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I've come across several articles stating TLC SSDs have better reliability and longer lifespan than QLC SSDs. However, they don't specify how much more reliable they are. I'm aware the real-world lifespan of an SSD is dependent on many factors, but a TLC drive can potentially be double the price of QLC, making this reliability difference important to quantify.

How much of a difference is there between the average reliability and lifespan of a TLC SSD vs a QLC SSD? Is there significant difference in failure rate or number of writes between TLC and QLC?

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    Impossible to answer quantitatively, as this depends heavily on the models of the disks you're comparing and also on pure luck when buying them.
    – harrymc
    Nov 13, 2023 at 19:53
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    @harrymc I'm aware it can't be quantitatively answered to an exact amount. I'm looking for a more general answer. IE: Is the failure rate difference on a small scale or are we talking QLC drives failing twice as often?
    – Stevoisiak
    Nov 13, 2023 at 19:55
  • Cpt.Whales answer is about the best you can get. The decreased endurance of the higher bit-per-cell technologies is somewhat offset by having larger capacities and having more intelligent wear levelling. Where older drive might have only levelled areas of the disk that had been TRIM'ed newer ones might prioritise that area under load but could wear level the rest of the disk as well and so wear level used but not freed blocks. The result is that overall the per-cell endurance decrease might not actually mean a decrease in operational lifetime.
    – Mokubai
    Nov 13, 2023 at 20:59

2 Answers 2

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In terms of pure durability, SSDs have a difference in how many program and erase (P/E) cycles their cells can be expected to endure before failing (general stats from a Western Digital article):

  • QLC: under 1000 P/E cycles
  • TLC: 500-1000 P/E
  • MLC: 10,000 P/E
  • SLC: up to 100,000 P/E

These can be offset by larger caches and workload type though. Cell wear is rarely the reason a consumer SSD will fail unless you're rewriting the entire thing multiple times a day. It's safe to say QLC is only slightly more failure-prone than other variants, given the same manufacturer, etc.


For general SSD reliability/performance metrics, I like Backblaze's bi-annual blog of their real experienced failure rates and types:

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    the TLC number seem quite low in your table, they should be around 3.000 to 5.000
    – Eumel
    Nov 14, 2023 at 17:27
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    What is the source for these numbers? They are quite confusing... "under a 1000" to me could mean the same as 500-1000, so according to this information either QLC is more reliable than TLC (~1000 vs 500), or they are about the same.
    – Bakuriu
    Nov 14, 2023 at 18:42
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From a purely theoretical point of view and for identical flash cells, the difference is 2x.

Flash wears down from gradual degradation of of the gate insulation, leading to higher leakage currents. Because TLC has 8 different voltage levels and QLC has 16 voltage levels, the QLC flash cell can tolerate half the amount of leakage.

In practice SSD drives differ in many other ways too: transistor size, wear-leveling implementation, amount of reserved space, erase block sizes, error correction, use of SLC cache memory. These can offset or even matter more than the TLC vs. QLC difference.

In practice manufacturers will select their target write endurance and attempt to design the drive to meet them. Therefore you can expect a QLC drive rated 100 TBW to last just as long as a TLC drive rated 100 TBW, but to perhaps need more reserve blocks or error correction internally to achieve that.

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    Small nitpick, but I think a QLC drive rated 100 TBW will last as long as a TLC drive rated 100 TBW assuming they are the same sized drives.
    – Michael
    Nov 14, 2023 at 20:30
  • @Michael From an endurance per cell perspective, you want drive writes per day (DWPD). But from a "how much can I write to this drive" perspective, total/terabytes written (TBW) works, and is the more practical number for most consumer uses. This is all assuming a perfect wear-leveling implementation. The conversion factor between DWPD and TBW is, of course, the drive size.
    – Bob
    Nov 15, 2023 at 4:52
  • @Michael Good point, though I think it may also depend on usage patterns. E.g. a surveillance camera system constantly writing to drive at constant rate vs. typical desktop usage with little writes at all vs. database drive where amount of writes grows in relation to amount of data being stored.
    – jpa
    Nov 15, 2023 at 6:25

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