Speed of WAL send/receive when replicating 1/ Description:

I have a cluster include two machines


Master: postgresql 9.3 run on Centos 
Slave : postgresql 9.3 run on Centos 


Master ----> Slave : relpica from Master to Slave by asynchronous method (M send WAL, S receive WAL)

2/ Question:

How can I monitor (catch) speed of WAL (ex: 1MB/s) is sent from Master to Slave ?
You can use pg_stat_replication along with pg_xlog_location_diff. Checking the differences can tell you how many bytes of xlog have flown from the master to the slave.