Server has multiple users, every user has its own folder and can upload files.
How to prevent abuse and set a size limit for the user folder? It should allow disk space optimization - users should be able to share unused space.
What network file storage systems can provide such option? Ability to mount as a standart unix file system is important, I can't use Riak for example.
Operating system can be any POSIX - compatible, I use Ubuntu-server because it's easy to set up, but any other also will be fine.
Also (but it's optional and not a hard requirement) it would be nice to have such property. If data get lost - it should get lost completely but in one place (whole user folders should get lost but for small amount of users), than small piece but in many-many places (one or two files but in many-many user folders).
It is possible to set such quota on the application-level by checking size before storing file, but I can't use it because in my case file upload done not directly by application but via some third-party module I don't control.