storage selection for embedded devices

[Olivier Gautherot]

Philip, Warner,

On Tue, Oct 14, 2008 at 3:00 PM, M. Warner Losh <imp at bsdimp.com> wrote:
> In message: <48F4C02F.1060407 at syx.ca>
>            Philip Mullis <philip.mullis at syx.ca> writes:
> : I was wondering if anyone has extended experience in this area with
> : embedded devices.
> :
> : I have a fixed embedded image which runs happily out of a 1Gig compact
> : flash card.
> :
> : However I have some applications that I want to install to my device
> : that will perform writes alot to the cf.
>
> I've deployed CF cards into systems for a number of years (since
> 2000).  They are way more reliable than spinning media in the
> environments that I deploy my company's gear into.
>
> We have most of the CF dedicated to a read-only partition.  A small
> modification partition was also provided.

I wonder if you're talking about the same thing (may be just me...)

Philip, what frequency of writes are you talking about? I think this
is key to the discussion. Are you planning enough RAM to avoid swap?
Can your system count with a RAM disk and regular dump of the content
to FLASH? If this is the case, a USB stick should be a safe approach.

The algorithm Sandisk is referring to enhances the statistical
lifespan by shuffling the cells and using spare ones when the main
array wears out (trial-and-error algorithm). The typical lifespan of a
cell is 100k write cycles: try to evaluate whether this is compatible
with the use you plan for your device.


> I've also tried to wear out a CF part by writing to the part, both
> directly and through a filesystem, millions of times.  I was unable to
> keep a machine running long enough to cause a failure (my mistake was
> doing it in a lab where people liked to unplug things).

The technology has surely evolved since I last dealt with it in an
industrial environment. However, I would not swear by the "millions of
times" as such: Sandisk is famous for leveling the writes over the
whole array. Now, if your partition is relatively empty, your device
will support more cycles. In any case, using 10% of the FLASH blocks
can surely lead you to the millions of cycles without problem.


> : Ive read the sandisk wear leveling white paper, yet I also hear many
> : people such as professional photographers swearing by the write once
> : rule with cf cards.

That's paranoia, especially with todays technologies.


-- 
Olivier Gautherot
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