AnandTech Year In Review 2020: Solid State Drives

Intel and Micron Put Their Shared Past Behind Them...

In 2018, Intel and Micron decided to end their long-running memory partnership, going their separate ways first for NAND flash development after their 96-layer node, and then for 3D XPoint memory after completing development of its second generation. The split was completed in 2019, and in 2020 we saw the first post-divorce developments from both players. Intel has introduced 144-layer 3D NAND as their follow-up to the 96L generation, with a variety of consumer and enterprise SSDs announced earlier this month.

Micron used the breakup as the opportunity to make more significant changes, switching their 3D NAND design from a floating gate memory cell to a charge trap cell design with a replacement gate manufacturing process, much more similar to what everyone other than Intel has been doing since the transition from planar to 3D NAND. Micron initially made this switch with a 128L generation that was used as a low-volume trial, and quickly followed up with their recent announcement of a 176L generation. That has ostensibly already started shipping in some products, but Micron won't identify them and their 176L NAND won't really begin to have an impact on the SSD market until well into 2021.

... And Intel Makes a New Deal

After ending the Micron partnership, Intel's NAND and SSD business was the odd man out in the industry. They're the only player focused on 3D floating gate designs, and their product line is very strongly focused on the enterprise/datacenter SSD market, with consumer SSDs as more of an afterthought. They are also putting far more of an emphasis on QLC NAND than any other player in the SSD market: in May, they even confirmed that they would not be releasing any more consumer SSDs with TLC NAND. Some of this QLC focus is an attempt to promote their 3D XPoint-based Optane products for fast tier storage, but the fact remains that Intel's newly independent 3D NAND business is relatively small and specialized within the NAND industry.

Given its new station in the market, it isn't too surprising that Intel's NAND business became the next acquisition target. In October, SK hynix and Intel announced a deal for SK hynix to purchase Intel's 3D NAND business and their NAND-based SSD product lines, leaving Intel with their Optane products as their only storage offerings. This acquisition will be an unusually long, drawn-out process with the final part of the transfer planned for March 2025. Once the deal gets regulatory approval (expected sometime in 2021, probably later in the year), SK hynix will get ownership of Intel's NAND fab and the SSD business and IP (eg. Intel's NVMe SSD controller designs). Meanwhile, Intel will retain their 3D NAND IP and the employees operating their NAND fab. The NAND IP and fab employees will be transferred at the end of the process, in 2025.

Neither company has commented on how this acquisition will affect their long-term technology roadmaps, but it's easy to imagine that by 2025 Intel's 3D floating gate may be a dead-end and SK hynix would switch the Intel fab in Dalian, China over to whatever charge trap based process the rest of the SK hynix NAND fabs are using at the time. Even though the differing 3D NAND technologies may not be practical to maintain in the long run, in the short term the SK hynix and Intel SSD businesses are quite complementary. They're two of the smaller players in the NAND market, but with relatively little overlap. SK hynix sells twice as many SSDs as Intel but lower total capacity in GB, so clearly SK hynix will be acquiring a much larger presence in the high-capacity enterprise SSD market—which is also the most profitable market segment. Meanwhile, SK hynix has recently become much more competitive for consumer SSDs after their first several generations of 3D NAND made them a perennial also-ran in the SSD market and left their 3D NAND mostly relegated to mobile (smartphone) storage and memory cards. The combined SK hynix + Intel NAND business is on track to have the second-largest market share in the NAND market.

Related Reading: SK Hynix to Buy Intel’s NAND Memory Business For $9 Billion

More Layers And Faster Interfaces

As the major flash memory manufacturers roll out new generations of 3D NAND flash memory, we have seen some of both diversification and convergence of the technologies implemented by the various manufacturers. The layer counts for 3D NAND are starting to get a bit confusing: At the 96-layer node, Samsung stuck out with just 92 layers—they came up a bit short because they managed to avoid using string stacking in their manufacturing process. Beyond 96 layers, the roadmaps for layer count are all over the place. Kioxia and Western Digital announced 112L NAND at the beginning of 2020, but we haven't seen it hit the market yet. Intel jumped from 96L to 144L. Micron made a short stop at 128L before continuing on to 176L. SK hynix started shipping 128L and has also recently announced 176L. Samsung's 128L debuted with the 980 PRO. It takes months to over a year to get from a NAND manufacturer announcing a new node to sampling that NAND to ramping up production enough for it to appear in mass-market consumer products, so most of the consumer SSDs on the market today are still 96L models with some of the newer drives using 128L NAND. In 2021 we should see someone announce 200+ layer 3D NAND, but new products hitting the shelves will feature anywhere from 112 to 176 layers.

The layer count for 3D NAND is a decent indicator of each generation's density cost per GB, but another metric is becoming important: IO performance per die. The speed of IO between the NAND flash and the SSD controller can be a limiting factor for the drive's overall performance. For a PCIe 3.0 x4 SSD with an 8-channel controller, running each channel at 533MT/s with TLC NAND was sufficient to provide the ~3.5GB/s sequential read performance that saturates the PCIe link. Now that drives are moving to PCIe 4.0, both NAND and SSDs need to support higher IO speeds. There's also a trend toward using four-channel controllers for mainstream consumer SSDs and reserving 8-channel controllers for high-end consumer and entry-level datacenter drives, and this is also driving a need for higher IO speeds. We're now seeing most PCIe 4.0 SSD controllers supporting 1.2GT/s to 1.6GT/s IO speeds, with the latest NAND generations offering similar speeds. This big jump in per-die performance is how the SK hynix Gold P31 with a 4-channel controller was able to match the performance of other PCIe 3.0 SSDs with 8-channel controllers, while using far less power. NAND IO speeds will probably stall again for a generation or two after PCIe 4.0 goes mainstream, until PCIe 5.0 or later starts to catch on in the storage market.

Related Reading:

• Western Digital and Kioxia Announce BiCS5 112-Layer 3D NAND
• Micron to Start Volume Production of 128-Layer 3D NAND with RG Architecture This Quarter
• Micron Announces 176-layer 3D NAND
• SK Hynix Announces 176-Layer 3D NAND
• The Best NVMe SSD for Laptops and Notebooks: SK hynix Gold P31 1TB SSD Reviewed

3D XPoint and SLC NAND Remain Niche Technologies

At the very high end of the SSD market and almost exclusively for top of the line enterprise SSDs, there are memory technology options aside from the QLC and TLC NAND used in mainstream SSDs. 3D XPoint memory is an alternative to NAND flash memory, and was developed by Intel and Micron. Micron has technically commercialized 3D XPoint with their limited-release X100 SSD, but otherwise Intel's Optane products are almost synonymous with 3D XPoint memory. Intel's first product to use second-generation 3D XPoint memory was formally announced earlier this month: the Optane SSD DC P5800X, replacing the first-generation P4800X. Intel has actually been sampling this for months, and they have let some independent performance numbers slip out that back up their impressive performance claims—but we still don't have a good idea when to expect widespread availability, or what pricing is like. Intel's other Optane products are still using the first-gen 3D XPoint memory, including the second generation of Optane DC Persistent Memory DIMMs that are usable with Cascade Lake and Ice Lake Xeons.

After Intel launched 3D XPoint memory, Samsung responded by bringing back SLC NAND flash memory in the form of their Z-NAND, a latency-optimized 48L 3D SLC NAND. We haven't heard anything new on that front since the 983 ZET two years ago, but Toshiba (now Kioxia) eventually announced their own low-latency 3D SLC memory: XL-FLASH. That memory started sampling this year and is starting to find its way into niche high-end enterprise SSDs from several brands. We have a sample of the DapuStor Haishen3-XL H3900 on our enterprise testbed.

These high-end memories may sound like they offer exciting possibilities for high-end storage, but their impact on the market has been limited and very little has changed in the past year. Everyone is finding 3D TLC NAND to be fast enough for almost every purpose, and the niche for faster-than-TLC storage options will remain tiny. There are no revolutionary new memory technologies poised to shake up the market in the near future.

Related Reading:

• Intel Previews 4-Layer 3D XPoint Memory For Second-Generation Optane SSDs
• Intel Announces New Wave of Optane and 3D NAND SSDs
• The Samsung 983 ZET (Z-NAND) SSD Review: How Fast Can Flash Memory Get?

NVMe 2.0: Coming Soon, And Already Partly Here

Work has been underway for over a year on version 2.0 of the NVMe specification, but despite the major version number bump it won't be bringing much revolutionary new technology. Instead, the spec is being reorganized to split it into more digestible components with separate documents for PCIe transports and NVMe over Fabrics transports, and different documents for each command set. The NVMe Working Group has been trying to ratify and publish new features without waiting to bundle them all up in the big spec releases, so we already have specs for two new command sets aside from the familiar hard drive-like block storage command set: Zoned Namespaces (ZNS) and Key-Value Namespaces. We covered the ZNS spec in depth earlier this year. It represents a significant change in the abstraction layer between flash memory and application software and allows software to structure its IO patterns in a much more flash-friendly manner, enabling better endurance and performance and lower hardware costs. The SSD industry has been exploring options along these lines for quite a while, including with several proposals for Open-Channel SSDs. With the ZNS spec, it looks like they have finally settled on the right balance for exposing the underlying hardware details without requiring too much software complexity, and lots of companies in the enterprise storage space are on board. Western Digital was the first to announce a ZNS SSD as a commercial product with their Ultrastar DC ZN540.

The other new command set—for Key-Value SSDs—is a subject we haven't covered in as much detail. Conceptually, it's pretty simple: instead of providing the traditional block storage interface with 512-byte or 4kB sectors, the SSD allows reading variable-length values identified by keys rather than logical block addresses (LBAs). This adds only a tiny bit more complexity to the Flash Translation Layer of a typical enterprise SSD, but allows it to offload almost all of the functionality of a key-value database store like RocksDB. As with ZNS SSDs, KV SSDs require significant changes to host software, but in turn offer benefits to performance and endurance. The KV SSD concept doesn't seem to be getting as much interest across the industry, but having it standardized opens the door to wider adoption.

Both of these major new NVMe features are at least initially going to be almost exclusively used by the major hyperscale cloud computing companies, because of the scope of software modifications required. As the necessary modifications roll out to common server application software, these storage features may catch on more broadly in the server market, but there's no easy path for them to make their way into the consumer storage market.

Related Reading:

• The Next Step in SSD Evolution: NVMe Zoned Namespaces Explained
• Western Digital at FMS 2020: Zoned SSDs, Automotive NVMe And More