Memory replacement

John Watlington wad at
Fri Mar 11 12:28:49 EST 2011

On Mar 11, 2011, at 5:35 AM, Arnd Bergmann wrote:

> I've tested around a dozen media from them, and while you are true
> that they use rather different algorithms and NAND chips inside, all
> of them can write to at least 5 erase blocks before getting into
> garbage collection, which is really needed for ext3 file systems.
> Contrast this with Kingston cards, which all use the same algorithm
> and can only write data linearly to one erase block at a time, resulting
> in one or two orders of magnitude higher internal write amplification.
> Most other vendors are somewhere inbetween, and you sometimes get
> fake cards that don't do what you expect, such a a bunch of Samsung
> microSDHC cards thaI have I which are labeled Sandisk on the outside.

Those aren't fakes.   That is what I'm trying to get across.

> I've also seen some really cheap noname cards outperform similar-spec'd
> sandisk card, both regarding maximum throughput and the garbage collection
> algorithms, but you can't rely on that.

My point is that you can't rely on Sandisk either.

I've been in discussion with both Sandisk and Adata about these issues,
as well as constantly testing batches of new SD cards from all major
vendors.  Unless you pay a lot extra and order at least 100K, you have no
control over what they give you.   They don't just change NAND chips,
they change the controller chip and its firmware.  Frequently.
And they don't update either the SKU number, part marking or the
identification fields available to software.    The manufacturing batch
number printed on the outside is the only thing that changes.

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).


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