Micron has announced that it intends to ship a new, triple-level cell (TLC) NAND type for consumer devices, including SSDs. These new TLC devices will use the company’s cutting-edge 16nm NAND and, as the name implies, can store up to three bits of information per cell, up from two bits in standard MLC. The new TLC NAND will ship in 16GB capacity chips like previous MLC designs, but the dies are 28% smaller than the MLC variants.

Micron has previously produced TLC at higher process nodes, but the company hasn’t shipped it in consumer SSDs to-date.

The uncertain history of TLC

In theory, TLC drives offer the best of both worlds — improved storage densities for consumers, and superior prices for OEMs, thanks to higher storage capacities and lower chip costs. In practice, things haven’t turned out that way. The more bits of data you store per cell of NAND, the more charge levels you have to be able to distinguish. An SLC cell has two “values” — 0 or 1. An MLC cell, which stores two bits, has four values — we can represent them as 00, 01, 10, and 11. A TLC NAND cell, with three bits, has eight total values that must be stored.

What this means, in aggregate, is that TLC NAND doesn’t have the endurance of other types of NAND, with fewer P/E (program/erase cycles). It’s also slower — it literally takes more time to store data in the triple-bit cell structure.

Both of these issues can be ameliorated with proper drive controllers and wear leveling. Consumers who want budget SSDs are often happy to trade performance for capacity and price, and the truth is, even a budget SSD is still faster than a hard drive these days. The bigger problem, at least based on Samsung’s experience, is that some of these TLC drives have persistent problems with data access speeds.

The 840 and 840 EVO shipped to good reviews (particularly good in the 840 EVOs case) and were widely recommended for quite some time, until evidence emerged that showed the 840 EVO losing performance over time. Samsung has tried to fix the drive’s problems twice, now — we don’t know yet if the company has resolved the issues or not.

These issues are all exacerbated by the use of lower process nodes for TLC. NAND flash actually becomes slightly worse at lower process nodes, which makes it even more difficult to build decent triple-level cells. Given the problems we’ve seen with TLC SSDs to date, it’s hard to be too enthusiastic about these new announcements. Micron will have to demonstrate that its avoided the pitfalls that have snared Samsung. The company is also pushing ahead with its own plans to deploy 3D NAND in partnership with Intel, which seems much more likely to bear fruit long-term. While there are some challenges to building NAND in 3D stacks, it may be easier than pushing TLC designs to lower process nodes.