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When Samsung shipped the 840 Evo, it seemed as though the drive struck a perfect balance between affordability and high-speed performance. Those impressions soured somewhat after it became clear that many 840 EVO’s suffered performance degradation when accessing older data. Samsung released a fix last year that was supposed to solve the problem for good, but a subset of users have begun experiencing issues again. Earlier this year, the company announced that a second fix was in the works.

Tech Report now has some details on how the company’s second attempt to repair the problem will work. Apparently, the upcoming firmware will add a “periodic refresh” function to the drive. When the drive detects that data stored on it has reached a certain age, it will rewrite that data in the background. This fits with what we heard back when the problem was first uncovered — some users were able to solve it by copying the data to a different part of the drive.



The original problem with the 840 Evo was traced to shifting cell voltage levels. The drive controller expects cell voltages to operate within a specific range. As the NAND flash aged without being refreshed, those voltage levels passed outside their original programmed tolerances, and the SSD had trouble reading data from the affected sectors. The last firmware solution that Samsung released was supposed to solve the problem by reprogramming the range of values that the NAND management algorithms expected and could tolerate.

This solution seems to be of a different order. Instead of patching the problem directly by addressing the corner cases, Samsung is adding a refresh feature to prevent the situations that cause an issue to start with. While this may be the smarter way of fixing whatever is throwing off the results, it does raise some questions: Does Samsung’s TLC NAND have a long-term problem with data retention — and will this new solution hurt long-term drive longevity?

The good news, at least on the longevity front, is that even TLC-based drives proved to be capable of hundreds of TBs worth of write cycles, well above their listed parameters. Rewriting a relatively small portion of the drive’s total capacity every few months shouldn’t have a meaningful impact on lifespan. Samsung does note, however, that if you leave the drive powered off for months at a time, you may need to run its Drive Magician software — the algorithm is designed to run when the system is idle and can’t operate if the machine is powered off.

It’s not clear what the future of TLC NAND is at this point. Samsung has introduced the TLC-NAND backed 850 Pro, but that chip is built on the 40nm process node. Higher (older) process nodes were actually better for NAND flash when it comes to reliability and longevity metrics, which means it may buffer this problem from appearing in future products. To date, very few manufacturers have introduced TLC NAND at 2D (planar) geometries — it may simply not be worth it for most products.


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