When Apple updated the iMac and Mac mini in late 2012, it introduced a new storage option called Fusion Drive. Despite the name, it's actually two drives working in a special arrangement.
It pairs 128GB of flash storage with either a 1TB or a 3TB hard drive. Apple bills it as providing the high capacity of a hard drive with the performance of flash storage, in a way that has no impact on how you work and store things.
Pure SSD configurations remain an option on some Macs, but they're still costly for their capacities. Replacing a Mac mini's 1TB hard drive with a 256GB SSD costs £240, and a 768GB SSD on the 27-inch iMac is an eyewatering £720. Hence Apple's attempt to find a compromise between speed and capacity.
Fusion Drives cost £200 and £320 for the 1TB and 3TB versions respectively, though the latter is only available on the 27-inch iMac. In fact, Apple sells Fusion Drive short by listing only the hard drive component's capacity and omitting that of the SSD.
In terms of hardware, all that's added is a 128GB SSD. A substantial amount of what you're paying covers the fitting of the SSD, and the configuration of the two drives to work together.
There's no setup process on your part. The two drives that make up a Fusion Drive don't operate like a striped RAID array to spread the load between them. Nor do they work like hybrid drives, available on PCs for several years, despite their similar makeup. A hybrid drive's flash portion acts as a cache, but that's all it is: a cache. A copy of everything is on the hard drive.
How does Fusion Drive work?
A Fusion Drive's components appear as a single volume in the Finder, with their capacities merged. No adjustment needs to be made to how you work, as decisions about which of the two components is used to store a given piece of information are made for you.
Neither of the drives holds a complete copy of everything. When something needs to be stored, it's always first written to the flash storage. As long as plenty of free flash storage is available, OS X doesn't touch the hard drive, and the Mac will operate solely from its flash storage.
Behind the scenes, OS X silently monitors how your Mac is used. When free flash storage dwindles to only 4GB, OS X's long-term observations are used to decide what you're least likely to need from day to day, and it moves some of it to the hard drive. This keeps plenty of flash storage available so that high performance is maintained.
Relegation to the hard drive isn't a one-way or irreversible. OS X continues to monitor your activity, and if it discerns you're using something enough to warrant moving it back to the faster storage, it will do so. Something else will end up relegated to the slower drive instead.
The operating system stays on the SSD, but pre-installed apps such as iMovie and GarageBand don't enjoy this privilege. Very large files, such as videos or an iPhoto library aren't treated as monolithic. That would be inefficient, so Fusion Drive doesn't have to shift the whole of a file.
A developer, Patrick Stein, has published blog posts examining the working of Apple's technology. He discovered Fusion Drive works at a lower level, instead moving the blocks that make up files, and only some of them. Stein discovered if he read the first megabyte of a large file enough to warrant storing it on the flash storage, only that portion of the file was moved. The rest remained on the hard drive. The effect of splitting data between two drives is that both need to be connected to a Mac (which must be running OS X 10.8.2) in order to read it.
Under any circumstances, it's wise to keep an up-to-date backup of your Mac's contents. OS X's Time Machine feature will back up a Fusion Drive just the same as it would a hard drive, and it's a good idea to use it because the contents are more susceptible to loss due to the increased risk that either of the two pieces of hardware fails.
In our testing of two 21.5-inch iMacs - one with a hard drive and the other with a Fusion Drive - OS X's System Information app revealed that the hard drives in both had the same model number and rotational speed. The smaller iMac uses a 2.5-inch, 5400rpm drive. However, Apple has stuck with 3.5-inch, 7200rpm drives in the 27-inch iMac, which are capable of faster transfer rates. However, they fall far short of flash storage's capabilities.
Our benchmarks show the speeds reached by a hard drive and a Fusion Drive in two 21.5-inch iMacs. The gap in their performance is really quite significant.
Fusion Drive and Boot Camp
Since Fusion Drive depends on software technology built into OS X, Windows doesn't support it. This doesn't mean Windows can't be installed on a Mac with a Fusion Drive, but Boot Camp Assistant will only create a partition on the hard drive.
However, there is an issue with installing on a 3TB drive, whether that's a Fusion Drive or a garden-variety hard drive. Apple acknowledges that Boot Camp Assistant won't work with drives of this capacity. The maker of WinClone, an app which backs up Boot Camp partitions from within OS X, detailed the reason for this and how to overcome it.
Some commenters on the blog entry report stumbling at the final step, and even if it works for you, there's a side effect that might discourage you from trying it. It splits the hard drive portion of your Fusion Drive into three partitions. OS X continues to see the first one as part of the Fusion Drive, Windows can be installed on the second one, and the last 1TB of the drive becomes a separate volume. It's usable, but OS X no longer sees it as part of the Fusion Drive, so it loses the ability to include that capacity in its shifting around of data.
There's no word as yet about an update to Boot Camp Assistant, or if an improved version will ship with the next version of OS X.
Homemade Fusion Drive
Among Patrick Stein's posts about Fusion Drive, he published instructions on how to use the command-line version of Disk Utility to create your own Fusion Drive from any two drives connected to a Mac. The Mac must support Mountain Lion, which is needed for Fusion Drive support. Specifically, it requires the installer for OS X 10.8.2 from the Mac App Store.
There's little reason to create your own Fusion Drive unless one of the drives is capable of very fast transfer rates though. That depends not just on the drive but also its connection to the Mac.
To get a real benefit requires, among other things, a Mac with Thunderbolt, a Mac Pro with an SSD, or a willingness to make major internal changes to other models. For instance, on a MacBook Pro, you'll need to install a kit such as OWC's Data Doubler ($45, about £30, bit.ly/jjIab5), which converts the bay normally occupied by the SuperDrive to hold a 2.5-inch drive instead. The hard drive needs to be moved there, since that bay typically uses a slower connection that would impede an SSD's performance.
Even then, MacBook Pros introduced prior to 2011 use a SATA-II connection, which impedes the transfer rate of modern, SATA-III SSDs. Apple doesn't intend you to set up your own Fusion Drive.
The graphical version of Disk Utility doesn't provide the capability, but the command-line version accessed in Terminal does. On the next page we'll show you just what you need to roll your own Fusion Drive.
However, you won't get all the benefits of an Apple-provided Fusion Drive. TRIM is technology that prevents degradation in an SSD's performance over time as it's repeatedly overwritten. It's normally enabled only for flash storage provided by Apple, but TRIM Enabler (groths.org/trim-enabler) turns it on for other drives. We installed TRIM Enabler on our Mac's homemade Fusion Drive and encountered no issues.
However, doing so means you will be running OS X with low-level modifications that take you further away from configurations that are tested and approved by Apple.
Fusion Drive benchmarks
We measured these peak speeds from three storage options. Sequential data transfer is the best case scenario. Random transfers involve accessing locations spread around a drive. Hard drives suffer most because of their moving parts.
Make your own Fusion Drive
Adding a Fusion Drive to a Thunderbolt Mac isn't cheap, but it's less costly than replacing your existing Mac If you run into a software problem on your Mac while running a homemade Fusion Drive, support might be refused at a Genius Bar or a service provider until your Mac is restored to a regular configuration. Use Time Machine for protection.
With your backup drive attached, start your Mac in the Recovery System by holding Command+R at the startup chime. In the option that restores from Time Machine, verify the most recent backup is listed. Backing up OS X this way gives a straightforward recovery if needed. A Boot Camp partition can be preserved during the process, but back it up for safety.
Windows Vista and later OSes have a backup feature, but require an external drive formatted for Windows. Alternatively, Winclone ($20, about £14, twocanoes.com) backs up Boot Camp from within OS X. It's possible to remove the Recovery System when setting up Fusion Drive, but that can be avoided by specifically targeting a partition.
When creating a Mountain Lion install disk, take a USB flash drive you don't need to re-use, so the OS X installer, Terminal and Disk Utility are always to hand. Installing Mountain Lion to a drive from scratch will recreate a Recovery System on it.
Removing a Fusion Drive configuration requires starting from an install disk, opening Terminal. Use the command diskutil cs list and copy the code listed next to Logical Volume Group to the Clipboard. Type diskutil cs delete tap space, then paste the code and press Return. The internal hard drive is then independent of the SSD.
Get ready for Fusion Drive
Some essential steps before you start to build your own Fusion Drive
1. Back up your Mac
Open the Mac App Store. You'll need to purchase and install Mountain Lion if you don't already have it. Otherwise, click Purchases at the top of the window, locate Mountain Lion in the list and click its Download button.
Once complete, the Applications folder will contain a file named 'Install OS X Mountain Lion'. If you already use Time Machine, open its System Preferences pane, click the Options button and check that you haven't excluded system files, your user account, or any other folders from the top level of the hard drive from its backups.
If you don't already use Time Machine, connect a Mac-formatted drive, flick the switch in Time Machine's preferences to On and choose the drive. Click the Time Machine icon in the menu bar, choose Back Up Now and wait until the backup is complete.
2. Prepare a USB flash drive
You'll need a USB flash drive with a capacity of at least 8GB, whose contents can be erased. Connect it and open Disk Utility. Select the drive in the left pane - that's the row that shows its capacity and model, not a partition already on the drive - then click the Partition tab. Set the Partition Layout item to '1 Partition'.
If the drive is bigger than 8GB, you can create more to use for other purposes, but there needs to be a partition that's at least 5GB large on which to create an install disk. It doesn't matter what you call this partition. Click the Options button below the partition layout. From the three options presented, choose 'GUID Partition Table', then click OK. Click Apply towards the bottom right of the window and wait for the drive to be repartitioned and mounted.
3. Create a Mountain Lion install disk
Browse to the Applications folder and -click the Mountain Lion installer. Choose Show Package Contents, then browse to Contents/SharedSupport and double-click InstallESD.dmg. Wait for the integrity of the disk image to be verified.
In Disk Utility, click the Restore tab. Drag 'Mac OS X Install ESD' from the left pane into the Source box, and the partition on the USB flash drive into the Destination box. Click the Restore button. You might be asked for your account credentials to proceed. It takes a while for the OS X installer to be copied to the flash drive.
In System Preferences, click Startup Disk at the far right of the row labelled System. Among the available startup disks you should see 'Mac OS X Install ESD' with 'OS X, 10.8.2' below it. Click it, then click the Restart button.
How to make your own Fusion Drive
1. Look up essential details
When your Mac restarts, choose Disk Utility from the options. Eject then disconnect everything except the internal hard drive, the SSD and the USB flash drive. Quit Disk Utility. Choose Utilities > Terminal. Type diskutil list and hit Return.
Partitions are grouped by the disk to which they belong under rows starting /dev/diskX (where X is a number). Two rows below those ones, use the Size column to work out which is the internal hard drive and the SSD. To avoid wiping Boot Camp and the Recovery System, look for the partition with the name of your OS X volume on the hard drive. Note its identifier, such as diskXsY.
2. Create a Logical Volume Group
Type diskutil cs create Fusion then a space, then the identifier of your SSD, another space, then the identifier of your OS X volume. Press Return to create a logical volume group that identifies the storage that will be used by the Fusion Drive.
Type diskutil cs list and press Return for a summary of what was created. In the row starting '+-- Logical Volume Group', drag the pointer through the long alphanumeric code to select it. Press Command+C to copy it to the Clipboard. Make a note of the free space listed four lines below the code.
3. Create a Fusion Drive volume
Type diskutil coreStorage createVolume then a space. Press Command+v to paste the code. Type another space, then jhfs+ "Macintosh HD" and another space. Type the capacity noted previously. If it's in gigabytes, use the form XXX.Xg, or X.Xt for terabytes. Press Return to create the volume.
When done, look for a line that says 'Finished CoreStorage operation'. Quit Terminal, then open Disk Utility to see a device named Fusion and the volume just made on it. You can install a fresh copy of OS X or restore from Time Machine. The former is the best starting point. Migration Assistant can later transfer users, apps and files from your Time Machine backup.
