iMac Fusion Drive speed test

Discussion in 'iMac' started by EricDraven, Jan 21, 2014.

  1. EricDraven macrumors newbie

    Dec 10, 2013
    Hi everyone,
    I'm using an iMac late 2013 with 1TB Fusion Drive.
    I run speed test with Blackmagic Disk Speed Test and get a result:

    350MB/s write
    650MB/s read

    Now, Im wonder how long you guys test the Fusion Drive to get the best result cuz sometimes during the test I see the read and write speed go up and down. Sometimes both go down to around 2xxMB/s. Is it normal?

    Thanks in advance!
  2. Bear macrumors G3

    Jul 23, 2002
    Sol III - Terra
    Yes this is normal. It means you spilled over to reading and writing directly from and to the HD portion of the FD.
  3. EricDraven thread starter macrumors newbie

    Dec 10, 2013
    Thanks so much for your reply Bear.
    I have 2 questions:
    • I wonder how long should I test speed with Black Magic cuz seems like it runs endlessly.
    • What happen if I fill up 1TB Fusion Drive?
  4. iamgalactic macrumors regular

    Apr 21, 2010
    that won't happen unless the SSD part is actually full - Eric, do you have more than 128GB on your hard drive?
  5. EricDraven thread starter macrumors newbie

    Dec 10, 2013
    Nope, I just have Mavericks, and some apps on my 128GB SSD. As you read above I just run Black Magic disk speed test for about 5 mins.
  6. jmgregory1 macrumors 68000


    Jun 24, 2010
    Chicago and a few other places around the world
    The real question is, does your normal usage feel slow?

    If not, what does it matter what the speed is according to a test? If it does seem slow, perhaps there is an issue.

    I have an early '13 21" iMac with FD and I've never thought, "gee this is slow" and I'm comparing it to my ssd mba.
  7. EricDraven thread starter macrumors newbie

    Dec 10, 2013
    Thanks for your reply,
    My usage is good and I open this thread just to know more about BlackMagic disk speed test and how to test it properly.
    Do you test your Fusion Drive with BlackMagic? Does the speed sometimes goes up and down?
  8. jmgregory1 macrumors 68000


    Jun 24, 2010
    Chicago and a few other places around the world
    I haven't tested either my MBA or iMac. They perform great and I don't need to know empirically how they are working. It's kind of like putting your car on a dyno. It will tell you the power but only actually driving it will tell you how fast it is.
  9. PJL500 macrumors regular

    Nov 27, 2011
    Tricky to speed test a Fusion Drive... would like a technical run down of how it could be done.
  10. JustMartin macrumors 6502a

    Feb 28, 2012
    As others have said higher up the thread, the question is why you would want to speed test your drive. With the kind of mixed media you have in a fusion drive, it's better to ask whether it meets your requirements. For example, if you regularly write more than 4Gb to the disk in one go, then you will lose the SSD advantage for those writes that exceed 4gb only you know if that's important to you.
  11. Samuelsan2001 macrumors 604

    Oct 24, 2013
    That is just testing the speeds of the SSD, that is exactly the speeds you would expect from a 128GB ssd. Delete the app and forget about it unless you have a problem.
  12. gnasher729 macrumors P6


    Nov 25, 2005
    Very important to realise that your Fusion Drive isn't designed to get high benchmark scores, but to work fast in real life usage. You have a combination of 128 GB SSD and one TB hard drive. Fusion Drive tries to do as much work as possible with the SSD drive while using the hard drive as a cheap place to store tons of data that you rarely use.

    As a result, a Fusion Drive is fast when the SSD drive is used, and slow when you access data that has ended up on the hard drive because it was rarely used. It also does work when you are not actually using your computer, so it can _seem_ to run fast while you are using the drive heavily, and then does a lot more work behind your back when you have your cup of coffee. With a benchmark that runs forever, it can't do that so it looks slower than in real life.

    With your numbers, it's clear that the speed test never even touches the hard drive. Other speed test software manages to use the hard drive and nothing else and gives a lot slower results. But both are totally meaningless because they don't test real usage.

    Summary: "Properly" testing the speed of a Fusion Drive is very, very hard, and you won't find any benchmarking software that can do it. Not "properly".

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