X-NAND Technology Explained – QLC Capacity at SLC Speeds

The field of storage has seen some rapid advancements in the past decade. For the longest time, hard drives were the primary and the only storage medium used in consumer PCs. At the dawn of the previous decade, there was the revolutionary introduction of a new form of storage medium known as Solid State Storage. Now the concept wasn’t unfamiliar but the implementation at the start was unrefined, to say the least. Not to mention the costs of different types of solid-state drives were through the roof when compared with a standard mechanical hard drive and thus, hard drives were still the default medium for storage in consumer PCs.

Later in the decade the advancement and progression in the field of solid stage storage sped up tenfold. Newer NAND flash technologies were brought to the market, faster and more efficient controllers were baked in, the raw numbers of the drives shot up exponentially, and the drives became cheaper and cheaper as well. A lot of these changes have to be attributed at some level to the progression and advancements in the field of NAND flash. The different types and configurations of the NAND flash allowed the manufacturers to lower the cost of the drive itself while still maintaining large capacities and high speeds. Before we unravel the secrets of X-NAND, we have to recap what NAND flash actually is.

NAND

As explained in our advanced guide to buying an SSD, NAND flash is a type of non-volatile memory that does not require any power to retain data. NAND Flash stores data as blocks and relies on electrical circuits to store data. When no power is available to the flash memory, it uses a metal-oxide semiconductor to provide an extra charge, thus keeping the data.

This form of solid-state storage is often coupled with something called a DRAM cache. This is a faster but smaller storage medium that works in tandem with the NAND flash of the drive in order to deliver the high speeds that SSDs are famous for. Whenever the system instructs the SSD to fetch some data, the drive needs to know where exactly the data is stored inside the memory cells. For this reason, the drive keeps a sort of “map” which actively tracks where all the data is physically stored. This “map” is stored on a drive’s DRAM Cache. It is important to understand that NAND flash works best when paired with a DRAM cache.

NAND Types

As X-NAND is also a new type of NAND flash, first we need to recap the types of NAND Flash that already exist in SSDs of today.

• Single Layer Cell (SLC): This is the very first type of flash memory that was available as flash storage. As the name implies, it stores a single bit of data per cell and is therefore very fast and long-lasting. However, on the flip side, it is not very dense in terms of how much data it can store which makes it very expensive. Nowadays, it is not commonly used in mainstream SSDs and is limited to very fast enterprise drives or small amounts of cache.
• Multi-Layer Cell (MLC): Despite being slower, MLC gives the choice to store more data at a lower price than SLC. Many of these drives have a small amount of SLC cache (adequately named the SLC caching technique) to improve speeds whereby the cache acts as a write buffer. MLC has also been replaced nowadays by TLC in most consumer drives, and the MLC standard has been limited to enterprise solutions.
• Triple-Level Cell (TLC): TLC is still very common in today’s mainstream SSDs. While it is slower than MLC, it allows for higher capacities at a cheaper price due to its ability to write more data to a single cell. Most of the TLC drives employ some sort of SLC caching which improves performance. In the absence of a cache, a TLC drive is not much faster than a traditional hard drive. For normal consumers, these drives offer good value and a fine balance between performance and price. Professional and prosumer users should consider enterprise-grade MLC drives for even better performance should they see fit.
• Quad-Level Cell (QLC): This is the next level of storage technology that is promising higher capacities at even cheaper prices. It also employs a caching technique to provide good speeds. Endurance can be a bit lower with drives using QLC NAND, and sustained write performance can become lower once the cache fills up. However, it should introduce more spacious drives at affordable prices.

These are the current forms of NAND Flash that currently exist in SSDs of today. As manufacturers are always innovating and improving these designs in order to improve performance and, more importantly perhaps, cut costs, we have also seen the introduction of something known as 3D NAND in SSDs.

As covered before, 2D or Planar NAND has only one layer of memory cells, whereas 3D NAND layers cells on top of each other in a stacked manner. Drive makers are now layering more and more stacks on top of each other which leads to denser, more spacious, and less expensive drives. Nowadays, 3D NAND Layering has become really common, and most mainstream SSDs employ this technique. These drives cost less than their planar counterparts because it is cheaper to manufacture a denser, stacked flash package as compared to a 2D one. Samsung calls this implementation “V-NAND” while Toshiba named it “BISC-Flash”.

This technique also allows the manufacturers of the drive to produce SSDs with higher capacities at lower prices in large volumes.

What is X-NAND

X-NAND is theoretically the combination of the best things about SLC and QLC. At its core, the concept tries to bring the best of both worlds in one place and that really is what is needed to push the NAND Flash technology segment forward.

X-NAND architecture was presented by the CEO of NEO Semiconductor at the Flash memory Summit for 2021. This new architecture promises to combine the speed of SLC Flash with the density and low pricing of QLC Flash. Compared to the conventional QLC NAND, the Read Time is improved by up to 30%, the Program Time by 37%, the Read Throughput by up to 27 times, and the Write Bandwidth by up to 14 times. These are astronomical improvements when we compare it to what we have available today, making X-NAND a truly enticing architecture to look out for in the near future.

Advantages of X-NAND

Andy Hsu, the CEO of NEO Semiconductor, explained the potential benefits of X-NAND in the three-day virtual Flash Memory Summit for 2020. Following are some of X-NAND’s important advantages over current flash technologies.

Speed

The best thing about X-NAND is the potential amalgamation of the two best things we find in SLC and QLC NAND nowadays. Currently, users have to make a choice between the capacity and affordability of QLC, or the raw speed of something like an MLC drive (since SLC is not commonly used to make consumer SSDs anymore). Since X-NAND promises to combine the speeds of SLC with the capacity of QLC, we have no reason to doubt that this new technology is going to deliver some ridiculous speed numbers.

Capacity

Currently, QLC is the NAND Flash type of choice when it comes to manufacturing high-capacity SSDs at reasonable prices. This is because due to the architecture and density of the QLC flash, it is possible to store more data in the flash than you can manage to store in a similarly-equipped MLC or even TLC drive. Bringing the capacity benefits of the slower QLC NAND to the higher-speed SLC speeds will potentially produce an SSD that combines the best of both worlds as we eluded to earlier.

Affordability

There is no certain information regarding the pricing of X-NAND as of the time of writing, but if the current pricing situation of SLC and QLC NAND is anything to go by, X-NAND has the potential to be as cheap as QLC in the near future. QLC is the slowest and most form of NAND in SSDs of today, and thus it is also the cheapest. While it may be a bit of a stretch to say that X-NAND will definitely match or undercut the QLC drives of today, the potential is definitely present and it is undeniable. The budget SSD segment is already very competitive and with X-NAND it has the potential to become even more crowded.

Mechanism behind X-NAND

While consumer QLC drives rely heavily on SLC caching (having a small amount of SLC NAND on board in order to speed up processes), X-NAND finds a way for the flash to maintain the SLC performance for an extended period of time. This is done by simultaneously allowing SLC and QLC write modes which is not a process implemented in current QLC drives.

As can be seen in this performance chart, the write throughput of a modern QLC drive falls off the cliff after a certain period of time has passed. This is due to the SLC cache being full and the drive having to rely on its much slower QLC NAND to move the data. Compare that to the X-NAND graph line which stays at 100% throughout the test, and the difference is night and day. Here we can really appreciate the performance benefits of X-NAND which bring SLC-level speeds to a more affordable price range and capacity level.

X-NAND achieves these gains by going from a 16KB page buffer per plane to a 1KB page buffer per plane, but with sixteen times the planes, as cited in one example. This can be further understood by dissecting some of the terminologies used here. A plane tends to be the smallest unit of interleaving for flash, with one or more planes per flash die. The page buffer holds data in transit between the bus and the flash. A flash die is divided into planes containing bit lines or strings of cells so planar division can reduce the bit line’s length and that helps boost performance. Write performance can be increased quite substantially by using this process.

Future Applications

The future certainly seems bright if we take a look at the potential of X-NAND. While it is certainly difficult to predict whether or not X-NAND will be an actual viable product in the market any time soon, the road ahead seems to be pretty well-paved for the introduction of this technology. X-NAND will definitely be one to shake the market of solid-state storage if it makes its debut in the current market situation.

Keeping in mind the potential for further improvements and polish, X-NAND can definitely be a viable candidate for data center and enterprise applications in the future. The most important thing in a data center setting is definitely the safety and redundancy of the data. If the minds behind X-NAND can figure out how to increase the endurance and reliability of this NAND, then that can definitely be a market segment where X-NAND can have an impact in the near future.

As far as consumer PCs and gaming applications go, there is also a lot of potential in this space as well. Currently, potential SSD buyers are definitely torn between the speeds of MLC/TLC and the capacity and pricing of QLC NAND. Pricing will definitely play a big part in the success of X-NAND in the consumer desktop market but we can expect it to get better once the architecture gets more mature and the process of manufacturing becomes more streamlined.

Conclusion

While it may sound too good to be true, X-NAND is a revolutionary new technology that aims to combine the best parts of SLC and QLC NAND types. Although it may not be as simple as that right now, the potential for this technology cannot be ignored. Not only is this something that can be a big advancement in the field of data centers and edge computing, but also in the market for consumer desktop PCs and gaming machines. X-NAND is still in its infancy right now and there is no product on the market using this NAND flash as of the time of writing, but it should be exciting to see what the minds behind X-NAND have planned for its eventual launch into the market.