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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?

Stevoisiak
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2 Answers2

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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:

Cpt.Whale
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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.

jpa
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