NAND FLASH wear-out

[Mitch Bradley]

Benjamin M. Schwartz wrote:
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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.
>   

The data sheet says "DATA INTEGRITY: 100,000 Program/Erase cycles (with 
1bit/512byte ECC), 10 years Data Retention".

I don't know what that means.  These parts are sold into the consumer 
electronics/consumer PC market.  Low cost / short product life cycles 
reign supreme.  We live in the modern throwaway world.  Handwaving and 
marketing reign supreme.

Not only that, but products are routinely developed by ripping off 
designs from other companies, cloning, rebranding, you name it.  Data 
sheets are copied from other companies.
> 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.
>   

That is correct, except that data can be lost if an erase block happens 
to die during a read cycle, losing enough bits that the ECC cannot 
correct it.


>> 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.
>   

Indeed.  I believe that David told me that the JFFS2 support for moving 
newly-found bad blocks to the bad block list is not fully implemented, 
so we may still have some work to do.
>   


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