Meet Fusion Drive

Available as a build-to-order option on both the new Mac mini and the new iMac is Apple’s own take on SSD caching, Fusion Drive. In true Apple fashion there are only two Fusion Drive configurations available: 1TB and 3TB. The 1TB option is only available on the upgraded Mac mini ($799) or any of the iMacs, while the 3TB Fusion Drive is a 27-inch iMac exclusive.

In all of these cases, the Fusion Drive is a combination of a 1TB or 3TB hard drive (2.5” or 3.5”) and a 128GB Samsung PM830 based SSD. In the Mac minis this SSD is a 2.5” drive, while in the iMacs it’s the same custom interface that’s used in the MacBook Air and MacBook Pro with Retina Display. For my testing I used a 1TB Fusion Drive in a 27-inch iMac.

Fusion Drive Options
  Mac mini (2012) Mac mini (2012) Mac mini server (2012) 21.5-inch iMac (2012) 27-inch iMac (2012)
Base System Cost $599 $799 $999 $1299/$1499 $1799/$1999
1TB Fusion Drive - +$250 - +$250 +$250
3TB Fusion Drive - - - - +$400
Largest Standalone SSD - 256GB
- 768GB

The size of the SSD used in Apple’s Fusion Drive is much larger than what we usually find in a caching setup. Most OEMs ship with 8 - 24GB of NAND, and even then the drives rarely use a good controller. In the case of Apple’s Fusion Drive, Samsung’s PM830 continues to be one of the best combinations of performance and reliability we’ve ever tested. While I would’ve personally picked something like the Link A Media or Intel S3700 controller due to their excellent performance consistency, the PM830 is probably a more proven and/or affordable option for Apple.

Right off the bat Fusion Drive is different than most of the hybrid/caching solutions we’ve seen, but where it really diverges from the norm is in the software component. This isn’t simply Intel’s Smart Response Technology running under an Apple brand, instead we’re looking at virtualized storage courtesy of OS X’s Core Storage. First introduced in Lion, Core Storage is a logical volume manager that allows the OS to treat multiple physical disks as a single volume.

Apple originally used Core Storage to enable full disk encryption in Lion, but its use has been expanded to Fusion Drive in Mountain Lion. The creation of a Fusion Drive is simple. If you have multiple drives you can create a Fusion Drive yourself using some simple Terminal commands. When you buy a Fusion Drive equipped Mac, Apple does everything for you. Subsequent system and backup restores on your Mac with FD will maintain the Fusion Drive facade, even if you’ve purposefully destroyed the array.

Unlike traditional SSD caching architectures, Fusion Drive isn’t actually a cache. Instead, Fusion Drive will move data between the SSD and HDD (and vice versa) depending on access frequency and free space on the drives. The capacity of a single Fusion Drive is actually the sum of its parts. A 1TB Fusion Drive is actually 1TB + 128GB (or 3TB + 128GB for a 3TB FD).

The latest version of Disk Utility will present a Fusion Drive as a single drive, labeled Macintosh HD from the factory. Apple doesn’t attempt to hide the FD underpinnings however, looking at System Report or using a third party utility like iStat Menus you’ll get statistics on both drives:

If you’ll notice, the 128GB SSD is reported as having a 121.33GB capacity. Since OS X 10.6, Apple has reported capacities in base 10 but if you do the math based on the capacity in bytes you’ll get an idea of how much space is set aside as spare area:

Apple Fusion Drive, SSD Spare Area
  Total NAND Exposed Capacity Spare Area
Apple Fusion Drive 128GB SSD 128 GiB 113 GiB 15 GiB

Approximately 11.7% of the 128GiB of NAND is set aside as spare area, which is no different than what you get with a 128GiB SSD in a standard Mac, but a bit higher than the usual 6.7% spare area you get with most of these drives. The added spare area will help improve performance consistency, but it’s still a bit shy of what I like to see on Samsung SSDs (~25%).

You can create Boot Camp or other additional partitions on a Fusion Drive, however these partitions will reside on the HDD portion exclusively.

Introduction Fusion Drive: Under the Hood
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  • name99 - Friday, January 18, 2013 - link

    Yeah, and USING Momentus XT sucks. The experience is horribly uneven.
    Enough stuff comes up fast that you get used to that, but enough stuff comes up slowly that it's REALLY noticeable because you're used to the occasional bursts of speed.

    I've used Momentus, I've used Fusion. There is no comparison.

    In fact (true story) after I replaced the broken HD in a friend's MacBook Pro with a Momentus she told me a week later that she thought the computer was still broken because it seemed to behave so strangely, sometimes feeling really fast, then a little later feeling so slow.

    Now, if Momentus were kitted out with
    - 64GB (maybe even just 32GB) of
    - FAST flash (not the cheap crap used in USB thumb drives) AND
    - cached writes
    it might work well. But that's not the product that Seagate is selling.
  • Death666Angel - Friday, January 18, 2013 - link

    2 of your 3 points are very correct. But they do use SLC which is not the cheap stuff.
  • name99 - Friday, January 18, 2013 - link

    If they do use decent flash, then why don't they cache writes?

    I always assumed it was because their flash (like USB thumb flash) was so crappy that it was slower for random writes than the HD was.
  • kyuu - Saturday, January 19, 2013 - link

    Because there's a lot more to it than just using the right NAND. Also, for the 2nd-gen Momentus XT they were going to release a firmware update that would enable write caching. I'm not sure if that ever happened, haven't followed up on it recently.
  • kyuu - Saturday, January 19, 2013 - link

    That's because the MacBook/MacOS sucks. Not the Momentus XT's fault.

    Been using a Momentus XT in a Windows machine for a long time, had no problems with it being "uneven".

    Also, they sure as hell don't use cheap flash "used in USB thumb drives".
  • ShieTar - Saturday, January 19, 2013 - link

    How would the HDD know what is a file? The OS will just command a drive to write a given data block to Sector X.

    The drive may treat X as a logical address, and reorder data internally, but it has no clue if it is writing a complete file or parts of it, or just writing zeros as ordered by some secure erase software.
  • Subyman - Friday, January 18, 2013 - link

    Any word on how much the migration process increases read/write quantity over a manually managed setup? As for ssd life being longer than hdd life, if we take into account that almost all writes will hit the ssd first and then some will transfer to the hdd this means the hdd is accessed less often. This could level the mean read/write to failure rate to make the hdd even with the ssd, unless migration has an effect that I'm not considering.
  • dimmer - Friday, January 18, 2013 - link

    Did you enable TRIM or not?
  • name99 - Friday, January 18, 2013 - link

    It's a Mac for gods sake. It comes configured correctly (yes, with TRIM enabled) out the box.
  • alanh - Friday, January 18, 2013 - link

    For me, the biggest problem is the added difficulty of doing an upgrade or replacement of storage if it starts getting full or goes bad. From what I've read, the only option is to do a full backup, replace one of the disks, and then do a full restore. I have an '11 MBP with SSD and the DVD replaced with a large HD, so I could, in theory move to a Fusion drive, but it just seems like a risky and annoying proposition.

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