RE: Unstable bits and JFFS2

Atlant Schmidt
2012-04-13T16:40:37Z

Artem:

> Yeah, thanks for correcting. Yeah, read/write-disturb may make bits
> to become unstable, but we assume this is a slow process which will
> gradually make more and more bits flip and ECC will take care of
> that. So I think Matej can exclude this.

Unfortunately, ECC can only fix those PEBs that
are actually read. If one of the PEBs in your
filesystem is being used entirely to contain
obscure data that only gets read once in a
blue moon (i.e., very rarely), then multiple
read-/write-disturbs can hit accumulate in
that PEB and when it is finally read, it may
already contains too many errors to be
corrected by the ECC.

We've seen this exact scenario occur with the
current UBI/UBIfs and will be implementing a
userland "scrubbing" system to ensure that
every PEB gets read at least once in a while
(e.g., weekly or whatever interval seems right).

Atlant

-----Original Message-----
From: Artem Bityutskiy [mailto:dedekind1@gmail.com]
Sent: Friday, April 13, 2012 12:28
To: Atlant Schmidt
Cc: 'Matej Kupljen'; Brian Norris; linux-mtd
Subject: RE: Unstable bits and JFFS2

On Wed, 2012-04-04 at 06:54 -0400, Atlant Schmidt wrote:
> Matej:
>
> > Artem, you said that this unstable bits only happen
> > during power cuts, is this right? Would those appear
> > also on simulated power cuts, the ones that integck
> > can produce?
>
> (Note: This isn't Artem replying.)
>
> Unstable bits can happen anytime, but *REAL* power-cuts
> while writing are certainly a great way to produce them;
> after all, if you've only transferred half the electrons
> to the floating gate when power goes away, well, that
> Flash cell is now a roll-of-the-quantum-dice each time
> you read it.
>
> Simulated power cuts (that just stop the software
> processing at arbitrary and random points) can't
> produce this effect.
>
> But any time a read-disturb or write-disturb takes place,
> there's some probability that a Flash cell will be left
> with a "near-threshold" charge on the floating gate, so
> unstable bits are a fact of life that must be faced by
> any software that drives NAND Flash memory chips. This
> is, of course, especially true of MLC chips and even
> more-true for TLC chips (with three bits per cell).

Yeah, thanks for correcting. Yeah, read/write-disturb may make bits to
become unstable, but we assume this is a slow process which will
gradually make more and more bits flip and ECC will take care of that.
So I think Matej can exclude this.

--
Best Regards,
Artem Bityutskiy

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