Database Systems: Why is it hard to scale a database, in layman's terms? - Quora
为什么数据库难以扩展具备高伸缩性？
The real-world analog here is that databases are often stored on hard drives, and hard drives are of finite space, and you can only pack so many hard drives on a computer (in a data center, on a rack). Data centers are now very, very large in order to contend with this common problem but if you are an extremely voluminous library, you may exceed this constraint, and overflow your data center rack's ability to hold hard drives, or even your entire data center and need a new one (rare). Still, the point is that no matter how much room you start off with, you may exceed it, and you may not be able to linearly keep adding more "units" of room (i.e. shelves in the same building) and you might have to take some sort of step jump, like leasing another building or renting another rack or building another datacenter.