During its iPad mini launch event today Apple updated many members of its Mac lineup. The 13-inch MacBook Pro, iMac and Mac mini all got updated today. For the iMac and Mac mini, Apple introduced a new feature that I honestly expected it to debut much earlier: Fusion Drive. 

The idea is simple. Apple offers either solid state or mechanical HDD storage in its iMac and Mac mini. End users have to choose between performance or capacity/cost-per-GB. With Fusion Drive, Apple is attempting to offer the best of both worlds.

The new iMac and Mac mini can be outfitted with a Fusion Drive option that couples 128GB of NAND flash with either a 1TB or 3TB hard drive. The Fusion part comes in courtesy of Apple's software that takes the two independent drives and presents them to the user as a single volume. Originally I thought this might be SSD caching but after poking around the new iMacs and talking to Apple I have a better understanding of what's going on. 

For starters, the 128GB of NAND is simply an SSD on a custom form factor PCB with the same connector that's used in the new MacBook Air and rMBP models. I would expect this SSD to use the same Toshiba or Samsung controllers we've seen in other Macs. The iMac I played with had a Samsung based SSD inside. 

Total volume size is the sum of both parts. In the case of the 128GB + 1TB option, the total available storage is ~1.1TB. The same is true for the 128GB + 3TB option (~3.1TB total storage).

By default the OS and all preloaded applications are physically stored on the 128GB of NAND flash. But what happens when you go to write to the array?

With Fusion Drive enabled, Apple creates a 4GB write buffer on the NAND itself. Any writes that come in to the array hit this 4GB buffer first, which acts as sort of a write cache. Any additional writes cause the buffer to spill over to the hard disk. The idea here is that hopefully 4GB will be enough to accommodate any small file random writes which could otherwise significantly bog down performance. Having those writes buffer in NAND helps deliver SSD-like performance for light use workloads.

That 4GB write buffer is the only cache-like component to Apple's Fusion Drive. Everything else works as an OS directed pinning algorithm instead of an SSD cache. In other words, Mountain Lion will physically move frequently used files, data and entire applications to the 128GB of NAND Flash storage and move less frequently used items to the hard disk. The moves aren't committed until the copy is complete (meaning if you pull the plug on your machine while Fusion Drive is moving files around you shouldn't lose any data). After the copy is complete, the original is deleted and free space recovered.

After a few accesses Fusion Drive should be able to figure out if it needs to pull something new into NAND. The 128GB size is near ideal for most light client workloads, although I do suspect heavier users might be better served by something closer to 200GB. 

There is no user interface for Fusion Drive management within OS X. Once the volume is created it cannot be broken through a standard OS X tool (although clever users should be able to find a way around that). I'm not sure what a Fusion Drive will look like under Boot Camp, it's entirely possible that Apple will put a Boot Camp partition on the HDD alone. OS X doesn't hide the fact that there are two physical drives in your system from you. A System Report generated on a Fusion Drive enabled Mac will show both drives connected via SATA.

The concept is interesting, at least for mainstream users. Power users will still get better performance (and reliability benefits) of going purely with solid state storage. Users who don't want to deal with managing data and applications across two different volumes are still the target for Fusion Drive (in other words, the ultra mainstream customer).

With a 128GB NAND component Fusion Drive could work reasonable well. We'll have to wait and see what happens when we get our hands on an iMac next month.

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  • derPat - Wednesday, October 24, 2012 - link

    The Fusion drive reminds me the Hierarchical Storage Management of Irix I was using in the good old days ... HSM is still in use but is usually relegated to the big iron.
    It would be nice if apple will support multiple tiers of storage.
  • iwod - Wednesday, October 24, 2012 - link

    For Random Write we have the 4GB sorted most of that. The problem is Random Read. Where do we do Random Read most since these needs to be inside SSD. For others Seq Read Write HDD isn't that much slower at all.

    Again, i think the experience will be hard to measure with any benchmarks tools. I am looking forward to Anand's review/
  • halbhh2 - Wednesday, October 24, 2012 - link

    I just suspect this is going to "just work" (as in just work well).

    And a mystery to me is why hasn't Seagate gotten it together, after years, to get really great performance from their XT series. I mean, how long does it take to get it right? A few months I bet is enough if you actually try.
  • epobirs - Wednesday, October 24, 2012 - link

    The problem for Seagate and other HD makers is getting the full value out of the SSD volume meant moving out of their comfort zone and into higher levels of the OS than their usual product. It also meant much higher price points if they were going to incorporate really large amounts of flash memory. This makes them nervous since they have very price sensitive customers in the big PC OEMs.

    I'm sure they'll keep at it but they've missed their window and that bird has flown. What is happening now is new computer models are coming with mSATA port/slots to allow a notebook to have an SRT drive along with a high capacity HD. Dell, for example, ow including a 32 GB mSATA cache as standard on several models.

    This isn't as sophisticated as what Apple is doing. It remains to be seen if Intel and/or Microsoft will offer the functionality to enable PC OEMs to match the Fusion feature set. Much of it should be trivial, such as pre-loading the SRT cache volume with the OS files and any other favored items, although what you'd really need to do is mirror the appropriate sectors on the hard drive. At least until a file system level version of SRT is offered.
  • MGSsancho - Wednesday, October 24, 2012 - link

    Sounds like ZFS to me.
  • Bob-o - Wednesday, October 24, 2012 - link

    :-) Apple certainly could have delivered the same benefits much earlier, had they not rejected ZFS. . .
  • ThreeDee912 - Thursday, October 25, 2012 - link

    Although Apple ended up ditching ZFS after running into licensing issues, they started working on the basic underpinnings for a possible new filesystem, and included it in 10.7 Lion, calling it Core Storage.

    While Apple's still using Core Storage with the ancient HFS+ filesystem, the new FileVault 2 disk encryption system uses Core Storage to present the encrypted drive as a virtual normal one to the system.

    I'm guessing Fusion Drive is also using Core Storage.
  • orthorim - Wednesday, October 24, 2012 - link

    Thanks for providing some actual information on how this works. I've been googling around for a while, apparently none of the other so-called "tech" publications are interested enough to actually ask... Anandtech stands out, once again. Thanks!!!
  • piroroadkill - Thursday, October 25, 2012 - link

    Just like all the other tiered/caching SSD and small NAND drive systems that have been around for ages.

    Still, it is of course a nice idea. But it is in no way a "breakthrough concept".
  • inplainview - Thursday, October 25, 2012 - link

    "Just like all the other tiered/caching SSD and small NAND drive systems that have been around for ages."

    Exactly and where are these systems? While they may not be new, they weren't mainstream. They lived in the domains of the mommies cellar dwellers… While not new Apple at least made it mainstream… This is what they do...

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