Samsung EDU/EPP Discount: 2TB Samsung 870 QVO 2.5" MLC V-Nand Internal SSD Expired

There's no quality difference in the chips. Flash memory stores data by placing a charge at a certain voltage inside a cell. QLC just uses more voltage levels per cell. SLC has 2 voltage levels (1 bit). MLC has 4 (2 bits). TLC has 8 (3 bits). QLC has 16 (4 bits). And the planned PLC has 32 voltage levels per cell (5 bits).

Because of the finer gradations of voltages at higher bit counts, it's slower to write. You have to be more careful placing the proper charge in the cell, and reading it back. Sometimes they even resort to reading it multiple times, and averaging the result to get the "correct" voltage measurement.

Because the charge slowly leaks out over time, the voltage gradually drops, eventually resulting in data loss. The more bits per cell, the smaller a drop before the data is lost. And the shorter the longevity of the data (unless the drive regularly refreshes the charge, which decreases the lifespan of the cell).

These are all consequences of using more voltage levels per cell. Nothing to do with the chips themselves.

Note that this puts these high-capacity QLC and PLC SSDs between a rock and a hard place. They're too slow for enthusiast use (unless you do a ton of SLC buffering). And they have poor reliability for long-term storage. So if you don't want to use them if you need to fast data access, and you don't want to use them if you need to write a little data and leave it there a long time, what exactly are you supposed to use them for? The caching performance is gonna have to be stellar, or they're going to have to prove its long duration endurance for QLC to be accepted.

The cost savings is not that big a deal either.

Say SLC costs $100 per TB of flash cells (NAND)
MLC would be $50, a $50 savings per GB compared to SLC
TLC would be $33, a $17 savings per GB compared to MLC
QLC would be $25, an $8 savings per GB compared to TLC
PLC would be $20, a $5 savings per GB compared to QLC

QLC is pretty close to the point where I would just rather pay the extra $8 (about $15 at retail) for the better speed and reliability of TLC. (The endurance issues with TLC have largely been canceled out by increases in drive size. A 1 TB SSD has 4x the write endurance of a 250 GB SSD, 8x that of a 125 GB SSD. Maybe that might be a niche for QLC in a decade, when a 10 TB SSD costs $100, and endurance is a non-issue.)

Devil's advocate: because it's cheaper memory and it's not intended to survive enthusiast use like TLC and MLC (higher quality chips). This is a business decision. You wouldn't sell a cow and warrant that it could fly.

Still waiting to replace my 1TB nvme ssd I bought three years ago for $69. Samsung, SK Hynix and Micron please stop colluding to milk the consumer at 1-2TB kkthx. We are ready to be cheated at 4-8TB.