Why files though, and why move them into different directories? You said billions a day. With files, the physical drive must be taking a beating. Not to mention potential issues with directory file limitations(based on OS and file system). Why not use some kvdb?
>>hknmtt+(OP)
As I understand (correct me if I'm wrong, it's been forever since I've worked with filesystems) - file renames are very cheap as the actual data does not get moved, simply a journal gets updated