that the numbers are relative to modern (15-20nm) MLC NAND, the endurance should be in the order of a few million P/E cycles; though the marketing materials are claiming up to tens of millions of write cycles. If we assume 3 million write cycles (1000x of what modern MLC has), a 256GB 3D XPoint based drive would have a total write endurance of 768 petabytes. That's equivalent to 420TB per day for five years, or 4.9GB per second. For storage applications that currently rely on NAND, 3D XPoint will eliminate any potential endurance concerns, but it's not durable enough to challenge DRAM in that front since DRAM endurance is essentially infinite. Whether 3D XPoint provides enough endurance to replace DRAM ultimately depends on the application, but especially in certain enterprise workloads there's a need for DRAM.

3D XPoint latency should be in the order of 10s of nanoseconds, but the companies didn't specify whether this is read or write latency. Judging by the graphs provided by Intel, it seems to be read latency because NAND write latency would measured in milliseconds (typically 1-2ms for a full page write), whereas the graph puts NAND latency at tens of microseconds that is in line with NAND read latency. Write latency is likely higher than that, probably at least 100s of nanoseconds or even a few microseconds given Intel and Micron's claims of "up to 1000x faster than NAND", but what complicates things is that 3D XPoint is accessible at the bit-level whereas NAND is page-level, so comparing the latency of the two without extended context is quite difficult. In any case, 3D XPoint performance should be closer to DRAM than NAND, but since Intel and Micron aren't discussing any specific latencies yet it's too early to make any final conclusions.

Meanwhile unlike many next generation memory technologies out there at the moment, 3D XPoint is the furthest along and doesn't only exist on paper or in a lab. Intel and Micron are currently sampling the first generation die that is being produced at the companies' jointly owned fab in Lehi, Utah. The die is 128Gbit (16GB) in capacity, whereas the products that startup memory companies have in production are in the order of dozens of megabytes. The die is built on a 20nm node and consists of two layers, and in the future scaling will happen through both lithography shrinks and by increasing the number of layers.

The Utah fab has been producing 20nm NAND for now since Intel didn't invest on the 16nm shrink and all initial 3D NAND production will take place in Micron's Singapore fab, but it's unclear whether the full fab with its 20,000 wafers per month capacity will be dedicated to 3D XPoint from now on. My guess would be that 3D XPoint will gradually take over the full wafer capacity in Utah depending on how the market reacts to the new technology and how high demand Intel and Micron are seeing. 3D XPoint does require some new equipment for manufacturing since 3D XPoint deals with a whole new set of materials, but Intel and Micron said that the transition is quite similar to a new NAND node and allows some of the existing equipment to be used. 

The companies aren't quoting any price per gigabyte yet, but since the whole function of 3D XPoint is to fill the gap between DRAM and NAND, it will also be priced accordingly. A quick look at NewEgg puts DRAM pricing at approximately $5-6 per gigabyte, whereas the high-end enterprise SSDs are in the range of $2-3. While client SSDs can be had for as low as $0.35, they aren't really a fair comparison because at least initially 3D XPoint will be aimed for enterprise applications. My educated guess is that the first 3D XPoint based products will be priced at about $4 per gigabyte, possibly even slightly lower depending on how DRAM and NAND pricess fall within a year.

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  • dlop - Friday, July 31, 2015 - link

    I'm still using only 5400 RPM HDDs. I'm waiting for DRAM-less SSD with 2-bit MLC memory, power loss protection and 5 year warranty. Reply
  • jamyryals - Friday, July 31, 2015 - link

    Buy an enterprise SSD, they have those features. Reply
  • dlop - Saturday, August 01, 2015 - link

    Enterprise SSDs are too expensive for low-end home desktop PC. Removing DRAM would make them cheaper because less capacitors should be needed. SSDs could be probably cheaper if they use solid capacitors which are used for motherboards. Bigger size of those capacitors isn't problem for desktop PC. Reply
  • Zan Lynx - Sunday, August 16, 2015 - link

    You didn't specify cheap in your first comment. And you know that you can't have everything. If you want cheap then you give up something else. Like 5 year warranties and power loss protection. Reply
  • dlop - Monday, August 17, 2015 - link

    Intel 535 is DRAM-less SSD with 5 year warranty without power loss protection. Solid motherboard capacitor cost about $1. DRAM-less PCIe or M.2 SSD with motherboard capacitors shouldn't be much more expensive than Intel 535. Reply
  • MrBowmore - Friday, July 31, 2015 - link

    Intel 750? Reply
  • MrBowmore - Friday, July 31, 2015 - link

    Not DRAM-free but with big capacitators that fills the disk with enough power to write down the cache. Most of the ram is for the indexing table anyway. Mabye you just hate DRAM? Reply
  • JKflipflop98 - Monday, August 03, 2015 - link

    Awfully picky for someone willing to put up with such a crappy drive for so long. Reply
  • toooskies - Friday, July 31, 2015 - link

    Not one mention of the mobile market, when they're an ideal place to replace DRAM + NAND. The fact that it's non-volatile will cut idle power usage, and you save PCB space by including it all in a single chip. Obviously database servers will be huge, but the place we're likely to see this stuff on the consumer market is in a cell phone. Reply
  • Freakie - Friday, July 31, 2015 - link

    Bottom of the Products & Applications page talks about mobile devices. It'd be great for all of the low-end to mid-range smartphones but the high end ones that do benefit from fast RAM are likely to keep using RAM while the more mainstream phones could potentially switch to Xpoint. Reply

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