I have already published an article discussing the differences and working of different types of NAND Flash cells. In that article, we discussed that SLC and MLC NAND SSDs are saturated for enterprise environments. For consumer markets, we mainly have two options i.e. QLC and TLC SSDs. So, this article is going to compare both.
Most high-end and some mid-range SSDs will have the TLC flash memory while the low-end drives will have QLC NAND flash. It is really important to understand the difference because your drive’s performance, endurance, and reliability will majorly depend on the type of its NAND flash.
I am not going into the technical intricacies of how TLC and QLC SSDs work because everything is discussed in the article I have mentioned above. I will just give you an idea of what you should expect from a TLC drive vs a QLC drive. Before starting, I would say that if you don’t have serious budget constraints, it is better to go for the TLC SSDs. QLC drives are generally preferred for raw data storage, backups, etc. Let’s talk a little more about it. You can hear this great podcast about this topic.
Memory Cell division in SSDs
The basic and the smallest storage unit in an SSD is a floating gate transistor. There are millions and even billions of these transistors in a single SSD NAND flash chip. To achieve higher speed and parallelism, these cells are combined in bigger groups i.e. pages and blocks. A page is normally 4KB in size. It is the smallest unit that can be read or written on an SSD. The block is the smallest erasable unit, on the other hand.
Now, you would imagine that a 4KB page would have 4 x 1024 = 4096 bytes. Now, because each byte has 8 bits, the total individual cells inside a single 4KB page will be 4096 bytes x 8 (8 bits in 1 byte) = 32,768 memory cells. Yes, this is true for the SLC (Single Level Cell) SSDs but not for all types of SSDs.
The MLC (Multi-Level Cell) SSD can store the same amount (4KB) of data in half of the cells. This is because a single cell is being utilized for 2-bit values at a time. How this happens is, again, discussed in this article. So, if we do the same calculation for an MLC SSD, it would be like this:
Total bits in 4KB = 32,768 bits (as calculated earlier). Number of MLC cells = 32,768 bits ÷ 2 bits/cell = 16,384 cells.
For TLC (Triple Level Cell) SSDs, the calculation will go like this:
Number of TLC cells = 32,768 bits ÷ 3 bits/cell ≈ 10,923 cells.
And, lastly for the QLC (Quad Level Cell), here is the calculation:
Number of QLC cells = 32,768 bits ÷ 4 bits/cell = 8,192 cells.
What is TLC NAND Flash in SSDs?
TLC or Triple Level cell uses a single memory cell to store 3 bits of data. You can see that the storage density has been increased by 300% from the SLC. But, there are some drawbacks for the end users. TLC requires 8 distinct voltage states to represent the 3 bits (since 2³ = 8).
Now, TLC NAND typically offers around 1,000 to 3,000 program/erase (P/E) cycles, lower than MLC but higher than QLC. Because the storage density is higher than the MLC, it provides storage at a much cheaper cost. The Read and write speeds are slower compared to MLC, due to more complex voltage management, but it’s generally faster than QLC.
All in all, TLC strikes a good balance between price, performance, capacity, and endurance, in normal consumer-grade SSDs. However, it has very less application in the enterprise environment because of lower read/write cycles.
What is QLC NAND Flash in SSDs?
The QLC (Quad Level Cell) NAND flash in SSDs comes with around 400% higher storage density than the SLC flash. But, the performance and endurance decrease a lot because of much complex circuitry. QLC requires 16 distinct voltage states to represent the 4 bits (since 2⁴ = 16). The typical P/E cycles supported by the QLC NAND flash cells are between 100 to 1000. This is because of the more distinct voltage levels and continuous usage of the cells.
Because of the highest storage density, QLC drives allow more data to be stored in the same space, making it the cheapest per gigabyte among NAND flash types. QLC has slower read/write speeds compared to TLC and MLC. This is mainly because of the heavy management overhead of 16 different voltage levels.
Which one should you choose?
1. For performance-oriented users
If you have performance goals for your drive and the computer in general, it is better to pick a TLC SSD. TLC SSDs have comparatively better raw speed than the QLC SSDs. Although the overall performance will depend on many other factors, the TLC drives generally offer better specs in terms of performance, endurance, and latency. So, if you are a gamer or anyone who wants a drive that offers the best performance, it is good to go with a TLC drive. Video editors, graphics designers, software developers, gamers, and 3D artists, should go for the TLC SSDs.
Not only performance, the TLC SSDs give better endurance ratings like TBW, MTBF, and DWPD. Because of the higher P/E cycles, the TLC drives will be much more reliable than the QLC SSDs.
2. For casual and storage-oriented users
If you are a casual user or looking for a drive for raw data storage, QLC SSDs are better than the TLC. For backups, archiving, and storing media files, QLC serves offers a good balance of price. Generally, the TLC SSDs would cost you around 0.10 to 0.20$ per GB of storage. While QLC drives can cost below 0.10$ to 0.05$ per GB.
Also, the lower endurance rating of QLC can easily be mitigated by using it for long-term data storage. For instance, a QLC drive with 4TB data stored on it which is generally read makes more sense instead of employing an expensive TLC drive. However, it is always suggested to go for TLC over QLC if you don’t have any budget constrains.
It isn’t that the QLC are very poor in performance. You will generally find the QLC drives with their own DRAM to compensate for the slow raw speed. So, even with an QLC, you can still get decent performance with any kind of system.
3. Longevity and Reliability considerations
Drive longevity can be a big deciding factor if you do professional work on your computer. Also, if you store sensitive information on your drive, you’ll probably want to mitigate any risks of data loss and invest pretty well in the storage drive. In this case, it is definitely a good option to go for the TLC SSDs.
Also, if your work includes heavy write operations like virtual machines, database management, software development, etc, your drive would wear out a lot faster. In this case, as well, choosing the TLC NAND flash is a good option.
4. Budget Considerations
As I discussed earlier, if you have a good budget, you should just go for the TLC SSD. But, for low budgets, a QLC SSD can save a lot of money for you with more data storage capacity. For example, it is easy to find a 2TB QLC SSD for almost the same price of a 1TB TLC SSD. In most cases, the high-capacity QLC will be cheaper than the low-capacity TLC SSD. So, for those with moderate data read/write tasks, saving money on QLC SSD is always a smart move.
I hope this helps!