This morning, Apple introduced the new iMac, with a build-to-order storage configuration called “Fusion Drive”. But what is Fusion Drive? Although it could be an off-the-shelf hybrid drive, I believe it is a software driver in Mac OS X.

Update: Apple’s Knowledge Base article on Fusion Drive for Mac Mini and iMac essentially confirms my conclusion: Fusion Drive is a function of CoreStorage and is a Logical Volume Group, not a piece of hardware! Now the question is, can you create your own DiY Fusion Drive using an off-the-shelf SSD?

What Do We Know?

We only have two data points so far. First, the Apple web site has this to say about the Fusion Drive:

“Available as a configurable option at the Apple Online Store, Fusion Drive is a breakthrough concept that combines the high storage capacity of a traditional hard drive with the high performance of flash storage. With Fusion Drive in your iMac, disk-intensive tasks – from booting up to launching apps to importing photos – are faster and more efficient. That’s because frequently used items are kept at the ready on speedy flash storage, while infrequently accessed items go to the hard drive. The file transfers take place in the background, so you won’t even notice. As the system learns how you work, Fusion Drive makes your Mac experience even better. And you don’t have to do a thing.”

Next, Phil Schiller described the Fusion Drive at length in the October 23 announcement:

“It’s 128 GB of flash storage, and added to that, your choice of either a 1 or 3 TB hard drive. And in software, they are fused together into one logical volume. We do this so that you can get much faster reads and writes than a hard drive can. But best of all it’s built into OS X Mountain Lion so it’s automatic; there’s nothing to set up, nothing to manage, nothing to do. It just works. “So how does it work? Well when you order your iMac, and you choose the fusion drive, you get it and there’s one drive made up of the flash and the hard drive. of course the operating system completely fits into flash, so we keep it there for maximum system performance. In fact all the software that comes preinstalled on your iMac automatically fits into flesh. And then as you load up applications and documents or migrate them from a previous computer, you will fill up all the storage and use the space that you have. “And then automatically, as you use your computer, OS X’s Fusion Drive is figuring out what you use the most and what will benefit from being on flash. So for example, if you don’t use iMovie all that often but you do use a number of high-powered numbers spreadsheets, it will move those onto flash…”

Until we see the Fusion Drive in action, we can’t be sure exactly what this is, but there are some clues in there…

Another clue is the interior photo of the iMac itself. It shows an SSD stick as well as a hard disk drive. The Fusion Drive could combine these two discrete elements in software rather than presenting them as separate drives.

One remaining question is whether the Mac Mini’s Fusion Drive is the same as the iMac’s. It would be trivial for Apple to equip the Mac Mini with a Seagate Momentus XT and call that “Fusion Drive” but I don’t think they would undermine their new offering like that. Instead, I bet the new Mac Mini includes a blade SSD slot like the iMac, MacBook Air, and Retina MacBook Pro.

Fusion Drive Is Not a Hybrid Drive Cache

Many jumped to the conclusion that Fusion Drive is a so called “hybrid drive”, variations on which companies like Seagate already sell. Since Apple works with Seagate, they could indeed just take a hybrid drive and stick it into the iMac and Mac Mini. But is this the case?

Seagate does not sell a desktop hybrid drive, nor do they produce one with 3 TB of capacity or 128 GB of flash. So if it was a Seagate hybrid, it would have to be one we haven’t seen yet. It could also be a Toshiba, since they have announced their own line of hybrid drives, but that would also be an unknown part.

It is not an OCZ RevoDrive Hybrid, either, since that is a chunky PCIe card that wouldn’t fit in the iMac. Integrating such a drive into the motherboard seems unlikely, too, since that would present quite an engineering challenge for a build-to-order-only option!

In fact, Fusion Drive is unlikely to be any sort of hybrid drive technology for a simple reason: Phil Schiller’s description suggests it is a tiering product, not a cache. All existing hybrid drive implementations (including OCZ, Seagate, and Intel’s SRT) are caches, storing a copy of all data on the hard disk drive and only using the flash memory for temporary storage.

Fusion Drive is Paired Storage Done Right

Re-reading Schiller’s comments, it seems clear that he is describing an automatic, file-based tiered storage solution, not a hybrid cache. Fusion drive “fuses” an SSD and HDD into “one logical volume” and moves files between them based on access. It is pre-loaded with the default Mac OS X install and moves data over time between both separate units.

If Fusion Drive was a cache, it would be an awfully large one. Even with 3 TB of disk, 128 GB is simply excessive for caching. Intel SRT doesn’t even support caches larger than 64 GB, after all! Recall that Apple’s smallest blade SSD is 128 GB, however, and we see why the flash portion is that particular size. It’s not a cache, it’s a full SSD in the blade slot!

Schiller emphasizes that Fusion Drive is built into Mac OS X Mountain Lion. He doesn’t say it’s a hardware feature or part of the iMac. And it’s also an option on the Mac Mini, a computer that uses a totally different drive (2.5″ form factor) from the iMac!

Lots of MacBook Pro and iMac users (including me) have opted for “paired storage“, installing both a hard disk drive and an SSD in a single machine. Although this works great, moving data between these devices is a manual process and probably not appropriate for the average non-technical user.

Apple is seizing the paired storage concept and integrating it with the operating system. They present the hard disk and SSD as a single volume (Schiller even uses the proper technical term, “logical volume”) and take care of the data movement automatically.

We do not know if Mountain Lion uses the new CoreStorage volume manager to manage the Fusion Drive or if it’s handled at a higher level. If it was in CoreStorage, it would likely be block-level, with “hot blocks” moved between the SSD and HDD rather than whole files. It is also conceivable that Apple could use the Spotlight/Time Machine Metadata Server to move files based on access time. Or perhaps it’s a combination of the two!

Stephen’s Stance

Fusion Drive is typical Apple: The introduction focused on user benefit but cut the details short. Even so, it seems likely it is a software feature of Mac OS X rather than third-party hardware transplanted to the platform. It really is innovative for a desktop operating system to implement automated tiered storage, since most previous (failed) attempts have been limited to small caches of data.