A few nano questions

Wolfgang Spraul wolfgang at sharism.cc
Sat Jun 5 05:42:02 EDT 2010


Ruben,

> I read two Toshiba presentations on NAND+ubifs... it looks a little
> scary, yes :) But, I'm not sure if SD cards ahve the same wear off
> effect.

An SD card just has a normal NAND chip, and another small IC that can be
an ARM core with memory etc. and performs roughly what ubi/ubifs are doing.
Here's an excellent post from Bunnie Huang of Chumby fame about SD cards
http://www.bunniestudios.com/blog/?p=918

> effect. I remember from somewhere that the average NAND life is
> 10000-20000 rewrite cycles

I am not sure what those advertised numbers mean. It may have more to do
with liability (legal) concerns, what they promise their customers etc.
I doubt the bits will suddenly all just fail after some number X of deletes.
But I really don't know. I think like 99% of us we are all just guessing.
I have been trying for years to have dinner, even just a coffee talk, with
a real NAND expert, someone working in the (very few) actual NAND factories.
But who knows, maybe only a few hundred such people exist in the world?
There are only a handful of fabs left, and they all cost billions of USD to
build, and whatever know-how they acquire is guarded. If anybody knows
someone really knowledgeable about NAND, please let me know. We are evaluating
some products like power meters or water meters that would need to run well
for 20 or 30 years. Even longer ideally, why not?

I have seen datasheets where in the first revision, it said 10K cycles, in
the second revision it said 5K, and in the third revision it just said "TBA".
Don't know what to think of that.

> I wonder what will happen to the nano when the NAND starts to fail...
> I guess, if there is enough working room in it to write the boot
> system (as reading does not damage NAND disks), it will always be able
> to boot from the SD, will it?

The boot rom in the CPU can only boot the first 2 (I think) sectors from
NAND, so if those 2 blocks are unreadable it cannot boot from SD either
anymore. The boot-from-USB will still work though :-)
If this really happens in 20 years we will find a way to unbrick
it - guaranteed!

> I wonder if we will still be using our nanos to show our children (or
> grandchildren, depends on your age) what Linux was like, 20 years from now.

Hmm. I hope we can still update it with new software, no?

Wolfgang




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