Response time between shared buffer cache and operating system

I am using Postgres 8.1.4 on Linux. I am interested in the calculating the following for a specific application:

How long it takes the operating system to fulfil a page demand, ie, reading the page from disk or from the OS cache to the Postgres shared buffer.

Also how long it takes the bgwriter to flush a page from the shared buffer into the OS cache or disk.

These can be averages or detailed info that I can analyze with other tools

Are there any functions/views available that log this information, if not how should I change the source code.