No surprise on memory

Jim Gettys jg at laptop.org
Thu Dec 18 14:39:39 EST 2008


On Thu, 2008-12-18 at 09:13 -1000, Mitch Bradley wrote:
> John Gilmore wrote:
> > Swapping to the soldered-in NAND chips is a very bad idea.  It will
> > tend to wear them out rapidly.  Even if you use load-leveling software
> > (e.g. swapping to a file in a jfffs2 filesystem), the problem is that
> > if you do start wearing out serious numbers of flash blocks, the
> > laptop becomes toast; it requires a soldering iron and spare chips to
> > fix it.

John, do the math: for the current chips (single level cells), life is
of order 10^5 cycles.  So you have 10^5 gigabytes of writing.  This
takes *a long* time.

Swapping is not an insane idea, once you have wear leveling.  We don't
do it now because JFFS2 cannot support swapping, and we don't have a
wear level beneath the file system.

UBI and Ubifs fix this, and it is something we can consider.

> >   
> 
> Well, maybe it's not as bad as all that.  When the NAND wears out, then 
> you can buy the SD card, thus deferring that purchase and taking 
> advantage of Moore's law in the interim.

Lots of people tend to forget, however, that warm (and/or cold) salt air
is a serious issue in many of the places we have to go.  Any connector
tends to die under these circumstances.

> 
> Note that I'm not advocating in favor of soldered NAND - in fact I've 
> been one of the leading proponents of migrating to an SD-based storage 
> solution.  I'm just pointing out that, if you're willing to buy an SD 
> card now (which is necessary for the SD-based swap solution), then you 
> are probably willing to buy one later.
> 
Soldered down SD, however may be an intermediate point; may fewer wires
than a conventional chip.

-- 
Jim Gettys <jg at laptop.org>
One Laptop Per Child



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