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Jffs2 compression and paging

I apologize if this is a duplicate post of a question.
 A developer I am working with attempted to send this
question but he was having problems subscribing to the
list for some reason.  I am on the list so I will
repost his question.  

Here is the original message:

I have an obscure question about jffs2...

Though I couldn't find any documentation indicating
that it does, jffs2 
appears to implement some kind of default compression.
 If I take a 
collection of files on my ext3 filesystem and build a
jffs2 image from 
them using mkfs.jffs2, the resulting image is
substantially smaller than 
the collection of files by themselves.  Is there some
kind of 
compression going on or is it just an artifact of the
difference in the way the 
filesystems store the data?

If there is no compression going on, then my next set
of questions are 
moot, but if there is...

If there is some kind of compression, how does it
interact with the 
demand paging system?  When the pager requests the 3rd
page of a file, 
does the entire file (or some chunk of it larger than
just that page) have 
to be uncompressed before the page can be delivered? 
If so, are the 
uncompressed pages just left in ram?  What happens if
there isn't enough 
ram to uncompress into?

The reason I'm asking is because I've observed
behaviour that is 
inconsistent with my understanding of demand paging in
linux.  When I load a 
program, the entire program and  all libraries that it
depends on 
appear to get loaded instead of just the initial page
or two.  The memory 
usage of the process appears consistent with this
determination, and I'm 
certain that at the point the code starts waiting for
input that it has 
not attempted to execute code in every page of the
library (or itself).  
I don't have a good explanation for it and I
ultimately became curious 
if there's some special interaction with jffs2 that
was causing it.  
Anyone have any ideas?


David Muse

Thank you,
Jim Williams

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