"virtualises files when your hard disk is full."
I guess the reason is to avoid to sync GBs if not more of data, especially when you local disk has chances of being smaller, and the connection not so fast. If you can just sync metadata, and then read/write file on demand, it looks "faster".
Their idea is probably to work like SMB (or the like) does, you see remote files, they can be cached locally when needed, but you don't get a full sync unless you explicitly require it for some - or it will truly fill your disk if you're an heavy user.
But if the OS and software are well designed, only when trying to access the Dropbox "namespace" code should try to access the remote data. And if metadata are already synced there's really no reason they shouldn't available in user space for a quick check. Sure, if you're going to look into file contents then there will be a lot of kernel calls to access the files - but then the bottleneck will be the network speed and latency, not the context switches - which also are now handled by specific CPU instructions to minimize the overhead. Anyway, context switches are there to protect the kernel from user space code - the overhead is due to access checks and other security features.
Biting the hand that feeds IT
