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RE: $subject




>>> >And how do we determine how many times we have to test? If we test it
>>> >twice is that good enough? Is 56 times enough? IMHO, it would be more
>>> >straightforward to just go ahead and erase the sectors,
> 
>> You are correct of course. This is a trade off. The more times
>> you read, statistically the higher chance that you will catch
>> a flipped bit. Of course, even reading many times is a decreasing
>> value function, as at one cross over point, the time to read n times
>> multiplied by the probability of missing a "bad" sector will be
>> greater than the sector erase time.

>That's not a simple calculation. In the 'erase it anyway' case you have
>to bear in mind that you've just used up 1/100,000 of the chip's lifetime.

>Consider a device with 128KiB erase block size. If you read a block and
>it appears to have any random zero bits, you're going to erase it, so
>you're OK. It's only if you get all the way to the end and by chance you've
>read 1,048,576 ones that you've even missed the first hurdle.

>I don't think this is going to be even a drop in the ocean. If we think
>it could possibly be a problem, we could scan the potentially-clean sectors

>twice - we can even do it in the background after mount, leaving them on
>a 'probably_clean_list' till we've scanned them. 

>What is the probability that two scans through 1Mib will return all ones

>if in fact the erase block was only half-erased? 

>Compare that with the probability that the sky will fall on your head 
>anyway. Do we still care?

If I understand you correctly, you are arguing the case that reading
the same "questionable" sector twice is good enough even if you are
paranoid,
and that we will catch at least 1 flipped bit, and the probability of
missing
that is so low that we probably don't care for that case.

If that is what you are saying, then I agree with it, and cast my vote
in favor.

Vipin

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