TLC (triple-level cell) and QLC (quad-level cell) are NAND flash designs that store three and four bits per cell respectively. Fitting more bits in each cell lowers the cost per gigabyte but makes the cell harder to read and write reliably.
Why it matters
A TLC cell must distinguish 8 voltage levels, while a QLC cell must distinguish 16. Smaller gaps between levels mean slower writes, more error correction, and fewer program/erase cycles before wear-out. For this reason QLC drives usually have lower rated endurance and rely on a fast cache to stay quick.
Most SSDs set aside part of the flash to act as an SLC cache, writing one bit per cell temporarily. Once that cache fills during a long transfer, speed can drop sharply.
Example
Drive endurance is quoted in TBW (terabytes written). A hypothetical 1 TB drive rated at 600 TBW can absorb about 164 GB of writes every day for 10 years. For ordinary desktop use, daily writes are usually far lower, so QLC is often acceptable for storage and game libraries.
Common mistakes
- Judging a drive only by its peak speed; sustained write speed after the cache fills is what matters for large transfers.
- Using QLC for heavy write workloads such as databases or constant video capture.
- Treating "TLC" as automatically fast; controller and cache design matter too.
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