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.
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.