Shared Buffer Size

Hi All,

I am little confused about the internal working of PostgreSQL. There is a
parameter shared_buffer in postgres.conf and I am assuming that it is used
for buffer management in PostgreSQL. If there is a need to bring in a new
page in the buffer and size exceeds the shared_buffer limit, a victim dirty
page will be written back to the disk.

However, I have read on many links that PostgreSQL depends on the OS for
caching. (http://www.varlena.com/GeneralBits/Tidbits/perf.html#shbuf)

So my question is, the actual limit of the shared buffer will be defined by
OS or the shared_buffer parameter in the postgres.conf to figure whether a
victim dirty page needs to be selected for disk write or not?

Thanks!