NAND flash memory cells can hold one or more bits of data each. SSDs originally came with single-level cells (SLC) and then double-level cells (DLC, also known as MLC), but most flash manufactured today is multi-level with either triple-level (TLC) or quadruple-level (QLC) cells. Each cell can hold a certain amount of voltage by which to discern the value it holds, therefore more bits means more possible voltage states. This means that the difference, or threshold, between states is smaller as the number of levels increases.
Smaller thresholds correspond to less margin for error so endurance tends to be lower with higher-level cells. This is because flash wears down over time and can become more difficult to read, until eventually the cell is unreadable. More bits also lead to reduced performance because cell programming must be more precise and narrower thresholds require additional error correction for reads. Therefore, TLC tends to have both better performance and endurance than QLC.
The amount of bits per cell does increase capacity, however, not only allowing for higher capacity drives but also bringing down the price per gigabyte. Many techniques exist to get the most out of TLC and QLC flash, such as pseudo-SLC or SLC caching. Modern QLC actually has plenty of endurance, and performance shortcomings can be avoided with the assistance of intelligent controllers. In the future, penta-level cells (PLC) will expand capacity options even further.
|Cell Type||# of Levels/Bits||# of States||Program/Erase Cycles|
|SLC||1||2||Up to 100,000|
|MLC/DLC||2||4||Up to 10,000|
Most of our drives are based on TLC, but our Rocket Q, Rocket Q4, and Rocket XTRM-Q are based on QLC. We focus on TLC for performance with the QLC drives generally offering more capacity and lower cost per gigabyte. However, we have some of the highest capacity TLC SSDs on the market, including the 8TB Rocket 4 Plus. Our goal is to offer a wide range of storage options, but also to push the limits of performance and capacity.
See our storage products here.