Putting Fusion Drive’s Performance in Perspective

Benchmarking Fusion Drive is a bit of a challenge since it prioritizes the SSD for all incoming writes. If you don’t fill the Fusion Drive up, you can write tons of data to the drive and it’ll all hit the SSD. If you do fill the drive up and test with a dataset < 4GB, then you’ll once again just measure SSD performance.

In trying to come up with a use case that spanned both drives I stumbled upon a relatively simple one. By now my Fusion Drive was over 70% full, which meant the SSD was running as close to capacity as possible (save its 4GB buffer). I took my iPhoto library with 703 photos and simply exported all photos as TIFFs. The resulting files were big enough that by the time I hit photo 297, the 4GB write buffer on the SSD was full and all subsequent exported photos were directed to the HDD instead. I timed the process, then compared it to results from a HDD partition on the iMac as well as compared to a Samsung PM830 SSD connected via USB 3.0 to simulate a pure SSD configuration. The results are a bit biased in favor of the HDD-only configuration since the writes are mostly sequential:

iPhoto Library Export to TIFFs

The breakdown accurately sums up my Fusion Drive experience: nearly half-way between a hard drive and a pure SSD configuration. In this particular test the gains don't appear all that dramatic, but again that's mostly because we're looking at relatively low queue depth sequential transfers. The FD/HDD gap would grow for less sequential workloads. Unfortunately, I couldn't find a good application use case to generate 4GB+ of pseudo-random data in a repeatable enough fashion to benchmark.

If I hammered on the Fusion Drive enough, with constant very large sequential writes (up to 260GB for a single file) I could back the drive into a corner where it would no longer migrate data to the SSD without a reboot (woohoo, I sort of broke it!). I suspect this is a bug that isn't triggered through normal automated testing (for obvious reasons), but it did create an interesting situation that I could exploit for testing purposes.

Although launching any of the iMac's pre-installed applications frequently used by me proved that they were still located on the SSD, this wasn't true for some of the late comers. In particular, Photoshop CS6 remained partially on the SSD and partially on the HDD. It ended up being a good benchmark for pseudo-random read performance on Fusion Drive where the workload is too big (or in this case, artificially divided) to fit on the SSD partition alone. I measured Photoshop launch time on the Fusion Drive, a HDD-only partition and on a PM830 connected via USB 3.0. The results, once again, mirrored my experience with the setup:

Photoshop CS6 Launch Time (Not Fully Cached)

Fusion Drive delivers a noticeable improvement over the HDD-only configuration, speeding up launch time by around 40%. A SSD-only configuration however cuts launch time in more than half. Note that if Photoshop were among the most frequently used applications, it would get moved over to the SSD exclusively and deliver performance indistinguishable from a pure SSD configuration. In this case, it hadn't because my 1.1TB Fusion Drive was nearly 80% full, which brings me to a point I made earlier:

The Practical Limits of Fusion Drive

Apple's Fusion Drive is very aggressive at writing to the SSD, however the more data you have the more conservative the algorithm seems to become. This isn't really shocking, but it's worth pointing out that at a lower total drive utilization the SSD became home to virtually everything I needed, but as soon as my application needs outgrew what FD could easily accommodate the platform became a lot pickier about what would get moved onto the SSD. This is very important to keep in mind. If 128GB of storage isn’t enough for all of your frequently used applications, data and OS to begin with, you’re going to have a distinctly more HDD-like experience with Fusion Drive. To simulate/prove this I took my 200GB+ MacBook Pro image and moved it over to the iMac. Note that most of this 200GB was applications and data that I actually used regularly.

By the end of my testing experience, I was firmly in the category where I needed more solid state storage. Spotlight searches took longer than on a pure SSD configuration, not all application launches were instant, adding photos to iPhoto from Safari took longer, etc... Fusion Drive may be good, but it's not magic. If you realistically need more than 128GB of solid state storage, Fusion Drive isn't for you.

The Application Experience Final Words
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  • StormyParis - Friday, January 18, 2013 - link

    The device is technically nice, however the price is wayyy too expensiveat around $450 for 128GB+2TB:

    Apple's 128GB SSD+ 2TB HDD "Fusions drive" is about $450 ($400 as an upgrade)

    A regular 256 GB SSD is $170
    A regular 3TB HD is $150.
    regular equivalent for Apple's price: 256 SSD+ 2x3TB HDD = $470

    You can get twice the SSD storage, and 3 times the HDD storage, for about the Apple price. This will take up more physical space, but also offer you way more storage space, both on the SSD side (plenty of space for your OS, apps, and live data files) and HDD space (3TB + 3TB backup, or 6TB JBOD for your archives and media)
    Reply
  • jeffkibuule - Friday, January 18, 2013 - link

    Hence the DIY route. Reply
  • Galatian - Friday, January 18, 2013 - link

    Thanks for pointing this out! I was very interested in getting a new iMac, as I love working on my 13" rMBP, but since I still like to game I really wanted to use the SSD on bootcamp as well. It is a huge step backward for all those people who want/need bigger SSDs. The only other option Apple offers is the 768 GB version for a whopping 1300€ upgrade price. There is no other size in between. Apple has lost me on that one. Reply
  • BrooksT - Friday, January 18, 2013 - link

    So Apple is charging $150 to take away the headache of managing files. What you call "gouging" is what many people call "charging for a service." Reply
  • Death666Angel - Friday, January 18, 2013 - link

    Again I have to wonder what you do with your PC. It appears as though you have no idea what is on your PC, because even saving a .doc in a specific folder is "managing files". Do you just save everything to desktop? Or download everything from the cloud and then start it from the browser download window? Reply
  • name99 - Friday, January 18, 2013 - link

    And I wonder if you have EVER actually
    (a) viewed how NORMAL people use computers,
    (b) used a Mac more recently than ten years ago.

    Apple realized a long time ago that people has MASSIVE difficulty dealing with the file system and naming/finding files. Practically every large advance in Apple tech has been to deal with this.

    iTunes and iPhoto are about dealing with "songs" and "photos", not "song files" and "photo files". Both create an environment where you never have to give a damn about where your songs and photos are stored or named in the file system. iPod follows the same path, and iOS is the logical endpoint, with no user visible file system.
    And this is not new --- did you EVER expect that you should have to manually organized your email files into folders?

    In OSX Apple first tried to simplify things with automatically provided folders for the most common situations (Documents, Downloads, Music, Movies, Pictures).
    Now, in Mountain Lion Apple is, through a combination of different features (version storage, automatically opening apps at reboot, auto-storing files in iCloud) trying to make it so that, more and more you don't have to name documents created in apps like TextEdit. You CAN, but you don't HAVE TO.

    Insisting on manually controlling the placement of your files really does start to come across as no different from insisting on writing assembly code.

    Look, I'm not an idiot. I have multiple external hard drives connected to my multiple machines, and I have a purpose for the different hard drives.
    The difference between you and me is that I'm not so deluded as to imagine that my needs are in any way typical, and that everyone else would be better off being forced to do things like me.
    Reply
  • Death666Angel - Friday, January 18, 2013 - link

    I see how other people use PCs. They are managing their stuff, even if they just put documents in a document folder, photos in a photo folder etc. I don't even know what not managing files would look like. Hence why I asked.
    I don't use Apple products.
    I manually organize my Emails to folders in Thunderbird.
    I also don't use the Windows libraries or the media player library or any of that stuff. I have no found a program that gave me the ability to organize "non-files" (songs, photos etc.) in a way I found easy to use, well arranged etc. I also like my files to be program/OS independent.
    I don't think I'm the typical user. I never said as much.

    Your last point though tells me that you don't understand the meaning of my post here. Read the other post that I did not replying to anyone. That should make it clear that I am against this technology. But if someone says he is not managing files, that just sounds dumb (real world equivalent: I don't clean up my closet and have no idea where what is).
    Reply
  • KoolAidMan1 - Friday, January 18, 2013 - link

    So many spergs in tech forums, it is unbelievable.

    I'm certain that most of you have low level autism and have no idea how people in the real world are.
    Reply
  • Death666Angel - Friday, January 18, 2013 - link

    I' pretty sure I'm not. What I am sure of, though, is that you are a dick. :P Reply
  • kyuu - Saturday, January 19, 2013 - link

    People who do the simplest management of their files (saving photos to a Photos folder, music to a Music folder, etc.) have "low-level autism"?

    The argument that basic file management is hard or a "headache" is absurd. It takes no time at all, and managing files and apps between an SSD and an HDD is no more difficult or time consuming than that.
    Reply

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