1. run "df -h" to see my disk image is almost not free space available. It is the reason that my websites often crashed.

2. apt-get install ncdu

3. ncdu to see which directory has the large files.

4. Find one cache folder is over 11G. Desided to delete it.

5. rm -rf command is very slow

6.So I tried another one: apt-get install rsync

7.mkdir empty directory.

8.run the command. rsync -a --delete empty/ cache/

9. Done. Much quickier than rm. I get my free space back now:)