The Fusion Drive difference
Apple introduced its Fusion Drive tech in 2012 as a way to gain some of the benefits of speedy yet pricey SS Ds, but at a lower cost and without sacrificing the large capacity of affordable (but slow) hard disks.
The tech binds one drive of each type and presents them as one. It tries to keep the data you use most often on the SS D. It also keeps a small amount of the SS D free, in which to quickly create new files. Data it thinks you won’t need soon is moved to the hard disk.
At first, Apple used a 128GB SS D in Fusion Drives, but has since reduced it to 24GB or 32GB. As a result, today’s Fusion Drives may have to move data more often.
When using a Mac with only a hard disk, you’ve likely encountered the wait cursor, or beach ball, when trying to do something on its disk. A common reason is that the disk has gone to sleep while idle, so you have to wait for it to spin up again.
That also happens with a Fusion Drive when required data is on its hard disk. You might encounter this less on
one with a 128GB SS D, as there’s a better chance what you need is stored on that. We put enough on the 21.5–inch iMac’s 1TB Fusion Drive to “fill” its SS D.
Blackmagic’s Disk Speed Test, which alternately reads and writes 5GB of data, had peak read and write speeds of 850.5MB/sec and 1.4GB/sec respectively, but mean averages were 379.2 and 534.1MB/sec.
The averages are dragged down by regular drops to the kinds of rates expected from a plain hard disk — too often they were barely above 100MB/ sec. So, we strongly recommend you buy an iMac only with a pure SS D, despite the extra cost, to avoid the performance compromise of current Fusion Drive specs.