SD/MMC cards, a year later

Tiago Marques tiagomnm at
Thu Aug 19 18:23:39 EDT 2010

On Thu, Aug 19, 2010 at 9:21 PM, John Watlington <wad at> wrote:

> On Aug 19, 2010, at 1:59 PM, Tiago Marques wrote:
> > Thanks for the update. Can you disclose the brands of the mentioned SD
> cards?
> > It's always useful to know what not to buy.
> I probably could, but variations between models from one manufacturer
> may be greater than variations between manufacturer.   I suggest that
> anyone thinking of purchasing a number of SD cards test the ones they
> are considering buying, instead of relying on outdated test data.
Ok, thanks. I'm half way down that path already, wanted to save some work.

>  > I'm assuming none of these cards don't have static wear levelling. Did
> any manufacturer provide you details on that or are they only using spare
> blocks for "repairability"?
> They all have dynamic wear levelling, where blocks are actively moved
> in order to balance the wear across all block in the device.

Nice to know. Unfortunately I've come across some flash devices that seem to
have none.
Correct me if I'm wrong. Dynamic wear levelling is the one done on the free
blocks and not the ones that have data, right?

>From your tests, 10TB on 2GB cards would imply 5000 write/erase cycles with
static wear levelling, which is pretty good. An SSD I bought a while ago
also had 5000 W/E cycles and that number seems to be going down as they
migrate to smaller process nodes. Perhaps that's the case?
3 bits per cell MLC also doesn't spell good things IMHO, especially as
they're only reducing the die size in 20% for the same capacity(as per
Intel/Micron 25nm 3bpc flash).

> > Any plans for the use of NILFS? It seems to help with wear levelling,
> although
> > it is not a completely circular log FS.
> We are starting to look at it.
> Does anybody have experience shipping it ?
> even using it ?

I've stuck with EXT2 with noatime for now but was looking into it. The only
thing I didn't like was it being not fully circular and being arranged in
blocks. Other than that it seems fine and should help a lot with the random
write latencies, as you may know, which was my motivation for looking into

Best regards,

> Cheers,
> wad
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