Questions about NAND (double)bit errors

[Wolfgang Mües]

Hello Charles,

Charles Manning wrote:

> > About bad block detection: what is your oppinion about partitioning the
> > flash (the programs in a read-only partition, the data in r/w).
>
> This gets fs specific. With YAFFS (and I assume JFFS2, but consult an
> expert), grabage collection will force read-only files to get rewritten
> occasionally. Thus for ultimate reliability it is probably a GoodIdea to
> seperate the read-only stuff into a seperate partition. This is also a
> GoodIdea in that a smaller partition mounts faster (true for YAFFS and
> JFFS2). So if all your kernel + mount stuff is seperated from your rw stuff
> things will probably dgo better.

OK.

> > How about detection of ECC errors in read only partitions?
>
> ECC should be done on both rw and read-only partitions. Sometimes NAND gets
> read disturbs which would impact on read-only partitions.

My real question was: does YAFFS do regulary reads of all files in a R/O 
partition so that one-bit-errors can be discovered? Without reading, you will 
never find them...

> Also, write disturbs from writes to one partition can still corrupt a
> read-only partition on the same chip.

Bad news. Are you shure about this?
I know from the toshiba paper that write disturb is limited to the scope of a 
block. From other vendors, I don't have informations. 

> There's been some interesting discussion over in yaffs-land on this. If you
> don't subscribe to yaffs list then you can catch up on the yaffs archive.

I am reading the YAFFS mailing list for 1 year now. Very impressed by your 
constant engagement for YAFFS and the community.

Regarding YAFFS2 and the mounting time /need for scanning the whole NAND:
Do you think it will be possible to separate the directory information from 
the file data? So scanning will be:
- read the bad block marker and the "is directory information bit"
- if directory: scan it, building the data structures in RAM
- if data: you don't need it.

Obviously, this is only a benefit if reading the bad block marker is very much 
faster than scanning the whole block.

best regards
-- 
Wolfgang Muees                    Vor den Grashoefen 1
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