Problem with NAND memory data retention

[Andrea Adami]

On Thu, Apr 27, 2017 at 3:04 PM, Lukasz Majewski <lukma at denx.de> wrote:
>
> Hi Ricard,
>
> >
> > On Thu, 27 Apr 2017, Lukasz Majewski wrote:
> >
> > > I'd like to ask you for sharing your experience with NAND Flash
> > > devices
> > > - which may be older that their data retention time.
> > >
> > > I've a problem with Flash NAND memory (Samsung 128Mx8).
> > >
> > > In the data sheet [1] the manufacturer claims that it has 10 years
> > > for data retention. The device equipped with it is a bit older than
> > > 10 years.
> > >
> > >
> > > How one can diagnose that data retention time for NAND Flash has
> > > been exceeded?
> > >
> > > Is that:
> > >
> > > - The number of bit flip errors on a RW page so large that ECC
> > > cannot fix it?
> > >
> > > - Or are whole RW pages erased (0xFF)/corrupted/cleared (0x00)?
> > >
> > > - Or any other evidence?
> > >
> > > Thanks in advance for your help.
> >
> > I think the interpretation is that the manufacturer guarantees that
> > under the circumstances specified (temperature, supply voltage, erase
> > cycles etc) that the data will be retained for at least ten years,
> > beyond that it's just not specified. It doesn't mean that something
> > catastrophic will happen after ten years, i.e. ten years is not a
> > hard limit beyond which things start going haywire.
>
> Thanks for your explanation.
>
> >
> > I can't say I have experience with old NAND flashes loosing their
> > data, but technically I would expect that what happens is that
> > eventually the charge leaks away from the bit cells, causing bit
> > flips, quite simply, so it's the same mechanism that requires the
> > flash to be 'scrubbed' periodically to check if the data needs to be
> > rewritten.
> >
> > I.e. it's the first case in your list, the number of bit flips
> > increases.
>
> Unfortunately with my device I do experience the second scenario.
>
> I do observe pages read as zeros.


Hello,

I think I have seen the same issue: one defective device did not
respond to CFI QRY.

http://lists.infradead.org/pipermail/linux-mtd/2014-May/053872.html

These devices were sold in 2002-2003 so they are now >15yrs old and I
have two of them working flawlessy.

Regards
Andrea

>
>
> For example in the middle of a file - page sized (2K) data is read as
> zero. And this file is RO (no modification). And such zero read is
> persistent - happens all the time. The only way is to replace the whole
> file (erase it and write again).
>
> We use yaffs file system.
>
> And such errors pop up in random places.
>
> >
> > The fewer erase cycles that a given flash block has seen, the less
> > wear there has been on the insulation, and the better the retention
> > would be. The ten year spec is under the worst case condition of the
> > maximum number of erase cycles having been performed on the flash.
> >
> > /Ricard
>
>
>
>
> Best regards,
>
> Lukasz Majewski
>
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>
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