I'm setting up a pair of PostgreSQL servers. Because cash is short I can only afford one high class server and some crapy backup box. If the expensive high end box should catch fire, operations can live with the slower backup system for a while.
I was wondering how much performance does the backup server need? It receives WAL and only has to apply it (I prosume that is the way syncrhonized commits work?). Is WAL always easy to apply, or would for example a delete with many checks on foreign key constraints, also have to do these constraint checks on the slow backup box?
Is the PostgreSQL WAL logical or image based? I'm not looking for absolute numbers, more a kind of answer like: "Applying WAL over TCP in sync commit mode on the hot backup will be cheap in any szenario, except for blahblah".