Database buffer cache versus operating system cache
Unlike many traditional database products, PostgreSQL does not assume or even prefer that the majority of the memory on the system be allocated for its use. Most reads and writes from the database are done using standard operating system calls that allows the operating system's cache to work in its usual fashion. In some configuration WAL writes will bypass the OS cache, that's the main exception.
If you're used to a database where most system RAM is given to the database and the OS cache is bypassed using approaches like synchronous and direct writes, you don't want to setup PostgreSQL that same way. It will be downright counterproductive in some areas. For example, PostgreSQL's stores commit log information in the pg_clog directory. This data is both written to and read from regularly, and it's assumed that the operating system will take care of optimizing that access. If you intentionally bypass PostgreSQL...
