Re: Prevent Nand page writes on Power failure

[Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>]

On Thu, Feb 21, 2019 at 11:36:46AM +0100, Boris Brezillon wrote:
> On Thu, 21 Feb 2019 11:17:47 +0100
> Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> wrote:
> 
> > Hi Boris,
> > 
> > On Thu, Feb 21, 2019 at 09:10:55AM +0100, Boris Brezillon wrote:
> > > Hi Sascha,
> > > 
> > > On Wed, 20 Feb 2019 14:58:20 +0100
> > > Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> wrote:
> > >   
> > > > Hi All,
> > > > 
> > > > I have hardware here for which the normal way to turn off is just to cut
> > > > the power. When the powercut happens during a NAND page write then we
> > > > get more or less completely written pages during next boot. Very rarely
> > > > it seems to happen that such a half written page with only very few
> > > > flipped bits is erroneously detected as empty and written again which
> > > > then results in ECC errors when reading the data.  
> > > 
> > > This should definitely be fixed, maybe by lowering the bitflip
> > > threshold when doing the empty check. Do you know the ECC strength and
> > > the number of bitflips you have when that problem occurs?  
> > 
> > The problem is that these half written pages do not seem to be very
> > stable. It happens that the number of bitflips change with each read.
> > I have seen pages which can be read sometimes and sometimes not. It
> > really seems that half written pages must be avoided entirely.
> 
> But when they are correctly read, do you know how many bitflips they
> have?

Yes, I know this number, but as said, it varies when reading the same
page again.

> To be honest, I fear not all users will be able to be informed
> that powercuts are about to happen, and we need a way to fix that for
> everyone.

If you want that I'm afraid we can't fix this on this level. You can't
rely on the data when a powercut happens during write. It may look ok at
first, but bitflips can develop later. I just found an interesting read
here:
https://www.datalight.com/blog/2017/03/08/enemy-of-nand-flash-memory/

If you want to fix it for all users you have to track somehow which
pages you have written last and discard the data or copy it to another
block.

Sascha

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