Re: UBIFS Corrupt during power failure

[Jamie Lokier <jamie@shareable.org>]

Artem Bityutskiy wrote:
> On Wed, 2009-07-15 at 21:52 +0100, Jamie Lokier wrote:
> > I suspect that's quite common for NOR cells.
> > 
> > Here's a question: Does it ever happen for NAND cells?  Does the UBIFS
> > code assume that NAND erase only _ever_ converts 0 bits to 1 bits,
> > never 1 bits to 0 bits temporarily?
> 
> I'm not good in physics of those processes,

I believe it writes 0 on NOR cells to put the cells into a known
physical state first.  As you know, you can safely overwrite cells on NOR.

Then the erase changes them to a 1 state.  I suspect the erase pulse
would damage cells which are not in the known 0 state first, or might
put them into a state which isn't a proper 1, a sort of "too much
erase".

> but AFAIU during the erasure all bits are set from 0 to 1 on NAND,
> and this is a simultaneous process for all bits in the
> eraseblocks. But I cannot say for sure.

It's physically impossible for each cell to change at _exactly_ the
same speed, though they can be very close.

So if power is lost at the wrong moment, each of the cells will be in
a "half-erased" state, and some will be closer to 1 than others,
making a mixture of 0 and 1 bits in some pattern.

A chip could try to store enough residual power to finish a clean
erase when it loses external power, but I doubt if they do that.

Even if they all look like 1 bits after power failure, it's possible
that some bits are "half-erased" if the erase didn't finish, and
aren't reliable.  Does UBI notice this, and force the block to be
erased again even though it looks like all 1 bits?

-- Jamie