Welcome to LQ.
The first question I'd have is what distro are you using and which kernel? That would be helpful to know. In any case, your system is far beyond anything I've used myself, but if you are seeing poor disk performance it potentially could be because DMA has not been enabled.
This article gives a good overview.
With respect to memory "never releasing", that's not a problem, and it's how Linux
manages memory
HTH
Edit: D'oh, hdparm is for IDE drives, but
sdparm is the equivalent for SATA