On Tuesday 18 February 2025 21:18:03 Greenwich Mean Time Grant Edwards wrote: > On 2025-02-18, Philip Webb <purs...@ca.inter.net> wrote: > > 250218 Frank Steinmetzger wrote: > >> I recommend an external USB case with an NVMe SSD inside. > >> This may not be as compact and not as cheap, > >> but they are much much much faster and probably longer-lasting > >> than any USB stick, because their flash controllers are more > >> sophisticated > >> and the parts of higher fidelity. A not-too-cheap NVMe will do fine, > >> as long as it’s TLC, not QLC flash. > > > > Can you explain this -- TLC/QLC -- in more detail ? > > Tri-Level Cells, Quad-Level Cells. > > SLC (single-level cells) > > Each "cell" consists of a capacitor (constructed as a FET with an > isolated-gate). In the old days, the capacitor was either chared or > not: two levels. One bit per cell. Easy to make and reliable. If > you want more bits, layout more cells. That means larger dies, which > means more money. > > MLC (two-level cells) > > OK, let's also do four different charge levels! You get _two_ bits > per cell with the same area of silicon. Harder to do and not quite as > reliable, so we'll add a few extra blocks of cells and make software > deal with blocks that don't work. > > TLC (tri-level cells) > > Hey, what if we do 8 different charge levels! Now you get 4 bits per > cell. Even harder to do, even less reliable... > > > QLC (quad-level cells) > > [I think you see where this is going]
Yep, SLC > MLC > TLC > QLC in terms of performance, reliability, longevity and price. The phenomenal decrease in price per GB/TB over time is just that. Phenomenal. For reasons Grant explained it comes with measured reduction in reliability and longevity. Since they come with fast(er) DRAM cache which to some extent masks the underlying NAND performance, with other things being equal it makes sense to choose one with faster/larger DRAM. I think most consumer grade cards these days tend to be TLC and the cheaper ones QLC. QLC ought to outlast a conventional USB stick, but given a choice I'd opt for TLC.
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