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How the Fusion Drive on the New iMac and Mac mini will Work

October 24, 2012

UPDATE: Confirmed

Apple’s new Fusion Disk technology sounds amazing. It combines fast Flash or SSD storage with the more space abundant HDD but there is some confusion on how this will work. Many people are comparing it to hybrid SSD drives that have been available for some time that basically cache a small amount (less than 8GB) of data to the SSD part of the drive. That means it keeps 2 copies of data that it thinks you are going to use for faster access. The small amount of size means that not much can be cached and it would constantly be copying and deleting the files onto the SSD and as benchmarks have proven, not really give you a performance boost. The new Fusion Disk is going to work completely different.

During the presentation, Apple showed the insides of the new iMac that clearly point out a full size 3.5″ HDD and on the left side the Flash memory. The Flash may be a card like what is used in the MacBook Air and MacBook Pro w/ Retina or soldered on. Phil Schiller I think mistakenly said that both the flash and the HDD were part of the same part but they are clearly separate from the layout.

In Mac OS X 10.7 Lion, CoreStorage was added as a new way for Mac OS to handle the storage systems. Most people are not using CoreStorage unless you are using FileVault 2 in Lion or OS X 10.8 Mountain Lion. The way it works is CoreStorage actually creates a virtual disk on top of the physical disk and then presents it to the operating system. I have FileVault turned on on multiple disks and using the command diskutil cs list in the terminal you can see how it looks.

CoreStorage took some physical space from the actual hard drive and created a Logical Volume Group. Inside that group you have physical volumes. Right now, this is just a single physical volume. Under the LVG you have the Logical Volume Family and finally the Logical Volume, the thing you actually see in the finder as your HDD.

What the Fusion Drive process is going to do is create a LVG with both the Flash Memory and the HDD and combine them into a single Logical Volume that you will see in the Finder. Behind the scenes, CoreStorage will automatically place any system files on the Flash storage along with your most used files for faster access. Anything else will be stored on the physical HDD storage without you needing to know what files are where. But, rest assured, when you go to your Documents or Pictures folder, it will be there just like you left it.

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