OS X breaks if I apply HFS compression to the whole volume. (It won't boot.)
What do I need to exclude?
I've been able to compress large parts of it, but then I had to restore after I compressed too much. I guess I can try to answer this via trial and error.
(Space is extremely tight on my boot volume.)
I'm guessing that these need to (or should) remain uncompressed:
/mach_kernel /sbin /dev /bin /.??* /private
/etc /tmp /var
I would guess I can get away with compressing most if not all of
/var/folders /System /Applications /Library /Users /usr
(Educated guess; include the stuff needed to bootstrap the system.)
At some point I may try to figure this out via trial and error, and answer my own question, but why reinvent the wheel when there's StackExchange?