由Ubuntu 14升级到Ubuntu 16，正常情况下简单执行如下命令即可：

$ sudo apt-get update

$ sudo apt-get upgrade

$ sudo apt-get dist-upgrade

$ sudo do-release-upgrade

如果使用的是阿里云的ECS服务器，且系统盘原来安装的是Ubuntu12，现在的Ubuntu 14是由原来的12升级而来的，那么在执行do-release-upgrade命令时可能会产生如下错误：

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating theupgrade.

This can be caused by:

* Upgrading to a pre-release version of Ubuntu

* Running the current pre-release version of Ubuntu

* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bugusing the

command 'ubuntu-bug ubuntu-release-upgrader-core' in aterminal.

Restoring original system state

Aborting

Reading package lists... Done

Building dependency tree

Reading state information... Done

Building data structures... Done

=== Command detached from window (Sun May 21 15:33:442017) ===

这个错误是什么意思呢？执行如下命令，查看出错日志：

$ cat /var/log/dist-upgrade/main.log | grep failed

可以看到如下内容：

901 DEBUG failed to SystemUnLock() (E:Not locked)

116 ERROR Dist-upgrade failed: 'The essential package 'mount'is marked for removal.'

118 DEBUG failed to SystemUnLock() (E:Not locked)

这说明mount这个包被锁定了,被锁定的原因是服务器安装的初始系统ubuntu 12.04 LTS上阿里云设定的锁定，如果服务器的系统是先从ubuntu12.04 LTS升级到ubuntu 14.04 LTS，那么这个设置被保留在了系统里面。

执行如下命令，查看包管理策略：

$ apt-cache policy

可以看到如下信息：

Pinned packages:

mount -> 2.20.1-1ubuntu3

linux-image-3.2.0-29-generic -> (not found)

信息显示，mount被锁定到了版本2.20.1上,导致无法升级替换。

此时可通过执行如下命令移除锁定策略：

$ sudo mv /etc/apt/preferences.d/etc/apt/preferences.d.old

接下来继续执行升级命令：

sudo do-release-upgrade

一路回车，什么都不填，一切都不更改，大约20-30分钟之后，系统提示重启。
重启完成后，清理无用的安装包：

$ sudo apt-get autoremove

至此完成Ubuntu服务器从14到16的升级。