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 OS calls that allow the OS's cache to work in its usual fashion. In some configurations, 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 such as synchronous and direct writes, you don't want to set up PostgreSQL that same way. It will be downright counterproductive in some areas. For example, PostgreSQL's stores commit log information in the pg_xact directory. This data is both written to and read from regularly, and it's assumed that the OS will...
