Questions about NAND (double)bit errors
wolfgang.mues at auerswald.de
Thu Feb 16 03:30:00 EST 2006
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.
> > 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.
Wolfgang Muees Vor den Grashoefen 1
Auerswald GmbH & Co. KG D-38162 Cremlingen
Hardware Development Germany
Tel +49 5306 9219 0 Fax +49 5306 9219 94
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