Also, Samsung claims the 3D NAND is 40nm but we don't really know the that for sure since it's a totally new process and even Anandtech had a hard time figuring out the node size. Samsung has been known to push the truth a little bit.
Its not 40nm, but a 40nm
class node. So it can either be 49 or 40nm. In any case, it would make little sense to go to a smaller process fab, given the advantages of the 3D stacking.
And saving/reading 4 voltage states is much less than 8. In this case 8 is a really big number.
Its really not that big of an issue. The bigger issue is dealing with errors, which are inherent for smaller fab. It gets increasingly difficult to reliably program and read smaller and smaller cells. Even if its only 4 states (mlc).
Is 20nm MLC = 40nm TLC? Do we know this is a linear relationship as far as endurance and data retention are concerned?
This is just a very rought guess. But given the much much bigger cells, its bound to have much better endurance and data retention.
Remember, back in the ~50nm days, MLC was good for 10k+ p/e. So its not that big of a stretch to say TLC is good for 2-3k p/e at that fab (thats not even including benefits of 3d stacking). And that figure is right on par with 2D mlc at sub 20nm.
it's just that enough time with this technology hasn't passed for people (myself included) not to have doubts about TLC NAND.
Again, TLC is not the issue here. Very small cells are.
Think of this techologies as a leaking container. MLC container only has 1 small holes for water to go out, while TLC has 3 of them. If we have two containers with the same amount of liquid, TLC version will obviously run flat sooner.
Now we go and get much much bigger container (so 40nm class cell) and again punch the same 3 holes in it... Yes, its gonna leak the same, but because it has much much more liquid inside, it will take a lot more time to get empty.
Now compare a very small container with only one hole and that bigger container with 3 holes. If 3 hole container is big enough, it will match or even outmatch the 1 hole container in the time, it takes to run out of liquid.
Roughly thats how sub 20nm flash and 40nm class flash relate to each other.