Follow-up to wearing / caching question

[Matthew Cole]

The question posed by Martin Neilsen leads me to write in search of an
answer that I've been pondering for a few days.  I've been tasked with
approximating the lifespan of the flash (JFFS2) filesystem embedded in our
products.  Is there a best method for calculating the space required for a
fixed-size file over a given lifespan?  If we want our flash filesystem to
be available for an approximate lifespan of 20 years, given the
wear-leveling duty-cycle of JFFS2, and an average block endurance of 100k
write/erase cycles, would I need 150% of the file's size? 200%? 1000%?  The
worst-case answer should be acceptable, but obviously, the most-realistic
case is what we're aiming for.  The actual read/write duty cycle of the
application is quite variable, so that adds some complexity to the problem,
but a good guess for now would be that it writes the entire file out to
flash once a minute.  But as that is an independent variable, maybe someone
could help me solve for that over a span of duty cycles?

I'd very much appreciate any input that anyone could offer me on this
problem.

Matthew Cole
Software Engineer
DTV Systems
ATI Research, Inc.
62 Forest St.
Marlborough, MA 01752
 
mcole at ati.com
O: 508-486-1179
F: 508-303-3920
C: 508-241-7923