In the Fusion Drive example, 3TB of hard drive is combined with 128GB of flash. The union of space is 3.125 TB, compared to the 3TB you would get with a cache.

In the Fusion Drive example, 3TB of hard drive is combined with 128GB of flash. The union of space is 3.125 TB, compared to the 3TB you would get with a cache. That's just a 4% increase in space that the Fusion Drive gives you over what a cache would give you. You can call it whatever you want, but I'm going to call them similar.