Details emerge on Apple's Fusion Drive technology

At last week's event introducing the iPad mini, Apple also rolled out new iMac and Mac mini models that can be configured with a proprietary "Fusion Drive." This technology presents a solid-state drive (SSD) and a hard disk drive (HDD) as a single volume to the user. There are all kinds of labels being applied to the technology, however, it looks to me that Fusion Drive employs a kind of hierarchical storage management (HSM) that tracks file and application usage and migrates frequently-used items to the quicker SSD.

As a result you'll enjoy shorter startup times, and as the system learns how you work you'll see faster application launches and quicker file access. Fusion Drive manages all this automatically in the background.

In Apple's About Fusion Drive technical note, it's clear that this capability isn't a function of the drives themselves, rather it's based on Apple's storage management software. Fusion Drive is a built-to-order selection in the Apple Store.

According to the note, Fusion Disk systems come with a "special version" of Disk Utility. It warns that earlier versions of Disk Utility can't be used with a Fusion Drive. We don't know whether there is a unique version that ships with the Fusion system, or whether everyone receives the same software and the "special" Fusion capabilities are revealed in the presence of a Fusion setup. But it's the same difference.

The note also warns that current versions of third-party disk utilities may or may not be compatible with the Fusion Drive. This would include various file and volume monitoring tasks often controlled with preference panes. Time Machine deals with the Fusion Disk as a single volume.

In addition, there are limitations with Fusion Drive and Target Disk Mode. To mount the Fusion Drive, the host system must be running Mountain Lion version 10.8.2 or later. Earlier versions of Mac OS X can't mount a Fusion Drive in Target Disk Mode.

There may be issues with the sustained data capture rates with Fusion Drive. Here's an item from the FAQ:

I'm using video capture software and it reports frame drops during sustained captures. What could be wrong?

Your project's capture requirements may exceed the sustained capture rate available to your Fusion Drive. Try capturing a smaller segment of the source or reduce its resolution.

In an interesting Betalogue blog post, Canadian IT consultant and author Pierre Igot pointed to another item in the tech note, where Apple offers a series of steps to take if the Fusion Drive system doesn't start up and you see the flashing question mark or another alert. But as we read, the end result isn't reassuring or entirely satisfying:

To repair the volume, click Fix. CAUTION: Clicking Fix will erase your Fusion Drive.

Igot suggests that the flashing question mark is rare occurrence nowadays, but a sign of serious disk damage. He worries "that the flashing question mark problem might occur more frequently with the Fusion Drive than with current drives, but that it’s not indicative of a hardware failure and can actually be fixed."

Maybe I’m reading too much into this. Maybe Apple is just being cautious and making sure that the required information is available in the rare cases when such a failure might occur. On the other hand, Apple is a company that tends to remain in denial about problems until reports reach some kind of internal critical mass. So this particular tech note looks rather unusual to me. After all, nobody except Apple people with access to prototypes is using a Fusion Drive right now.

It's too soon to understand the Fusion Drive's advantages and potential issues in professional workflows. Mac managers should proceed with caution until more is known.

If you want the speed advantages of running both a SSD and HDD, but without the automatic performance management, check out upgrade kits from vendors such as Other World Computing's Data Doubler series. One works with the new Mac mini 2012.