In pictures: Taking apart Hewlett-Packard's Prolia...
The SmartArray controller is used to drive the hard disks. It takes up one of the PCI slots, leaving four free. Standard configurations include 256MB or 512MB of read/write cache. There is a battery upgrade available for those who want a Battery Backed Write Cache (BBWC) so that cached data isn't lost in the event of a power failure.
In pictures: Taking apart Hewlett-Packard's Proliant DL380 G5 server