This write-up is meant to suppliment the DRAMless SSDs write-up and explain QLC nand flash, and how it should affect your process shopping for an SSD. When you look at an SSD on a marketplace like Amazon, Wish, Best Buy, Walmart, etc, you may see it advertised with "3D TLC" or "3D QLC". If you're unaware what this means, it's describing the amount of data that can be held within a single memory cell. There's tons of documentation on the technical nuances on this, so I'll try and avoid going too much into detail.
Personally I hate the term SLC, MLC, TLC, QLC, since it can be used to mislead consumers (Samsung calling everything MLC). But each term describes how many bits can be stored in a single flash cell, a bit being a 1 or a 0, the funny numbers that make up all computational data. SLC can be described as 1LC, it can store one bit per cell, which means a single flash memory cell can either be a 1 or 0. In recent (not too recent) years we've been able to store more than one bit (1/0) within a single cell, these are: MLC (2 bits per cell), TLC (3 bits per cell), and most recently QLC (4 bits per cell) This means that if you had two flash dies with the same amount of memory cells, a QLC die would be able to hold 4x as much data as an SLC die. As a result QLC is cheaper for manufactures to deploy, you need less raw material to acheive the same capacity.
How does this affect performance? The main technical difference between each type of flash is how precisely the cell is charged or programmed. A QLC cell has 16 possible programmed states, meaning it takes longer to program (Charge) the cell to the desired level. As a result the largest performance metric that QLC nand flash hits is in write operations. In an SSD review this is would be most apparent in a sustained sequential write, when the SSDs SLC cache runs out. In the real world this translates to the copy of a large file transfer or copy, I wouldn't coorelate it with download speed since you're typically going to be limited by your internet bandwith before your SSD is a limitation. This poor performance is also the most apparant outside of the drives SLC cache, which means that the file transfers would have to be rather large in order to exhaust the cache and force the drive to write directly to QLC. Typically an SLC cache will shrink as the drive fills up, which is why QLC SSDs are known for slowing down as they fill (This is true for all SSDs which use SLC cache, but more exaggerated with QLC) It's important to note that not all QLC dies perform the same, for example IMFT 144 layer QLC (Found on the intel 670P) gets pretty close to some worse TLC ties in native write speed (around 400MB/S) You're primarily only losing out on sustained sequential write, which is not a major concern for the average gamer-consumer workload. However I would not recommend QLC drives being used in a write intensive workstation environment.
Another concern with QLC drives is their endurance, since a QLC flash cell needs to be programmed and read with more precision, defects caused by wear are more likely to cause a stuck or flipped bit. This is why you see QLC drives have a lower endurance rating (TBW) than a TLC drive. Under a normal consumer workload this isn't much of a concern, in your average gamer/light use environment the drive is still going more than likely going to outlast its useful lifespan to the end user. However this can become a concern in more specialized write-intensive use cases, that I would typically coorelate with a home-server or workstation environment where you're writing large datasets to the drive constantly. It's once again important to note that not all QLC dies have the same endurance, and some overtake older TLC dies in endurance. Over time this endurance will only increase as the technology matures. For now though it's safe to say that in the average-consumer/gamer/light-productivity use case, QLCs endurance is not a major concern.
TL;DR: QLC nand flash primarily affects sustained sequential write performance, which is a typically unimportant metric outside of more medium to heavy productivity focused use-cases. There is a real impact on endurance, but not enough to create a serious reliability concern in typical use. If you're just gaming and browsing the web, or even hobby grade photo/video editing, your SSD having QLC nand flash should not be a major concern.
If you have any other questions or want to contribute our discord server is here https://discord.gg/wTA2T5QG, questions on any computer hardware topic are welcome.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
