Summary
As a PC builder for over a decade, I’ve learned one thing: never trust advertised specs. Sure, PC components can reach those high speeds that are highlighted in red and are supposed to be catchy, but only under very specific conditions. Take SSDs, for instance: the advertised speeds on the packaging might not be sustained over time, and it’s not that the manufacturer is lying to you; it’s just that maintaining those speeds over time depends on the NAND flash configuration, which should be the biggest factor to weigh in for your next SSD purchase. Why advertised SSD speeds don’t tell the whole story NAND flash type determines the real-world performance SSDs replaced conventional physical hard drives with a flash-based mechanism with no moving parts, offering faster performance and instant data access. The average PC user will be fine with nearly any SSD over an HDD, but as times have changed and workflows have become more complex and demanding, SSD performance has become a big consideration. Now, you can buy an SSD purely based on its advertised speed, which might be 14,000MB/s read speed for a PCIe 5.0 drive, but that drive might not be capable of holding those speeds over a longer period of time. To actually determine the reason behind this, it’s important to know the difference between NAND flash memory types, and the four main types are:
- SLC: Single-Level Cell can store a single bit per cell, either 0 or 1.
- MLC: Multi-Level Cell holds 2 bits per cell.
- TLC: An extension of MLC, the Triple-Level Cell, can hold up to 3 bits at a time.
- QLC: Another type of MLC, the Quad-Level Cell, can hold up to 4 bits simultaneously. generally wear out faster than HDDs. This is where the NAND flash memory types come into play. Fewer bits per cell means more endurance and higher speed because the SSD controller (the processor that manages the memory cells) has less work to do to maintain the cells, and the program and erase cycles are also less limited as a result. NAND flash memory types that store more bits tend to be slower and have lower endurance because they require more precise, high-voltage charge states to store data. However, flash memory like SLC or MLC, which holds fewer bits per cell, is more expensive to manufacture than QLC or TLC because it has fewer cells and more bits per cell. How SLC caching fakes your SSD’s top speeds The burst performance speed disappears once the emulated buffer fills up Your SSD can emulate MLC flash memory as SLC, which is where the higher speeds come from. So, an SSD with QLC that emulates SLC might deliver faster speeds on the 250GB portion of a 1TB capacity, considering the entire space is empty. So you can get higher SLC speeds for the first few minutes of your write process, but the speed then drops as the SLC cache fills up. The SSD only slows temporarily; after the workload ends, it rebuilds part of the SLC cache in the background by moving and converting data. The exact behavior may differ depending on the type of MLC your SSD uses, whether it’s 4-bit or 3-bit per cell. So, when you see an advertised write speed on the SSD’s spec sheet, the highest speed mostly represents the SLC cache, and the performance can drop when that cache is filled in real-time workloads. For instance, an SSD with an advertised write speed of 5,000 MB/s may drop to 500 MB/s-1GB/s after the SLC cache fills. What to check before buying your next SSD A detailed spec sheet overview and benchmarks for long write speeds While the NAND flash memory type isn’t advertised prominently in the SSD listing’s headline, I recommend checking the detailed specifications. If the SSD still doesn’t mention that, I recommend checking reviews and sites like TechPowerUp, which has an extensive SSD database. Otherwise, cheaper SSDs usually use TLC/QLC flash cells. SSDs with 2 bits per cell strike the perfect balance between performance and price. With NAND prices skyrocketing due to the AI-led memory shortage, be mindful of your next purchase. Buying an SSD with a QLC configuration and high advertised speeds might feel like a great deal, but you’ll note the impact on heavy workloads when the pseudo-SLC cache runs out. Also, SLC SSDs are not usually sold to consumers, as they can be super expensive and are mostly used in enterprise settings, so I’d recommend not looking up SLC SSDs. Most SSDs also use a mixed configuration across different MLC extensions to reduce costs, so keep that in mind as well. Missing DRAM slows down your SSD further If you’re going for the fastest SSD available on the market, there are other factors to consider beyond just NAND flash memory cells. For NVMe drives, DRAM is a hidden spec that most people ignore; it provides its own pool of fast memory that maps all the data in the entire SSD. Without DRAM, the NVMe relies on a portion of RAM to map the data pool, which is slower. As a general rule of thumb, a 1TB drive has about 1GB of DRAM.