I call a guaranteed space allocation a "reservation". Reservations are possible, but they do impose more complexity and more processing overhead. We've had little if any strong pressure for reservations.

...

Applying quotas to an arbitrary subtree is tricky...

I agree with keeping things simple. If setting up reservations, or applying quotas to arbitrary trees adds to the complexity of the software too much relative to the number of people requiring it, then the current design of quotas is fine.

I don't know whether this level of detail in our design process is interesting to people or not, but hopefully it helps explain why quotas work they way they do.

I'd have to say that knowing why things are designed the way they are make a big difference, and its not often that users get the reasons behind decisions that can seem arbitrary from the outside.

-- Dave Heiland