Here's an odd question. Kind of an in the future thing.
I have two db's on two smaller boxes I'd like to combine to one big box. But one db is replicated and the other doesn't need it.
I know currently you must replicate the entire cluster. I was just wondering, would it be a possible way forward to support WAL logging per tablespace. (each tablespace wal's to different directory). Then, as a logical step, could we support replication per tablespace?
This is more popularly called logical replication. There was a talk about one of the bigger projects to get this incorporated into core at last year's PGCon. It's quite a complex feature to get implemented, so it's probably not going to be around for a while yet. Links to the talk and slides are on the conference website if you're curious to learn more http://www.pgcon.org/2013/schedule/events/577.en.html
In the mean time, there are several third-party solutions that can provide logical replication, the more popular being Slony & Bucardo. I've been working on one myself as well (Mimeo) that's not quite as advanced as those two, but a little simpler to configure if you just need a few tables replicated.