Re: master node can not be recovered

["Bityutskiy, Artem" <artem.bityutskiy@intel.com>]

On Wed, 2015-11-04 at 10:20 +0200, Artem Bityutskiy wrote:
> Right, and I hear that a lot of bug reports and frustration comes
> from
> this. This worked with SLCs we were using when implementing UBIFS
> (particularly, Samsung OneNAND was used, it was very high-quality
> NAND). Nowadays, this needs to be changed.
> 
> UBIFS logic is this. If there is a corruption, then it must be in the
> last used NAND page. Pages after this NAND page must contain empty
> space.
> 
> A small complication, which is not important now, is that UBIFS may
> operate with multiple NAND pages, this depends on what the driver
> tells
> is the min. IO size.
> 
> No the logic behind this was that we always write data from the
> beginning of the LEB, and continue to its end. In case of a power
> cut,
> we can only get corruption in the last NAND page (or more strictly,
> min. I/O unit) where we were writing to. The next NAND page and all
> the
> NAND pages after it should be empty. The previous NAND page and all
> the
> NAND pages before it should contain valid data (CRC OK).
> 
> Pretty simple. Worked well.
> 
> So what has to be changed in this logic? Obviously, the definition of
> empty space should be changed, it seems, because obviously not every
> driver wants/can ECC-protect the empty space.
> 
> What else?

And by the way, from my point of view, if someone wants to add MLC
support, just fixing this problem is a good practical step forward. I
see a lot of discussions about MLC support, and we even have some
arguments, and the whole thing looks complex. But this is a much
smaller isolated piece of work which would contribute to the MLC
support effort, and the implementer would learn UBIFS while
implementing this, and would be a lot more confident about making UBIFS
changes. I think I can sense this attitude: oh, UBIFS is complex, let's
try to do most in UBI, it seems simpler to deal with.

-- 
Best Regards,

Artem Bityutskiy
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