NAND FLASH wear-out

[Benjamin M. Schwartz]

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Mitch Bradley wrote:
> The part is rated for 100,000 *erase* cycles per block.  There are 64 
> independently-writable 2K pages per block.  Writing doesn't count in the 
> wear calculation - just erasing.
What does "rated" mean?  Specifically, I imagine this means that X% of blocks
are expected to survive 100,000 erases, where X is at least 50.  Even if X is
99, there will still be some blocks that fail early, perhaps extremely early.  I
do not know what the failure curve looks like for erase blocks.

I am by no means an expert here, but from conversations with David Woodhouse, I
have the following impression:
1. We can expect each XO to lose a few erase blocks over the course of its lifetime
2. JFFS2 is specifically designed to handle erase-block failures without losing
any data, so the only effect will be a minimal decline in the available disk space.

> The bottom line is that NAND wear-out is not likely to be an issue.  
> JFFS2 does a good job of spreading out writes, and even if it only did a 
> half-hearted job, that would probably be good enough.

I agree.  However, erase-block failure handling may be very important.
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