Memory replacement

[C. Scott Ananian]

On Sat, Mar 12, 2011 at 5:51 PM, Arnd Bergmann <arnd at arndb.de> wrote:
> I've had four cards with a Sandisk label that had unusual characteristics
> and manufacturer/OEM IDs that refer to other companies, three Samsung ("SM")
> and one unknown ("BE", possibly lexar). In all cases, the Sandisk support
> has confirmed from photos that the cards are definitely fake. They also

Please see the blog post I cited in the email immediately prior to
yours, which discusses this situation precisely.  Often the cards are
not actually "fake" -- they may even be produced on the exact same
equipment as the usual cards, but "off the books" during hours when
the factory is officially closed.  This sort of thing is very very
widespread, and fakes can come even via official distribution
channels.  (Discussed in bunnie's post.)

> However, they have apparently managed to make them work well
> for random access by using some erase blocks as SLC (writing only
> the pages that carry the most significant bit in each cell) and
> by doing log structured writes in there, something that apparently
> others have not figured out yet. Also, as I mentioned, they
> consistenly use a relatively large number of open erase blocks.
> I've measured both effects on SD cards and USB sticks.

You've been lucky.

> I believe you can get this level of sophistication only from
> companies that make the nand flash, the controller and the card:
> Sandisk, Samsung and Toshiba.
> Other brands that just get the controllers and the flash chips
> from whoever sells them cheaply (kingston, adata, panasonic,
> transcend, ...) apparently don't get the really good stuff.

You're giving the OEMs too much credit.  As John says, unless you
arrange for a special SKU, even the "first source" companies will give
you whatever they've got cheap that day.

>> How we deal with this is constant testing and getting notification from
>> the manufacturer that they are changing the internals (unfortunately,
>> we aren't willing to pay the premium to have a special SKU).
>
> Do you have test results somewhere publically available? We are currently
> discussing adding some tweaks to the linux mmc drivers to detect cards
> with certain features, and to do some optimizations in the block layer
> for common ones.

http://wiki.laptop.org/go/NAND_Testing

But the testing wad is talking about is really *on the factory floor*:
 Regular sampling of chips as they come into the factory to ensure
that the chips *you are actually about to put into the XOs* are
consistent.  Relying on manufacturing data reported by the chips is
not reliable.
  --scott

-- 
                         ( http://cscott.net/ )