Quad-level cell (QLC) flash memory, or QLC SSD (solid-state drive), is a capacity-optimised NAND memory technology that delivers a per-terabyte cost that matches or beats hard-disk drives (HDDs). As its name suggests, QLC SSDs store four bits per cell, delivering NVMe performance with higher capacities.
QLC SSD vs. SLC vs. MLC vs. TLC
QLC SSDs are the latest in a long trend of squeezing more bits per cell within a NAND flash device. Here’s a brief overview of how the technology has evolved over time:
Single-level cell (SLC) flash: One bit per cell, two possible voltage states
Multi-level cell (MLC) flash: Two bits per cell, four possible voltage states
Triple-level cell (TLC) flash: Three bits per cell, eight possible voltage states
Quad-level cell (QLC) flash: Four bits per cell, 16 possible voltage states
As can be seen, the number of possible voltage states doubles with each additional bit stored within a NAND cell. There’s an inherent tradeoff between capacity and complexity as you increase the number of bits you can fit within a single cell.
Greater electrical precision is needed to account for the added complexity of managing multiple voltage states during read/write. In practice, this can translate to a reduction in performance and longevity of the NAND device.
NAND endurance is measured in program/erase (P/E) cycles. So far, manufacturers have been able to produce QLC flash with 1,000 P/E cycles, which is orders of magnitude less than what’s possible with SLC SSDs (100,000 P/E cycles).
However, performance and endurance are relative. Flash is still orders of magnitude more performant than HDDs, and there are work-arounds for dealing with the endurance limitations of QLC SSDs (e.g., using SLC SSDs as a cache).
This same tradeoff slowed adoption of TLCs when they first appeared, but the technology improved over time, and today most enterprise flash storage arrays deliver high performance, reliability, and speed with TLC NAND. With the advent of QLC SSDs, it’s now possible to enjoy the speed of all-flash storage at a cost per capacity that can match or beat HDD storage arrays.
How do QLC SSDs work?
Flash memory stores data in individual memory cells made of floating-gate metal–oxide–semiconductor field-effect transistors (MOSFET).
Traditionally, each cell had two possible states—one or zero—according to a certain one voltage level. An SLC uses these two possible voltage states to store a single bit of information, with a 1 being when the charge is almost empty and a 0 when it’s almost full.
Two-bit MLC SSDs, on the other hand, use four possible charge values, or levels, per cell to store more than one bit of information, assigning a charge level to every possible combination of ones and zeros as follows:
- 11, when it’s close to 25% full
- 01, when it’s close to 50% full
- 00, when it’s close to 75% full
- And 10, when it’s close to 100% full
QLC SSDs simply expand on this concept by using 16 different threshold voltages to store up to four different pieces of information per cell.
What are the Benefits of QLC SSDs?
Common benefits of QLC flash memory include:
- A lower total cost of ownership (TCO) for read-centric loads
- A reduced data centre footprint
- An exponential increase in capacity with the speed of all-flash
However, QLC SSDs are less reliable than their flash memory counterparts (more on that in the next section).
What are the Disadvantages of QLC SSDs?
Durability is one of the primary disadvantages of QLC SSDs, and that’s because storing more bits per cell has proven to be very detrimental to write cycles.
While you can read to a flash cell as many times as you want, every time you write to it you degrade the charge traps’ floating gate insulators just a little bit.
Are QLC SSDs reliable?
As such, what QLC SSDs achieve in lower costs and increased capacity, they lose in endurance and reliability. Current QLC SSD cells can only endure about 1,000 program-erase cycles before they start to break down and become unreadable. SLC SSDs, on the other hand, can probably be erased and rewritten more than a hundred thousand times before they start to break down.
What about performance: Are QLC SSDs slower?
Higher-level SSDs are also slower. A TLC SSD needs to check eight different threshold voltages to attain a precise value, but a QLC SSD needs to check 16 different threshold voltages. As the number of possible values increases, the differences between them becomes harder to discern, and partly because of this, the read latencies for SSDs have nearly doubled with each additional bit of data.
Writing data is also way slower with QLC SSDs. SLC has a very large margin of error and can write data to a cell with just one large voltage pulse. But higher-level SSDs have to use cycles of a short pulse and then verification to precisely dial in the required threshold voltage. Currently, QLC SSDs can only write between about 80 and 160 megabytes per second, which is even slower than some mechanical hard drives.
Most QLC drives work around this issue via “caches”: to speed up file transfers, the drive will actually treat some of its cells as if they were SLCs instead of QLCs by only reading or writing a single bit of data to them. This works well until you start to transfer massive files or have big sustained workfloads, at which point performance will typically drop. The write performance of a TLC drive, which uses the same caching strategy, is about 10 times better than that of a QLC drive.
That said, the performance and relative excellent capacity of QLC SSDs still make them a great choice for many applications, including media streaming, databases, and backup.
Unless you’re writing tons of data, the reduced endurance shouldn’t be a problem, either. Most QLC drives are still rated for between about 100 and 300 terabytes of data written per 1 terabyte of drive capacity, which means a 4-terabyte QLC SSD will keep you going through writing about 80 days worth of 4K footage.
Which SSDs are QLC?
There are many QLC SSDs currently on the market. These include:
