Du answers the question: (roughly) how big are these files (the files in
these directories)?
It still does that reasonably well in Plan 9 (I'm not ruling out possible
improvements, but "it works for me!").
It doesn't answer questions about physical storage. On my Linux machines, I
do use it as a guide to
"which of my or the system's directories is exhausting the space on my
SSD", with du -s * | sort +0nr
[in rc on Linux: with sh you'd need to account for the stupid dotfile
convention]. On Plan 9, I more often
use it to see roughly how much data I'm going to move to a remote machine,
or whether I've left temporary
objects and executables, or whether to tar | gzip something. I sometimes
use du -a to list the names in a
hierarchy, but then I do the same on Linux (or I use find). On Linux, I
never use ls -R, partly because I'm
running p9p's ls, but mainly because the default format of /bin/ls -R is
amazingly useless (even worse than I remembered).