Linux（Ubuntu）在阿里云上的磁盘空间异常

在linux下，使用df -h命令查看磁盘空间使用量，显示已使用了100%，而实际用到的远没有那么多。

root@iZ94yakhyenZ:/# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       20G   20G     0 100% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
udev            990M  4.0K  990M   1% /dev
tmpfs           201M  384K  200M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none           1001M     0 1001M   0% /run/shm
none            100M     0  100M   0% /run/user
用du -h查看，根目录下的用量为6.8G。

root@iZ94yakhyenZ:/# du --max-depth=1 -h
4.0K    ./opt
9.6M    ./bin
384K    ./run
4.0K    ./srv
4.0K    ./lib64
0    ./sys
8.5M    ./sbin
34M    ./boot
4.0K    ./home
4.0K    ./dev
du: cannot access './proc/12212/task/12212/fd/4': No such file or directory
du: cannot access './proc/12212/task/12212/fdinfo/4': No such file or directory
du: cannot access './proc/12212/fd/4': No such file or directory
du: cannot access './proc/12212/fdinfo/4': No such file or directory
0    ./proc
283M    ./lib
24K    ./tmp
28K    ./root
4.0K    ./media
2.1G    ./mnt
803M    ./usr
16K    ./lost+found
3.7G    ./var
4.7M    ./etc
6.8G    .

系统为ubuntu14.04，是运行在阿里云上的虚拟机。