I have an entry level doplet (20GB) and have some files and an unfinished project. Today I tried to install VCN But the installation stopped halfway because there was not enough space. here is my usage,

root@aaa:/# du -s * | sort -nr | head
du: cannot access ‘proc/19923/task/19923/fd/4’: No such file or directory
du: cannot access ‘proc/19923/task/19923/fdinfo/4’: No such file or directory
du: cannot access ‘proc/19923/fd/4’: No such file or directory
du: cannot access ‘proc/19923/fdinfo/4’: No such file or directory
5496816 var
1330076 usr
1048580 swapfile
394848 lib
63980 boot
41200 root
11312 sbin
9328 bin
6688 etc
740 1.sql

And the df is,
root@aaa:/# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/vda1 20G 20G 0 100% /
none 4.0K 0 4.0K 0% /sys/fs/cgroup
udev 239M 4.0K 239M 1% /dev
tmpfs 50M 352K 50M 1% /run
none 5.0M 0 5.0M 0% /run/lock
none 248M 0 248M 0% /run/shm
none 100M 0 100M 0% /run/user
I want to know where is my disk space. and what is this proc folder?
Sorry for bad English.