我目前正在运行 Ubuntu 15.10，但正在尝试升级到 16.04。当我运行时sudo do-release-upgrade，进程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'.
我试过了，sudo apt-get clean但它并没有显着减少空间，此外，我不太明白为什么会发生这个错误。我的磁盘分区如下所示：
Filesystem Size Used Avail Use% Mounted on
udev 3.9G 0 3.9G 0% /dev
tmpfs 787M 9.8M 777M 2% /run …