什么是基于2种类型变量（在php中）实现加权随机选择的最佳方法？

Basically my dilemma is this. I have a list of x servers that host files. There is another server, that hosts the site's mysql db and application. When a file is uploaded (to the frontend server), the application checks to see which server has the most free space on it, and moves the file there. This works fine if you started with 2+ empty servers with identical amount of free space. If you introduce another server into the mix at a later point.... which will have more free space than the current servers, this method isnt so effective, because all the new files will be uploaded elusively to the new server, which would overload since it will be handling most of the new traffic till it catches up with the rest of the boxes in terms of free space. Basically my dilemma is this. I have a list of