Re: Lifetime of flash memory

[Sergei Organov]

linux@xxxxxxxxxxx writes:

>>>>> Due to the multiplexing scheme used in high-density NAND flash devices,
>>>>> even the non-programmed cells are exposed to a fraction of the programming
>>>>> voltage and there are very low limits on the number of write cycles to
>>>>> a page before it has to be erased again.  Exceeding that can cause some
>>>>> unwanted bits to change from 1 to 0.  Typically, however, it is enough
>>>>> to write each 512-byte portion of a page independently.
>>>>
>>>> Well, I'm not sure. The Toshiba and Samsung NANDs I've read manuals for
>>>> seem to limit number of writes to a single page before block erase, --
>>>> is 512-byte portion some implementation detail I'm not aware of?
>>>
>>> No.  I just meant that I generally see "you may program each 2K page a
>>> maximum of 4 times before performing an erase cycle", and I assume the
>>> spec came from 2048/512 = 4, so you can program each 512-byte sector
>>> separately.
>>
>> I've a file system implementation that writes up to 3 times to the first
>> 3 bytes of the first page of a block (clearing more and more bits every
>> time), and it seems to work in practice, so maybe this number (4) came
>> from another source? Alternatively, it works by accident and then I need
>> to reconsider the design.
>
> No, I'm sorry, I was still unclear.  The spec is 4 writes per page.
> I believe that the REASON for this spec was so that people could write
> 512+16 bytes at a time just like they did with small-block devices and
> it would work.

Ah, now (due to your explanation below) I see! But then how do you
explain that those old "small page" devices do support multiple writes
to a single page?

> But I do not believe there is any limitation on the pattern you may use,
> so your system should work fine.
>
> What confuses me is that I thought I said (quoted above; paraphrasing
> here) "there is a very low limit on the number of times you may write
> to a page.  That limit is large enough that you can do pagesize/512 =
> 2048/512 = 4 separate 512-byte writes."  I didn't intend to imply that
> that was the ONLY legal pattern.
>
> But from your comments, I'm getting the impression that you think I did
> say that was the only legal pattern.  If that impression is correct,
> I'm not sure how you read that into my statements.

Well, after your last explanation I'm not sure myself how I've read that
into your statements ;)

> (I wonder if the actual limit is the number of writes per BLOCK, and
> they just expressed it as writes per page.  I don't know enough about
> the programming circuitry to know what's exposed to what voltages.
> If the physics implied it, it would be useful flexibility for file system
> design.)

I doubt block is relevant here, otherwise there seems to be no reason to
introduce "page" as a write unit in the first place. It seems that write
voltage is applied to entire page, and bit 1 in the data indeed leaks
some charge (less than 1/4 of those bit 0 leaks). I think block in this
context is mentioned only because it's impossible to erase single page.

-- Sergei.
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