Code review comment for lp://qastaging/~justin-fathomdb/nova/constraint-scheduler

Soren: It's true that an exhaustive search can be expensive, which is why I
didn't code an exhaustive search (other than in the unit tests) :-) There's
a framework for 'Criteria', which the 'Solver' tries to solve as best it
can. You can plug in whatever Solver technique you think best (which might
actually be exhaustive for small sets), but I believe the Solver I've got
here is likely to be reasonable in practice. The approach is that it
identifies the 'most selective criteria' and then steps through those
results in order to find one where all the other criteria are no-more
unhappy (mini-max). I'll document the magic better! I haven't coded
heuristic algorithms yet because - frankly - we're nowhere near the scales
where it becomes necessary and we have no way to take advantage of
heuristics when we're sourcing data from a relational DB, and because there
are difficult questions around behavior when the heuristics fail to find a
good solution or any solution at all.

Sandy: I think the work is non-overlapping with the distributed & multi
schedulers, but I'll check them out in more detail. My goal is to support
more constraints in the scheduler (in particular, co-placement of volumes
and servers), and I'm going to work on this constraint to help motivate this
patch.

I'm going to check out the other branches, and code up a co-placement
Criteria so this isn't just work in the abstract!