我目前正在运行Ubuntu 15.10，但尝试升级到16.04。
当我运行sudo do-release-upgrade时，进程会中止，并显示以下消息：
Not enough free disk space
The upgrade has aborted. The upgrade needs a total of 2,206 M free
space on disk '/var'. Please free at least an additional 4,623 k of
disk space on '/var'. Empty your trash and remove temporary packages
of former installations using 'sudo apt-get clean'.
我尝试了
sudo apt-get clean 但它并没有显著减少空间，而且我不太明白为什么会出现这个错误。
我的磁盘分区是这样的：Filesystem Size Used Avail Use% Mounted on
udev 3.9G 0 3.9G 0% /dev
tmpfs 787M 9.8M 777M 2% /run
/dev/sda6 26G 14G 11G 58% /
tmpfs 3.9G 233M 3.7G 6% /dev/shm
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 3.9G 0 3.9G 0% /sys/fs/cgroup
/dev/sda4 923M 8.6M 851M 1% /tmp
/dev/sda3 3.7G 1.4G 2.1G 40% /var
/dev/sda2 78G 47G 28G 63% /home
/dev/sda1 256M 32M 225M 13% /boot/efi
tmpfs 787M 12K 787M 1% /run/user/121
tmpfs 787M 20K 787M 1% /run/user/1000
为什么升级程序需要在/var分区上腾出空间，即使操作系统并未安装在那里？
另外，以防有帮助的话，这是/var目录的内容（在我看来看起来相当正常）：
total 88
drwxr-xr-x 15 root root 4096 Aug 17 2015 .
drwxr-xr-x 23 root root 4096 Feb 4 19:38 ..
drwxr-xr-x 2 root root 4096 Jan 26 19:38 backups
drwxr-xr-x 22 root root 4096 Oct 23 22:24 cache
drwxrwsrwt 2 root whoopsie 4096 Feb 3 22:25 crash
drwxr-xr-x 79 root root 4096 Feb 4 19:38 lib
drwxrwsr-x 2 root staff 4096 Apr 17 2015 local
lrwxrwxrwx 1 root root 9 Aug 16 2015 lock -> /run/lock
drwxrwxr-x 13 root syslog 4096 Feb 4 18:45 log
drwx------ 2 root root 16384 Aug 16 2015 lost+found
drwxrwsr-x 2 root mail 4096 Apr 22 2015 mail
drwxrwsrwt 2 root whoopsie 4096 Apr 22 2015 metrics
drwxr-xr-x 2 root root 4096 Apr 22 2015 opt
lrwxrwxrwx 1 root root 4 Aug 16 2015 run -> /run
drwxr-xr-x 9 root root 4096 Feb 26 2016 spool
drwxrwxrwt 126 root root 20480 Feb 4 20:11 tmp
drwxrwxr-x 2 root plugdev 4096 Sep 3 2013 upek_data
非常感谢任何帮助！