a question about Direct I/O and double buffering

Hi,

A page may be double buffered in PG's buffer pool and in OS's buffer cache.
Other DBMS like DB2 and Oracle has provided Direct I/O option to eliminate
double buffering. I noticed there were discusses on the list. But
I can not find similar option in PG. Does PG support direct I/O now?

The tuning guide of PG usually recommends a small shared buffer pool
(compared
to the size of physical memory). I think it is to avoid swapping. If
there were
swapping, OS kernel may swap out some pages in PG's buffer pool even PG
want to keep them in memory. i.e. PG would loose full control over
buffer pool.
A large buffer pool is not good because it may
1. cause more pages double buffered, and thus decrease the efficiency of
buffer
cache and buffer pool.
2. may cause swapping.
Am I right?

If PG's buffer pool is small compared with physical memory, can I say
that the
hit ratio of PG's buffer pool is not so meaningful because most misses
can be
satisfied by OS Kernel's buffer cache?

Thanks!

Xiaoning