replication optimization: page writes only at the slave

Assumption: I have enough memory to cache all the database pages.
Goal:
Master never write pages. Slave replays logs from master and writes pages.
Benefits:
Reduce the page IO overhead at master, save money in EC2 cloud.

Question:
Can you give me some comments on this idea?
And I cannot turn of page writes in Postgresql.

I adjust the following parameters:
shared_buffers = 3GB

bgwriter_delay = 2000ms                 # 10-10000ms between rounds
bgwriter_lru_maxpages = 0               # 0-1000 max buffers written/round
bgwriter_lru_multiplier = 0             # 0-10.0 multipler on buffers 
scanned/round

checkpoint_segments = 256                # in logfile segments, min 1, 
16MB each
checkpoint_timeout = 1h              # range 30s-1h
checkpoint_completion_target = 1.0      # checkpoint target duration, 
0.0 - 1.0

However, I still witness large amount of page writes.
Can anyone tell where are the page writes come from?
Can I turn off that part of page writes by configuration?
If not, which part of source code should I adjust to achieve my goal 
(turn of page writes)?


Thanks!

Xin