Prior to now, Apple’s Fusion drive was only available for the high-end 21.5-inch iMac, 27-inch iMacs, and the top model Mac mini. Starting today, Apple is now offering the Fusion drive for built-to-order bottom model 21.5-inch iMacs.
Of course, if you’d like to get an iMac with a Fusion drive, you’ll have to have your Mac built-to-order. This special drive combines a solid state drive and a traditional hard drive into a single volume.
The real functionality comes from the software integration that intelligently moves the files you use the most to the solid state drive for faster access:
With Fusion Drive in your iMac, disk-intensive tasks — from booting up to launching apps to importing photos — are faster and more efficient. That’s because frequently used items are kept at the ready on speedy flash storage, while infrequently accessed items go to the hard drive. The file transfers take place in the background, so you won’t even notice.
You can customize an iMac to include a Fusion drive on Apple’s online store.
Source: MacRumors