QLC Flash Is The Problem

Here’s the issue. Making SSDs is expensive, and few people want to pay $200 for a 512 GB SSD when you can get “2000 GB” mechanical hard drives for less than $50. Bigger capacities sell.

SSD manufacturers are increasing storage capacities while keeping costs down—but this is bad for performance and endurance. Large SSDs may be getting cheaper, but there’s a tradeoff for each leap in SSD technology. We’re currently seeing the rise of Quad Level Cell (QLC) SSDs, which can store 4 bits of information per memory cell. QLC hasn’t replaced standard SSDs completely, but a few drives using it have made their way to the market, and they’ve got problems.

Specifically, SSD manufacturers have to find a way to fit more space into the same sized NAND flash chips (the actual data-storing part of the SSD). Traditionally, this was done with a process node shrink, making the transistors inside the flash smaller. But as Moore’s Law slows down, you’ve got to get more creative.

The ingenious solution is multi-level NAND flash. NAND flash is capable of storing a specific voltage level in a cell for an extended period. Traditional NAND flash stores two levels—on and off. This is called SLC flash, and it’s really fast. But since NAND essentially stores an analog voltage, you can represent multiple bits with slightly different voltage levels, like so:

The problem, as shown here, is that it scales up exponentially. SLC flash only requires voltage or lack thereof. MLC flash requires four voltage levels. TLC needs eight. And in the last year, QLC flash has been making a break into the market, requiring 16 separate voltage levels.

This leads to a lot of problems. As you add more voltage levels, it gets harder and harder to tell the bits apart. This makes QLC flash 25% denser than TLC but significantly slower. The read speed isn’t affected that much, but the write speed takes a dive. Most SSDs (using the newer NVMe protocol) hover around 1500 MB/s for sustained read and write (i.e., loading or copying large files). But QLC flash only manages between 80-160 MB/s for sustained writes, which is worse than a decent hard drive.

QLC SSDs Break Down Much Quicker

All SSDs generally have unfavorable write endurance compared to hard drives. Whenever you write to a cell in an SSD, it slowly wears out. Erasing a cell is supposed to rid it of electrons, but a few always stick around, causing a “0” cell to be closer to “1” over time. This gets compensated for by the controller by applying a more positive voltage over time, which is fine when you’ve got a lot of voltage room to spare. But QLC doesn’t.

SLC has an average write endurance of 100,000 program/erase cycles (write operations). MLC has between 35,000 and 10,000. TLC has around 5,000. But QLC only has a measly 1,000. This makes QLC unsuitable for frequent access drives, like your boot drive, that are written to very frequently.

Bottom line—don’t buy a QLC drive to use for your operating system’s system drive. They’re far too unreliable to be sure it won’t degrade in a few years. We would recommend using a large QLC drive as a replacement for a spinning hard drive, and use a fast SLC, MLC, or TLC drive as your primary OS drive. This may be a problem in laptops, where you don’t have the option, but QLC is still very new and hasn’t made its way into laptops yet.

Efficient Caching Hides These Problems

At this point, you might be asking why QLC is even a thing when it’s objectively slower and breaks much quicker than the other flash types. You obviously can’t market a downgrade, but SDD manufacturers have found a way to hide the problem—caching.

QLC SSDs dedicate a portion of the drive to a cache. This cache ignores the fact that it’s supposed to be QLC and instead operates like SLC flash. The cache will be 75% smaller than the actual drive space it takes up, but it will be much faster.

Data from the cache can be written to at the same speed as other high-end SSDs, and will slowly be flushed out by the controller and sorted into the QLC cells. But when that cache is full, the controller has to write directly to the slow QLC cells, which causes a considerable drop off in performance during long writes.

Take a look at this benchmark from Tom’s Hardware’s review of the Crucial P1 500GB, a consumer QLC SSD, which shows this problem quite clearly:

The red line representing the Crucial P1 operates at solid NVMe speeds, albeit a little slow compared to some of the higher-end offerings. But after about 75 GB of writes, the cache becomes full, and you can see the real speed of QLC flash. The line plummets to around 80 MB/s, slower than most hard drives for sustained writes.

The ADATA XPG SX8200, a TLC drive, displays the same characteristics, except the raw TLC flash after the drop off is still faster. Most other drives also employ this caching method, as it speeds up quick, small writes to the drive (which are most common). But sustained writes are what you’ll notice most—you won’t notice if a small file copy takes 0.15 seconds versus 0.21 seconds, but you will notice if a large one takes an extra ten minutes.

You could easily write this off as an edge case scenario, but that cache doesn’t stay 75 GB forever. As you fill the drive up, the cache gets smaller. According to Anandtech’s testing, for the Intel SSD 660p lineup, the cache for the 512 GB model is reduced to just 6 GB when the drive is mostly full, even with 128 GB of space left.

This means if you filled up your SSD and then tried to install a 20-30 GB game from Steam, the first 6 GB would write to the drive extremely quickly, and then you’d start to see the same 80 MB/s speeds for the remaining files.

Granted, you’re likely limited by download speed in this example, but in the case of updates (which need to download and then replace the existing files, effectively requiring twice the space) the problem would be much more apparent. You’d finish downloading, and then have to wait forever for it to install.

So Should You Avoid QLC?

You should definitely avoid QLC drives with 512 GB (and less, once it becomes cheaper to produce), as they don’t make much sense. You’ll fill them up much quicker, and the cache will be smaller when it’s full, making it considerably slower. Plus, they’re currently not much cheaper than the alternatives.

Despite its shortcomings, QLC flash isn’t too much of an issue when you look at the higher capacity drives. The 2 TB model of the 660p features a minimum of 24 GB of cache when it’s filled up. It’s still QLC flash, but it’s an acceptable trade-off for a cheap 2 TB SSD that operates really fast most of the time.

Given their gigantic capacities, QLC based SSDs can serve as a decent replacement for a spinning hard drive, provided you make regular backups in case it kicks the bucket. It’s optimal for something you access infrequently but want to be really fast when you do, and with a decently sized SLC cache, most sustained write operations will be reasonably fast until you fill the drive up.

Due to the reliability issues, you should avoid using it as a boot drive or for anything that gets written to very often.

There’s still a lot of advancement to be made in other aspects of manufacturing—better controllers capable of addressing more flash chips, cheaper flash chips as process nodes mature, and perhaps other technologies altogether. QLC flash isn’t becoming the standard anytime soon; currently, it’s just another option. Just make sure that when buying an SSD, you check the technical specifications and pay attention to the type of flash used to make them.