Another big reasons that databases with to circumvent this caching is that they really, really need to make sure that when a transaction is committed, that they know what is on disk. Having the OS pretend stuff is written to disk is great for performance, but not so great when you're trying to recover after someone pulled the power.
This is where you get DBs that use their own raw block devices, as well as keeping transaction logs on other regular filesystems for integrity.... Oracle does something similar no?
