I'm sure you've encountered a point when moving files to a solid-state drive (SSD) suddenly grinds to an unexpected halt, and wondered what was causing this to take place. After all, the manufacturer likely promised considerably higher speeds for the SSD, which lured you into a false sense of hope with drive performance. The reasons for these slowdowns can vary depending on the SSD and wider system it's connected to, but for cheaper flash storage, it can be caused by the SLC cache. This can cause transfer speeds grind to a crawl, sometimes reaching figures you'd typically see with hard drives.

This isn't a firmware bug or something wrong with the cache present on the drive, but rather how SLC caching works and is as designed by the manufacturer. SLC caching made it possible for affordable SSDs to become viable in a wide range of applications, but over the years, data demand and workloads have only increased in complexity, requiring more (and faster) storage. The limits of SLC caching have always been present, but it was largely avoided unless hitting the drive with some heavier writes. But these days, you can find yourself experiencing reduced performance quicker than initially thought.

Why SLC caching came to be

Where it all began

NAND was expensive, slow, and had limited endurance. When SSDs first hit the market for consumers, they were out of reach for the masses. This created a demand for more affordable drives with flash technology. We first started with what's known as a Single-Level Cell (SLC), which essentially stores one bit per cell. This is the fastest and most durable form of flash storage, but it's also incredibly expensive. Multi-Level Cell (MLC) is next up, doubling density and reducing endurance. Then there's Triple-Level Cell (TLC) and Quad-Level Cell (QLC), storing three or four bits per cell, respectively.

The more densely packed the NAND, the slower and less durable the drive becomes. It's a trade-off that allows for higher-capacity SSDs to be priced more aggressively for consumers to make the switch from mechanical drives. To help facilitate this trend, companies started to combine the speed and reliability of SLC with slower MLC, TLC, and QLC, creating a local cache on the drive to provide a notable uptick in performance without increasing the cost of the drive by changing the underlying storage technology. This worked well to begin with, back when data demands were a fraction of what they are today.

Unfortunately for those with TLC and QLC drives with the densest NAND, sustained loads lead to reduced performance. Without direct intervention from manufacturers within the design stage, most budget SSDs would provide write speeds that wouldn't provide much enticement to upgrade from an HDD. This is where SLC caching came into play since we all cared little for the physical limitations of NAND storage and simply desired quicker, more reliable, and flash-based drives for laptops, phones, and everything else.

SLC cache was almost too good to be true

Instead of using the entire TLC or QLC drive as designed, companies realized that one could dedicate part of the drive and use it as if it were SLC. Instead of storing three or four bits per cell, this specific part of the SSD would be used to temporarily store just one bit per cell. Data would be written quickly to this somewhat emulated SLC drive and later reorganized into more dense TLC or QLC storage once the write requests were complete. Using SLC caching worked well for average use, such as saving a document, downloading a single file, or moving small backups to a secondary location, but it quickly bogged down when things got tough.

Initially, performance looks solid with numbers matching what's marketed on the box. Cheaper SSDs with SLC caching would feel similar to more expensive models, allowing those with tighter budgets to enjoy similar bandwidth to those who could pay for SLC storage. Depending on how large the SLC cache is, which can vary on drives with dynamically adjusted cache levels, one could move numerous files to the drive without noticing a single second of slowdown. There's a difference between fixed and dynamic caching, too. While the latter sounds better since you're not reserving as much space on the drive for temporary storage, it also can't expand as much when the SSD is full.

Once you've worked the SSD hard enough with write requests, the controller will run out of cache to utilize, and that's when the house of cards collapses. This is especially apparent with QLC drives, which exacerbate the issue. Due to how dense QLC storage is, performance would suffer without a cache present for anything other than a light workload. This essentially renders the drive useless without the cache present and working at an optimum level. The unfortunate reality of SSDs and their reduced pricing means more and more systems use flash-based storage for data other than OS files.

Sequential writes don't tell the whole story

I'm talking extensive Steam game libraries, apps, large media files, and more. Unfortunately, this is a trap many can fall into. Companies don't help matters with sequential speed ratings for marketing. These speeds are almost always measured within the SLC cache. They aren't read when the drive runs out of SLC cache and needs to power through with TLC or QLC. That's why sustained write performance is also incredibly important for choosing the best SSD for your system. We can also play a part with this due to not completing filling up the cache and working the drive hard to showcase what can happen once this invaluable resource is fully spent.

What's more, SLC caching can be affected by firmware versions, so even updating the SSD can either improve or degrade sustained rates. This is why larger SSDs can appear faster, thanks to the larger dynamic cache, so if you were to compare two varying capacity drives with the same NAND type and controllers, you may see different results. Running these drives with heavier workloads will showcase how running out of cache can cause problems. Storing all your data may unearth some hidden design choice that may appear as though the SSD is failing, but in reality, it's operating as intended.

So when you're next shopping around for an SSD, be mindful of the NAND type as well as sustained speeds to get a better understanding as to how the drive will operate and whether it's the best data storage solution for your needs. Remember: this isn't an SSD failure, but more a trade-off of how flash storage works.