SSD: TRIM, SLC cache and QLC

In short: How fast and durable an SSD is depends on the cell type (SLC, TLC, QLC), a buffer (SLC cache) and TRIM.

In more detail: Flash stores bits in cells. The more bits per cell, the cheaper, but slower and less durable.

In Depth

• Cell types: SLC (1 bit), MLC (2), TLC (3), QLC (4). QLC is cheap and dense but writes more slowly and tolerates fewer write cycles.
• SLC cache: Part of the flash temporarily writes like SLC, hence briefly very fast. Once the cache is full (on large copies) the rate falls to the true speed, on cheap QLC SSDs to 100 MB/s or less.
• DRAM cache: High-quality SSDs have a little DRAM for the mapping table. “DRAM-less” is cheaper but slower on random access.
• TRIM: The operating system tells the SSD which blocks are deleted so it can tidy up in the background and stay fast when writing. Automatic on Windows and modern Linux (fstrim.timer).
• TBW: “Terabytes written” states how much you may write under warranty. For normal use it lasts years.
• Tip: Leave about 10 % free. See wear levelling.

See also: SMART values