[maemo-users] Limited life of flash memory

From: kenneth marken kemarken at broadpark.no
Date: Tue Dec 2 09:43:58 EET 2008
On Tuesday 02 December 2008 00:52:47 Mark wrote:
> On Mon, Dec 1, 2008 at 4:01 PM, kenneth marken <kemarken at broadpark.no> 
> > On Monday 01 December 2008 23:35:24 Sirio Negri wrote:
> >> Hi all! I found some days ago a few articles about the limited life of
> >> flash memory, specially for usb pendrive. The problem seems to be the
> >> limited times you can write on it. Does it exist also for rs-mmc?
> >> I know that the limit is very high (around 10 million of times) and it's
> >> not a big problem for a normal user. But if you install an OS on it,
> >> like on my Nokia 770, I think that the use of the memory card is bigger
> >> and, if there is that problem, its life is shorter.
> >> Thanks all for any info on it.
> >> Sirio Negri
> >
> > iirc, the math goes something like this:
> >
> > take the space of the card, divide by write speed, thats how long it will
> > take (in seconds) to do a write of the whole card.
> >
> > a 4GB card at 2MB pr second will take about half an hour to do a full
> > write...
> >
> > now, if the card has a write limit of 10 million, that means it will take
> > about 5 million hours...
> >
> > thats about 580 years of continual writing...
> >
> > yes, there are some cheap chips out there with poor wear leveling
> > (meaning that one can burn out a single sector by repeatedly writing to
> > it), but i suspect no name brand card will be that poorly made.
> Unfortunately, this makes some fundamental assumptions that aren't
> correct, such as that "the whole card" is involved when writing and
> that it's sequential. Actually, some areas of the card are going to
> get very heavy use and others very little, especially if an OS is
> installed on it. Even without an OS, some files are going to be
> written much more often than others. All it takes is for one critical
> area (not the whole card) to burn out in order for you to lose data,
> possibly even catastrophically. The bottom line is that 580 years
> would be *extremely* optimistic. That said, it's fairly safe to say
> that you'll either switch to a new device or get a larger and/or
> faster card long before the current one wears out.

ever heard of wear leveling? just about any product has it these days.

this means that while the filesystem things the files are in the exact same 
place as last time, the actual area of the chip being used is different, so 
even with specific files being hit hard (i know i have heard a horror storry 
about someone burning out the first sector of a no-name usb stick thanks to 
having the mounting set to updating the FAT for each change done, but that was 
some years ago when flash drives where a novelty), the wear should be spread 
out over the whole chip.

on internal chips, like on the 770 or the N8x0's, one can use a wear leveling 
FS. but that would mean that your writing directly to flash without some kind 
of controller chip sitting inbetween.

also, flash being flash, the error will shop up on write, not on read. and 
will not stop the drives readability. so being able to siphon out what data is 
in there can still be possible (iirc).

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