Question about Fusion Drive

Discussion in 'iMac' started by ThisBougieLife, Dec 24, 2016.

  1. ThisBougieLife macrumors 68000


    Jan 21, 2016
    SF Bay Area, California
    Do you guys find the Fusion Drive to be satisfactory? I'm used to the speed of all-flash in an MBP, so I'm not sure if I'd like the Fusion Drive. What I really want is a setup like my Windows desktop: 160 GB SSD with the system and all my applications on it, everything is fast in day-to-day computing. It also has a 2 TB HDD that I store my lossless music and downloaded movies on. The only time I notice that HDD kicking in is when I go to listen to music or watch a movie. Otherwise, it's as if the computer runs only on SSD.

    But I've read some reviews of the iMac saying that things constantly jump back and forth between SSD and HDD and that over time as you fill up the fusion drive, it gets slower and slower. What has been your experience?
  2. mpe macrumors 6502

    Sep 3, 2010
    I use several Macs at work and also at home with 512/1TB SSDs and 2TB/3TB Fusion drives. For what I do (Xcode, Lightroom, Photoshop) they feel the same. The Core Data layer in FD does a great job by using SSD caching and hiding HDD latencies and "learning" what to cache on the SSD and what can stay on HDD.

    My suggestion:

    If you keep writing gigabytes of new data every day and need absolutely consistent sustained write speeds with large transfer sizes or need to process RAW video files, go SSD-only.

    For anything else, particularly if you have normal sized system, data + a large media library, get the FD.
  3. kschendel macrumors 6502a

    Dec 9, 2014
    Well, why not do exactly that, then? Assuming we're talking about an iMac, go with an internal SSD, and add on an external USB-3 hard drive for extra space. USB-3 can run a single HDD at full speed easily.

    To attempt to answer your specific question, the Fusion drive is a compromise. It's a caching mechanism, and like any cache, works well for access patterns for which it's designed, and works poorly for some other access patterns. I don't think slowness will be a function of drive space used, it will be a function of locality of access (which might be loosely related). In other words, if you have 2 Tb of stuff and you're constantly reading or writing some random part of that 2 Tb, the cache will be less likely to hold what you want and you'll be more likely to see HDD speeds. If you are generally busy with some small subset of that 2 Tb, and only occasionally access other stuff, the cache will work well. Anecdotally it seems that the Fusion algorithms work well for what it sounds like you want to do.
  4. maflynn Moderator


    Staff Member

    May 3, 2009

    As I was when I bought my 2015 iMac. Yet, I was pleasantly surprised by the performance. My use case may not be the same as yours but I found the fusion drive to be quiet fast. Some things did run slower, like Virtual Machines (because the large size they were sitting on my spinning disk), and some things with photoshop, but overall it was fairly quick. I use Lightroom more then PS and I didn't use VMware Fusion much so I guess those were relegated to the spinning drive either for the size or lack of usage.

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