Fusion Drive is an optimized storage solution for Apple’s iMac and Mac mini desktops. Can mix fast SSD and standard HDD hard drive, provide SSD speed and bring the standard hard disk storage capacity.
Frequently accessed files are stored on the SSD, and files that are not frequently accessed are stored on a standard hard disk. Apple has also released the wind before, suggesting that will be in the future update to provide support. Today Federighi just confirms this matter.
Reference: MacRumors