In terms of basic SSD storage units, there is SLC, MLC, TLC and QLC, of which TLC is the most popular, although QLC will eventually replace them. In terms of I/O, there is SATA and NVMe.
Finally, the latest SSDs are also based on 3D NAND/VNAND technology. This can make SSD selection very confusing. Let’s break down the terms below.
SLC, MLC, TLC and QLC: These are storage cells and are shown schematically below.
In HDDs, the track is the building block of the memory that is stored. In an SSD, the same function is provided by a cell, which is essentially a gate circuit. How much each cell can store depends on the type of cell used in SSDs. The most popular are SLC, MLC, TLC, and QLC, which stand for single-level cell, multi-level cell, tri-level cell, and quad-level cell.
As the name implies, cells in SLC SSDs can only store one bit per cell, MLC stores two, TLC stores three, and QLC stores four. While this may seem like a “bigger is better” scenario, it is not. It is easiest to increase capacity (for the same price) with QLC drives because they require 1/4 the number of cells as SLC drives for the same amount of storage.
It takes more time to write multiple bits to a single cell, which also affects SSD endurance. This means that SLC SSDs are actually the fastest and most reliable, but they are much more expensive. Most commercial SSDs are TLC, which offers a reasonable compromise between performance, reliability and cost.
QLC is now becoming popular, and while it still lags behind TLC in terms of speed and endurance, it is a bit cheaper. This has led many OEMs to use them in pre-built PCs, so be sure to check the drive’s endurance (TBW or maximum TB written) and compare it to other TLC-based options before you buy.
Reviewed and edited by Zixiong Tang、
