[Yaffs] STMicro and NAND read-disturb

[Jesse Off]

Recently our founder had lunch with a STMicro engineer that knew quite a bit 
about NAND and specifically the read-disturb failure modes.  He told us that 
after 100,000 or so reads of a sector, the stored data is actually very 
likely to experience corruption.  Subsequent reads of the sector will return 
the same corrupted data, but the flash sector can be erased and reprogrammed 
(with the same data or otherwise) for another 100,000 or so reads.  Towards 
the end of the useable life of the flash, a written sector may only be good 
for 1000 (or less) reads before it looses value.  He also reaffirmed that 
the quality of NAND chips have peaked and we should expect the NAND "quirks" 
to get worse in the future.

When asked for market predictions on alternative NAND chips with embedded 
controller logic in the chip (such as OneNAND, etc..) STMicro seems to 
believe those chips will be confined to niche markets and the current 
architecture of the 2k sector NAND chip will be where all the volume will 
be.  I'm not sure if YAFFS is currently pursuing support of OneNAND, but 
from the sound of it it may not be a big deal either way.

Is it still the current behavior of YAFFS2 to retire a sector permanently 
that experiences a read-disturb?   We still seem to have a return rate of 
our boards about 1 every couple months with YAFFS having marked a large 
proportion of the flash sectors bad.  This is much worse than the failure 
rates we have seen for CompactFlash cards-- which are usign the same NAND 
chips inside as YAFFS.  In every case we've done failure analysis, the flash 
is almost 100% good after wiping the flash clean and unmarking the sectors 
bad.

//Jesse Off