A modern DB (eg: Drizzle) is going to delegate as much responsibility to the kernel as possible. This makes the code simpler, and this easier to optimize. That's just how it is.
I recall that major enterprise database engines like SQL Server have the ability, and for very high end specific performance , the desirability to store their data on unformatted raw disks. However, the lack of OS support means that this is not a recommended
except where every ounce of performance is required. Otherwise it is easier and cheeper to throw a couple of Gb ram at the problem.