Re: Some news for this: [PATCH] [MTD] BLOCK_RO: Readonly Block Device Layer Over MTD ?

Ricard Wanderlof
2007-11-22T07:58:38Z

On Wed, 21 Nov 2007, Jörn Engel wrote:

> > Won't ECC take care of spurious bit flips?

> Single-bit, yes. Once you accumulated a bunch of them in proximity, it
> may no longer work.

Yes that is true of course.

Does anyone know of any tests or evaluations run on NAND flash chips, more
than what can be found out from the data sheets?

We ran some tests on an ST 256 Mbit flash here to try and get some grip on
the various bit flipping mechanisms.

At least for this chip, it seemed that if a block has only been written a
few times, then there seems to be virtually no limit to how many times it
can be read without bit flips occurring (the test ran for three months
with continuous reading of the same block, during which over 4.5e+10
reads were performed). On the other hand, if the block was written the
maximum specified number of times (100 000 for this chip), bit flips would
start occurring after 15 million reads.

(We also wrote a block far over the specified limit, about 1.7 million
times. In this case, bit flips during read started occurring after about
200 000 reads.)

Of course, this was a random sample of a single chip type used in a
specific hardware environment, so it would be interesting to see if anyone
else has done any similar test.

/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