Technical details are scarce, but based on Schillers descriptions, the answer to both of those questions appears to be “no.” Apples Fusion Drive does not appear to function like an SSD-backed disk cache, but rather seems more like a file-level implementation of a feature that has existed for some time in big enterprise disk arrays: automatic tiering.

read the complete article @ Apple Fusion Drive—wait, what? How does this work? | Ars Technica.