State of read-only filesystems in NAND / MTD bad blocks handling when reading

[Thilo Fromm]

Hello, Atlant,

>> Please keep in mind that re-write events (or even bit scrubbing)
>> should be extremely rare since, well, we're r/o.
>
>  I'm not sure this assumption holds on modern NAND Flashes,
>  especially for the highest-density, lowest-cost MLC (and
>  now, even a few TLC!) devices. On these devices, the
>  difference between bits is down to 20-30 electrons on
>  the floating gate. These devices also have relatively
>  high rates of "read disturb" errors so there can't be
>  any such thing as a read-only Flash, only a read-mostly
>  Flash.

You're right, and I consider this an important factoid that I learned
from this whole thread. As a result I aim to add very basic scrubbing
support to blockrom. I'm still convinced, however, that it can be done
with very few code at the r/o MTD level compared to r/w UBI. Plus I
consider having a generic r/o fs image "directly" in an MTD block
device a strong use case for embedded systems. So it may be worth the
trade-off of having to maintain slightly more code.

Regards,
Thilo

-- 
Dipl.-Ing (FH) Thilo Fromm, MSc., Embedded Systems Architect
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