Woodhouse on flash storage

Chris Ball cjb at laptop.org
Mon Oct 5 11:27:52 EDT 2009

Hi Martin,

   > How might the outcome of these debates affect the use of the
   > microSD "internal" storage on the XO-1.5?

I don't think there's much we can use Dave's advice for given the
choice of microSD.  If a microSD card that had a passthrough mode
to access the raw flash existed, his argument would be that we
should consider using that mode.

   > Wikipedia[1] is a bit hard-for-me-anyway to use to understand
   > whether linux drivers access the SD card's flash "directly"[2]
   > (which I assume is what dwmw means by "talk directly to the
   > underlying flash medium") or via some USB mass storage interface
   > layer provided by the card (is this part - perhaps the highest
   > level - of the FTL that dwmw and T'so are talking about?).

Yep, it's like a mass storage interface -- that's why we can run our
standard filesystems on SD cards -- and that interface is indeed part
of the FTL.

You can think of an SD card as being purely bare flash (of the type
we use on the XO-1) plus the microcontroller that runs the FTL and
abstracts the flash away from the OS.  Oh, here's a cool image:


- Chris.
Chris Ball   <cjb at laptop.org>
One Laptop Per Child

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