If you’ve done any research or scoured tech forums or Reddit before buying an SSD, you’ve probably noticed that no one is a fan of QLC SSDs. While SLC, MLC, and TLC lag behind SSDs that use NAND flash, they’re not that bad. Here’s why.
QLC NAND is actually inferior to SLC, MLC and TLC
High capacity, but low performance and durability
What is true is that QLC NAND is clearly inferior to SLC, MLC and TLC. It all depends on how flash memory it works. The smallest building blocks of NAND flash are tiny memory cells that store data by changing voltage levels.
The highest quality flash, SLC (Single Level Cell) can store only one bit per NAND flash cell and supports only two voltage states: on or off. This makes SLC extremely fast because the memory controller only needs to recognize two voltage states. They are also extremely durable as they only have to go through two electrical states: on and off, allowing them to go through about 100,000 P/E (Program/Erase) cycles before becoming unreliable.
Despite its advantages, you will not find any SLC SSD on the market, because SLC NAND is very expensive to manufacture due to its low capacity. For example, a 1TB SLC SSD will cost the same as a 4TB TLC SSD. Of course, even if you found a buyer for such an SSD, you wouldn’t be able to sell them in large enough quantities to justify the investment, so manufacturers years ago decided to focus on other types of NAND flash that were cheaper to produce: MLC (Multi-Level Cell), TLC (Tri-Level Cell) and QLC (Quad-Level Cell).
MLC stores two bits per cell by encoding data at four voltage levels, making it slower and less durable than SLC. MLC is also quite expensive to produce, at least in capacities comparable to modern SSDs. While you may have found MLC SSDs in the past, they haven’t been manufactured in years.
TLC can store three bits of data per cell and is slower, but fast enough for most workloads. Although more expensive to manufacture than QLC, it remains affordable, hence TLC SSDs to judge mid- and high-end consumer SSD market.
Then there is QLC flashcapable of storing four bits in one cell, but requires sixteen different voltage levels. Because the controller must distinguish between multiple states, QLC NAND is much slower than other types of NAND because both reading and writing require greater precision.
QLC also has relatively low durability because its cells must handle these sixteen stress states, which causes them to deteriorate faster. As a result, QLC NAND can typically withstand only 1000 P/E cycles before failing.
The only advantages of QLC NAND over other types of NAND are higher capacity and lower manufacturing costs. These two factors alone have made QLC NAND very popular in budget and more recently mid-range consumer SSDs.
Early QLC SSDs had a number of compromises
Cheap, but not so cheerful
Early QLC SSDs weren’t great. Their durability was very low, performance dropped to HDD levels after the pseudo-SLC cache – a part of NAND configured to work like SLC – was exhausted. DRAM cacheand they were equipped with inferior controllers.
This resulted in serviceable consistent read and write performance as long as there was no need to write tens of gigabytes of data simultaneously. There would be low-level supervisors thermal regulatorand lack of DRAM cache combined with cheap controllers resulted in poor random read and write performance.
In short, they were a solid choice for mass storage (backups of games, videos, and data that aren’t written to or accessed frequently), but a poor choice for most workloads, such as boot drives, scratch drives for video editing, or warm storage.
The bottom line was that early QLC SSDs were cheaper than TLCs, but not enough to justify their lower specs. If you don’t need a 4TB SSD, you can get a TLC SSD for a little more money, which has double the durability, higher random performance, and DRAM cache. This was the main reason many people avoided QLC SSDs. But times change, and modern QLC SSDs can be a solid choice for most workloads.
QLC SSDs have come a long way
Modern QLC SSDs are solid performers
Modern QLC NAND still has lower stability than TLC, but that doesn’t really matter to most people. For example, let’s say you bought a 1TB QLC SSD with a capacity of 500 TBW (written in terabytes), which is a typical endurance rating for modern QLC drives. That means you can write 50 gigabytes of data to it every day and it should still last you around 27 years! Of course an SSD will die much faster, but I’m trying to emphasize that durability is not really an issue with QLC drives anymore.
Memory controllers have also come a long way. Modern budget controllers can easily handle QLC NAND and its sixteen voltage states. True, thermal throttling is still real in some models, especially PCIe 4 drives, but you can solve this with a $10 SSD cooler.
The lack of DRAM cache is another thing with modern QLC SSDs, as almost every DRAM-less drive released in the last few years is supported HMB (Host Memory Buffer) technology. HMB combined with improved controllers results in higher random performance that is very close to TLC drivers.
Finally, while early QLC SSDs typically had small SLC caches below 50GB, newer models offer caches that can soak up hundreds of gigabytes of data at once before write speeds drop to typical QLC levels.
A modern QLC SSD is suitable for most workloads
If you’re considering buying a new SSD, a QLC drive won’t be as steep a downgrade compared to TLC options like it was years ago. You can use it not only to store games and large files, but also as a boot drive and other similar use cases that involve a lot of reading operations.
On the other hand, I still recommend TLC drives for write-intensive workloads. Video editing, working with AI or large databases, and any other workload that involves a lot of intensive write operations will benefit from TCL NAND. I’d also avoid QLC SSDs if you know you’re going to fill the drive to the brim, as they can slow down more than TLC SSDs when full.
Before pulling the trigger, you should always do your due diligence and research the models you are interested in. Read both professional and user reviews, especially long-term user reviews, as a good portion of SSDs have multiple revisions, and newer ones are often equipped with lower-quality controllers and NAND.
Overall, QLC SSDs aren’t such a bad choice anymore. Sure, they’re still inferior to the TLC options, but they’re durable and fast enough for most people. If you have the money, buy a high-end SSD; just know that the QLC SSD will meet all your needs while being easier on the wallet.
