Consistency and availability are competing requirements. It is trivial to keep the files in a SageMathCloud project consistent if we store it in exactly one place; however, when the machine that project is on goes down for any reason, the project stops working, [...]. By making many copies of the files in a project, it's fairly easy to ensure that the project is always available, even if network switches in multiple data centers completely fail, etc. Unfortunately, if there are too many users and the synchronization itself puts too heavy of a load on the overall system, then machines will fail more frequently, and though projects are available, files do not stay consistent and data is lost to the user.
and boldly summarizes
The architecture that we have built could scale up to a million users.