Posted September 06, 2024
SLC (1 bit per cell) = 100% vs 0
MLC (2 bit per cell) = 100% vs 66% vs 33% vs 0
TLC (3 bit per cell) = 100% vs 86% vs 71% vs 57% vs 43% vs 28% vs 14% vs 0
QLC (4 bit per cell) = 100% vs 93% vs 87% vs 80% vs 73% vs 67% vs 60% vs 53% vs 47% vs 40% vs 33% vs 27% vs 20% vs 13% vs 7% vs 0%
The more they squeeze in, the fewer flash chips are needed to store the same amount of data. However, as you can see, the more they squeeze in, the lower the "headroom" between voltage states, the lower the endurance of the drives and the less time they can spend unplugged & unpowered before they start to lose their data. Older SLC flash used to be able to write 100,000 P/E cycles (times each cell of flash can be written to and erased before it wears out). MLC flash is still fairly decent at 10,000 P/E (more than enough for most people), TLC is down to 3,000 or so, still enough for most consumers. However QLC completely falls through the floor into barely just a few hundred P/E cycles, and often isn't that much cheaper in practise than TLC. They developed strong ECC (error correction) to help mitigate this with TLC but that was a one-time boost and there is no "magic beans" solution that will allow them to keep on squeezing in more and more voltage states without compromise.
As for the interfaces:-
SATA - Serial ATA. Uses the same interface as HDD's and ODD's and plugs into the motherboard via a cable. Slow (typical max 500MB/s) but you can used 4-6x of them per motherboard.
M.2 - The name of the physical socket for the new "slot-in" SSD's. It can "talk" in 3x interfaces (SATA, NVMe and USB). People mix up NVMe and M.2 all the time, but you can get SATA versions of M.2 drives as well as M.2 devices that aren't SSD's (eg, WI-Fi / 5G cellular "modem" cards in laptops). The number after an M.2 refers to the physical size. The "standard" size is 2280 = 22mm width x 80mm length. There are shorter ones, eg, 2230 / 2242 = 22mm width x 30/42mm length often used in some tablets, Steam Deck, etc.
NVMe / PCIe SSD - Essentially the same thing - the "faster" interface used on new drives. NVMe SSD means it will plug into the M.2 slots whilst PCIe SSD usually means the are a PCIe slot-in card (like a GPU). They both use PCIe speeds though.
tl:dr - If you're in any doubt, buy an NVMe TLC drive. If you find a TLC and QLC at nearly the same price, always go for the TLC one. And if you plan on using an SSD externally, be sure to plug it in at least every few weeks. It used to be the case they were rated for "52 weeks unpowered data integrity" but that was measured on MLC in a now 10 year old study. Some cheap QLC drives can only last as little as 2-6 months constantly unplugged before the data starts to "fade" and strong error correction is needed to "guestimate" back the data you wrote often resulting in speeds plummeting down to 10MB/s. For external data storage (not system drives), if you have any irreplaceable data (wedding photos, etc) very definitely don't assume (as some have) that 1x $300 SSD is more reliable than 2-3x $100 HDD's just because it's "solid state".
Edit: Another thing to be aware of is "DRAMless" drives. Most SSD's have a DRAM cache to store the Flash Translation Layer / partition table, but to save money there's some that don't. There's a feature called "Host Memory Buffer" that can use a small chunk of main memory (usually 60-100MB) in place of that, that works well but it can only be used on internal drives, ie, a DRAMless SSD in an external USB caddy will have no functional memory cache at all and performance and endurance will be a lot lower. So if you're planning on using SSD's as external backup drives, buy those with built-in DRAM to be on the safe side.
Post edited September 06, 2024 by AB2012
