A new firmware has been released for the Samsung 840 EVO and it appears to have fixed the issue with speed degradation of older data but… and you knew that was coming, it has some side effects.
Samsung has stated that there will be periodic refreshes of the data in the background either by the firmware of the SSD itself or by a tool in the Samsung SSD Magician. To quote Anandtech…
Ultimately Samsung’s second fix is a bit of a brute force solution to the problem, but at this point there doesn’t seem to be anything Samsung can do about 19nm TLC cell charge decay other than to refresh the data, as the problem is intrinsic to the NAND itself. Pre-release versions of the firmware show that this fix works, and conceptually this is much more likely to work over the long run than Samsung’s initial fix. The tradeoff is that it does consume P/E cycles to refresh the data, but by our own calculations even 5 years of refreshes at 1/week would only be 26% of the drive’s rated 1000 cycle lifetime.
And it seems that TLC NAND isn’t the only type of NAND that’s having a problem at process nodes of 19nm or less. The Crucial MX200 SSD which uses 16nm MLC 128Gb NAND flash chips appears to be having the same very issue that the Samsung 840 EVO had, namely NAND flash voltage drifting which causes read speed performance issues on older data.
What does this mean? Well, it means that pretty much NAND flash memory becomes unreliable as the process node shrinks past 20nm. NAND flash manufacturers have hit the wall when it comes to planar NAND. From this point on the only way to make NAND flash reliable is to reverse the process node shrinkage and go to 3D-NAND where NAND cells are stacked on one another instead of laid out flat.
Samsung started this trend in which they released their 3D-VNAND and reversed the process shrinkage to 40nm with their 850 line of SSDs. Other manufacturers are also doing the same with their own approach to 3D-NAND. Ultimately it comes down to the fact that NAND flash memory manufacturers have hit the wall in terms of shrinking NAND and that as you shrink the process node the NAND has far more of a chance of voltage drifting that can effect read speed performance as the data ages.