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).