System volume 'root' usage is 81 %

Also ran into these symptoms just a couple of days ago. First thing I did, was to identify the large files in rootfs. So did a find on large size files and excluded /data and /apps (dirs not part of rootfs), leaving the find to only touch upon the rootfs, mostly. There are probably better and other ways to do this (like the du -csh /var/* above), but this helped me isolate the large files:

find / -not -path "*/data/*" -not -path "*/apps/*" -size +100000k -print0 | xargs -0 ls -sd | sort -nr

Based on the results below, I found that 3rd party apps were storing a lot of data in MySQL's /var/lib/mysql/ibdata1. And another app was writing a lot to /var/log/syslog, as indicated by mdgm above.

find: `/proc/6993/task/6993/fd/5': No such file or directory
find: `/proc/6993/task/6993/fdinfo/5': No such file or directory
find: `/proc/6993/fd/5': No such file or directory
find: `/proc/6993/fdinfo/5': No such file or directory
706M /var/lib/mysql/ibdata1
124M /var/log/syslog
...

Together, these two large files alone accounted for about 830/4096 = 20% of rootfs, causing the message "System volume 'root' usage is 81%".

The result from the commands, suggested by mdgm, that you used above, seem to point in the same direction.

du -csh /var/*

577M /var/lib
0 /var/local
4.0K /var/lock
428M /var/log

Are you by any change running LogAnalyzer (or ran it in the past)?