Fusion Drive: Under the Hood

I took the 27-inch iMac out of the box and immediately went to work on Fusion Drive testing. I started filling the drive with a 128KB sequential write pass (queue depth of 1). Using iStat Menus 4 to actively monitor the state of both drives I noticed that only the SSD was receiving this initial write pass. The SSD was being written to at 322MB/s with no activity on the HDD.

After 117GB of writes the HDD took over, at speeds of roughly 133 - 175MB/s to begin with.

The initial test just confirmed that Fusion Drive is indeed spanning the capacity of both drives. The first 117GB ended up on the SSD and the remaining 1TB of writes went directly to the HDD. It also gave me the first indication of priority: Fusion Drive will try to write to the SSD first, assuming there's sufficient free space (more on this later).

Next up, I wanted to test random IO as this is ultimately where SSDs trump hard drives in performance and typically where SSD caching or hybrid hard drives fall short. I first tried the worst case scenario, a random write test that would span all logical block addresses. Given that the total capacity of the Fusion Drive is 1.1TB, how this test was handled would tell me a lot about how Apple maps LBAs (Logical Block Addresses) between the two drives.

The results were interesting and not unexpected. Both the SSD and HDD saw write activity, with more IOs obviously hitting the hard drive (which consumes a larger percentage of all available LBAs). The average 4KB (QD16) random write performance was around 0.51MB/s, it was constrained by the hard drive portion of the Fusion Drive setup.

After stopping the random write task however, there was immediate moving of data between the HDD and SSD. Since the LBAs were chosen at random, it's possible that some (identical or just spatially similar) addresses were picked more than once and those blocks were immediately marked for promotion to the SSD. This was my first experience with the Fusion Drive actively moving data between drives.

A full span random write test is a bit unfair for a consumer SSD, much less a hybrid SSD/HDD setup with roughly an 1:8 ratio of LBAs. To get an idea of how good Fusion Drive is at dealing with random IO I constrained the random write test to the first 8GB of LBAs.

The resulting performance was quite different. For the first pass, average performance was roughly 7 - 9MB/s, with most of the IO hitting the SSD and a smaller portion hitting the hard drive. After the 3 minute test, I waited while the Fusion Drive moved data around, then repeated it. For the second run, total performance jumped up to 21.9MB/s with more of the IO being moved to the SSD although the hard drive was still seeing writes.


In the shot to the left, most random writes are hitting the SSD but some are still going to the HDD, after some moving of data and remapping of LBAs nearly all random writes go to the SSD and performance is much higher

On the third attempt, nearly all random writes went to the SSD with performance peaking at 98MB/s and dropping to a minimum of 35MB/s as the SSD got more fragmented. This told me that Apple seems to dynamically map LBAs to the SSD based on frequency of access, a very pro-active approach to ensuring high performance. Ultimately this is a big difference between standard SSD caches and what Fusion Drive appears to be doing. Most SSD caches seem to work based on frequency of read access, whereas Fusion Drive appears to (at least partially) take into account what LBAs are frequently targeted for writes and mapping those to the SSD.

Note that subsequent random write tests produced very different results. As I filled up the Fusion Drive with more data and applications (~80% full of real data and applications), I never saw random write performance reach these levels again. After each run I'd see short periods where data would move around, but random IO hit the Fusion Drive in around an 7:1 ratio of HDD to SSD accesses. Given the capacity difference between the drives, this ratio makes a lot of sense. If you have a workload that is composed of a lot of random writes that span all available space, Fusion Drive isn't for you. Given that most such workloads are confined to the enterprise space, that shouldn't really be a concern here.

Meet Fusion Drive Management Granularity
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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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