UBIFS recovery fails

[Ricard Wanderlof]

On Wed, 19 Oct 2011, Atlant Schmidt wrote:

> All:
>
>> On modern SLCs (at least I first saw it on 34 nm SLC flash), those bitflips
>> can be _unstable_, i.e. they can appear and disappear randomly as you read
>> pages. I experienced this phenomenon only on pages which were being programmed
>> or erased during a power cut.
>
>  This makes perfectly good sense. During erasing or programming,
>  charge is being deposited-upon or removed from the floating gates
>  and that's not an instantaneous process, so it can be interrupted
>  while on-going, leaving a gate that's only half charged or half
>  discharged.
>
>  At that point, the floating gate may have charge on it that's
>  all-too-near the threshold voltage for the cell and any given
>  read of that cell could "go either way" depending on minute
>  variations in other conditions.

That makes sense, but it doesn't explain why the effect is appearently 
more pronounced on newer flashes than on older ones.

Of course, it could be that there is some other mechanism that also has 
had to be changed with shrinking geometries, such as for example (note: 
wildly speculating here) that the charge/discharge time for the individual 
bit cells is longer on modern flashes for whatever reason, causing them to 
be more sensitive to power cuts, as the programming operation (per bit) 
takes place over a longer time.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30