We suspect that a combination of cramfs and jffs will serve handhelds
very well... I can say from first hand experience that compressed ramdisks
and/or cramfs gets very old, very quickly: we really want a writable file
system, with (read) compression.
We have both cramfs and jffs cranking over right now on the iPAQ, but
haven't cut over to using either quite yet (but probably will in the next
few weeks sometime. We initially tried using cramfs, but had a flash
driver bug we didn't know about at the time so did ramdisk to get the
iPAQ to Usenix. But the current state is not ideal.
NOTE: we don't want/need general compression of data being written.
Most data being written (in terms of volume) is likely to be in already
compressed formats (e.g. note taking via audio which will then be
compressed before being written). You don't want to pay the joules
or performance to compress the data twice. Most of the flash is likely to
be executables or shared libraries.
A scheme Keith Packard proposed which would work very well for read only
data is somewhat similar to cramfs: just compress on 4K boundaries
and store an offset table, and mark the file as compressed when you are
done; do the obvious uncompression on the marked files on read. My intuition
is that this might be a performance win even on vanilla machines today.
So we argue that in fact full general compression of files automatically
behind the application's back will in fact be highly counter productive
on handheld devices. The stuff we will write the most of will already
be compressed...
Ted says Keith's scheme can be done with a stacking file system: this would
get it for all file system types, which strikes me as a win. We'll do
it someday, if no one gets to it in the meanwhile (it will probably still
be months before we get to that point).
- Jim
-- Jim Gettys Technology and Corporate Development Compaq Computer Corporation jg@pa.dec.com- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Sat Jul 15 2000 - 21:00:20 EST