The never-ending push for faster personal solid-state storage drives with larger capacities gave NAND manufacturers a complex equation to solve. The late 1990s brought widespread adoption of SSD storage based on NAND flash, and at the time, they stored a single bit per cell, so they were known as SLC SSDs. But scaling the number of bytes per millimeter quickly ran into physical limitations, as the transistors and other electronic structures on NAND can only be shrunk down so far before physics takes over, and you can't store the data bit reliably.
To fix this, manufacturers did a few things. The cells started to be stacked so that more could be put in the same area, giving more capacity without changing the form factor of the SSDs. And they figured out a way to add multiple bits of data per cell, with MLC allowing two bits, TLC allowing three, and QLC allowing four. Sounds good, right? More capacity, same space, but in practice, the new structures led to electron leakage (causing data retention issues), and increased wear as the thinner structure was easier to damage.
Nowadays, it's almost impossible to find an SLC or MLC SSD. They are expensive to manufacture, and the hard cap on capacity means they are no longer suited to the large amounts of data the average user creates, let alone the data amounts that enterprises create. Of the remaining two, QLC has a bad reputation, but that was partly from growing pains when the technology was still maturing, and it's no longer the case that I won't recommend TLC drives. The new models are fine for most users, and that's mostly due to improvements in the process.
What is a QLC SSD anyway?
Time for some engineering knowledge
All SSDs store data similarly, and it's easiest to visualize when talking about a single-level cell. Each cell has a control gate, a floating gate, and an n-channel that serves as a source and drain for electrons. By passing a voltage across the control gate of a cell, electrons get pulled from the n-channel into the floating gate, and that charge creates the bit storage.
In SLC NAND, these cells are connected in series, so it's a little more complex to read stored data, as you have to activate the control gate for the individual cell, while passing a higher voltage of 6V or so across every other cell in that string. That's with only two charge levels on the floating gate, but the engineers realized they could stack more charge levels on each cell and store more data.
MLC, TLC, and QLC were born. MLC has four charge levels per cell to store two bits. TLC has eight levels to distinguish the three bits it can store. QLC has sixteen differing charge levels per cell to cover the permutations of carrying four bits of data in that cell. More data per cell, more data per NAND chip, more data per SSD, without increasing the footprint of the physical device.
-
Crucial P3
-
Crucial P310
- Storage capacity
- 1TB or 2TB
- Hardware Interface
- NVMe PCIe Gen 4 x4
- TBW
- 220 per TB
- DRAM
- No
- Warranty
- 5 years
- Controller
- Phison E27T
-
Crucial BX500
- Storage capacity
- 1TB
- Hardware Interface
- Solid State
- Sequential read
- 540 Mb/s
The case against using QLC NAND
With complexity comes issues
Stuffing that many bits into one cell has several consequences, above the fact that you've just stored more data in one cell. QLC isn't as durable as any other NAND type, and it can be affected more by environmental temperatures. The added complexity means the controller takes longer to read the data on any given cell, significantly reducing the read and write speeds. This is mitigated by using a pseudo-SLC cache that uses part of the QLC flash (roughly 10% on any given SSD) as single-cell storage to store data while transferring it.
So, we've got QLC NAND with slower write speeds thanks to the complex design. Lower endurance thanks to the weaker structure and propensity for electron leakage. Slower performance over time as the pSLC cache fills up, and a big question mark over the long-term reliability of the stored data.
But nothing is quite that simple
While early QLC did have issues with slowdowns and reliability, the technology has matured to a point where the average user won't be able to tell the difference between TLC and QLC. It's no longer the kill, in the kill, kiss, marry equation, and as long as you pick a recent drive and check reviews, I'm confident enough to recommend it.
QLC isn't quite as bad as you think
Don't believe the bad reputation quite so quickly
While it's true enough that QLC NAND is slower than the other types, it's not enough that you'll notice if you're working or browsing the internet. You probably won't notice it when gaming either, because it's still many times faster than the fastest hard drives. If your drive has a DRAM cache, it'll be even faster, and modern SSDs use some of your system RAM as a cache by default, mitigating the slowdowns. You'll only notice for ultra-low latency applications or when you need high IOPS for database retrieval.
The quality of the SSD's controller also plays a significant part, and that's why I'd suggest buying QLC from trusted brands like Crucial, Samsung, Kingston, Sabrent, and SK Hynix. Write-heavy tasks can still slow the drives down, but most people's daily usage is read-heavy, and QLC is fine for this. And the other thing? The data center is full of QLC NAND, and enterprise users are picky about using the right tech for the job. If QLC was really that bad, it wouldn't be used, even if TLC is more expensive and can't store quite as much data.
QLC doesn't have to be on the do-not-buy list anymore
QLC SSDs are a world apart from the first models that came out. The only big drawback I can still levy against them is the need to keep significant amounts of free space for the pSLC cache otherwise, the drive will slow down significantly. Longevity could be an issue, but I've not personally seen SSD wear issues with QLC, and as long as the controller is designed properly with mitigation methods, you can happily use QLC SSDs for most tasks.