Fusion drive experience

Discussion in 'iMac' started by mpe, Feb 9, 2016.

  1. mpe macrumors 6502

    Sep 3, 2010
    I am considering ordering my iMac with 2TB or 3TB Fusion Drive. I normally use my computer for two main tasks:

    1. processing RAW files from DSLR in Adobe Lightroom. My library has over 300 GB in total and growing as I import gigabytes of pictures every month, processing them cataloguing, retouching, exporting to JPEG and archiving for occasional future use. I would actually prefer if these RAWs go to HDD portion of the Fusion once processed. On the other hand I want my Lighroom catalog (metadata), previews and ACR cache to stay on SSD so that I can quickly browse my files search them, etc. Is the fusion drive smart enough to do that?

    2. Software development. I am checking out projects using git (often up to few gigabytes of small files), indexing them, searching through them, compiling in Xcode. In total I have about 20GB of source files. I would prefer if these stay of the SSD all the times. I certainly don't want to find out that my sources were demoted to HDD after I import a few gigs of files from my DSLR...

    And now my question:
    • Given by my specific use case. Do you think that the Fusion drive the right choice for me? I certainly have more than 128GB of data. On the other hand I am fine with having most of my files on the HDD supposing that my critical files are on SSD. My other alternative would be 512GB SSD, but it is pricey and given by the speed how I keep adding images I am afraid it will be small soon. 1TB SSD is quite expensive. I would like to avoid external enclosures, drives, etc.
    • Is there any perceivable latency when using Fusion drive? I mean like opening finder window when some files are on SSD, waking-up from sleep, etc.
    • Does Power Nap feature work on Fusion drive? Apple docs is not clear about this.
  2. dphershman macrumors newbie


    Dec 26, 2009
    I have a 3 tb fusion drive on my 2015 iMac where I store and access about 2 tb of photos on it for projects in Lightroom, Photoshop and Premiere. I don't have the technical knowledge to say what is stored where but LR PS and PR all run very fast on this machine. I also use a 2012 MBP laptop that I recently upgraded to a medium sized SSD, it doesn't seem to be any faster than the Fusion drive in on the iMac. (Similar lags in LR previews, file I/O) and so on . The Fusion drive was a no brainer for my photo library as I saved about $1000 by going with the largest Fusion paired with a 3TB backup drive with Time Machine. An SSD would have required extra outside storage and more elaborate backup system.
  3. gian8989 macrumors 6502

    Oct 23, 2015
    with all the photos you have i would go with ssd + external raid (raid5 is the best but cost a lot, raid 1 is a cheaper solution and it protects your data but slower than fusion drive).

    The fusion drive is the cheapest solution. Here you have 2 alternatives:
    - let the imac decide where to put files (it will probably move the photo you'll be working on in the ssd portion and than place it again in the hdd )
    - split the ssd and hdd so the imac will use only the ssd (128gb) and you'll use the hdd like an external one (I don't really like this solution).

    So it depends mainly on your budget:
    - ssd + raid --> expensive
    - fusion drive + external hdd --> cheap
  4. dimme macrumors 65816

    Feb 14, 2007
    SF, CA
    I have a 2TB fusion drive on my Late 2015 5K iMac, I also have a late 2013 MBP with a SSD. I love SSD's but I found with my MBP I was always moving files around because of the lack of disk space. And transferring to my server or external drive is a pain in the A%% I find my fusion drive to be just as fast most of the times. I did notice the other day when I started up Photoshop after not using it for a few days it took a extra 2 second or so to start up, to me that is not a big deal. Startup and waking from sleep is very fast. The only time I notice a difference is copying files larger that 4GB you can see when the buffer fills up, but again for the convenience of having all my photos stored locally it is great. IMO if you have a raid attached directly to your computer you still need a backup.
  5. taedouni, Feb 9, 2016
    Last edited: Feb 9, 2016

    taedouni macrumors 65816

    Jun 7, 2011
    I have a 2TB Fusion Drive and Power Nap shows in settings.
    I recently got this iMac. I have a late 2013 Macbook Pro with a 512 GB SSD. I don't have enough content on this iMac to give you an answer based off my experience. But Apple claims to keep your most frequently used files on the SSD. I would assume that Apple's algorithm for determining which files belong on the 7200RPM portion would automatically keep large media files on that (just purely looking at file type). Even if some data that you'd prefer be on the SSD happens to be on the 7200 RPM, I don't think that it would be a deal breaker simply due to the fact that you'll be at home and I assume would have more time to wait a bit more. As for me, if I am on the go with my Macbook Pro I need stuff done ASAP.

    Oh yeah, and to answer your question. Honestly I think that it's worth it. I personally don't think that it's worth spending a premium to only get 512 GB SSD, on the other hand 1TB is expensive ($700 additional). If the highest 27" model came with a 512 GB SSD by default then I may have gotten that. But no way I can recommend on paying extra just to get 512 GB.
  6. Starfia macrumors 6502a


    Apr 11, 2011
    I've been using a 2012 iMac with a 1TB Fusion Drive for about two years.

    Power Nap: yes, available. (Presumably available on current iMacs as well.)

    Latency: versus a hard drive, it's much, much faster – enough that I might not have suspected it wasn't Flash if I hadn't known. Startups are seconds long, and I only find myself waiting for large, media-filled projects to open (again, a matter of seconds). I've never found myself idling while waiting for an Xcode project to open.

    I haven't ever used a Flash-only Mac, so I don't have that frame of reference – if you've been using one, you might notice differences I haven't been able to; that said, it seems very much worth the additional storage to me.
  7. maflynn Moderator


    Staff Member

    May 3, 2009
    I have a late 2015 5k iMac with a 2TB Fusion drive. I have powernap off, as I see no benefit for leaving it on for my needs and purposes. I'm also someone who grew tired of dealing with moving files and folders from the internal drive to the external one. For example iTunes can run off the external drive but at times it would reset and look for everything on the internal drive. I'd need to then repoint it, and let it do its thing for a few minutes. Not a huge deal, but it can be a pain when you hit play and no music is coming out of the speakers.
  8. Fishrrman macrumors P6


    Feb 20, 2009
    OP wrote:
    "I would actually prefer if these RAWs go to HDD portion of the Fusion once processed. On the other hand I want my Lighroom catalog (metadata), previews and ACR cache to stay on SSD so that I can quickly browse my files search them, etc. Is the fusion drive smart enough to do that?"

    If you want "absolute control" over what goes where on the two internal drives, I suggest you consider "manually splitting apart" the fusion drive into a "two drive standalone" setup.

    The procedure doesn't look to be that hard to do, and doing so will give you the level of control that you require.

    I believe both the 2tb and 3tb fusion drive models give you a 120gb SSD portion.
    The difference will be in the HDD portion:
    - 2tb 7200rpm drive on the 2tb fusion model
    - 3tb 7200rpm drive on the 3tb fusion model.

    "Breaking" the fusion drive will wipe out the contents of BOTH drives, so you must be ready to back up the original "factory OS install" as your very first move, then "do the split".
    After the split, you can just "re-clone" the OS to your SSD.

    I -HIGHLY RECOMMEND- that you use CarbonCopyCloner to do the backup and re-installation.
  9. mpe thread starter macrumors 6502

    Sep 3, 2010
    Thanks. I am coming from ssd-only Mac (rMBP 2014) so I am used to pure-SSD performance. I only hope that I won't get random hiccups with Fusion.

    As for splitting the drive. I kind of like the idea of automatic moving of the stuff from SSD to HDD and back and not having to think about it, always get SSD caching for writes, .... I only hope it is smart enough to work just as I want it to work. Is there any way of tweaking it what should always stay on SSD? Apple claim that the OS always stays on SSD so there must be a way. Do they set some sort of extended attributes or something?
  10. r6mile macrumors 6502

    Feb 3, 2010
    London, UK
    In my 2008 Mac Pro I decided to set up my 250 SSD and 500GB as a single 750GB fusion. Having to manually move stuff and work out where to store files seems like a massive headache, for what seems like very little gain.
  11. Weaselboy Moderator


    Staff Member

    Jan 23, 2005
    Fusion works at the block level and not file level, so the OS moves the most uses blocks of data to the flash storage portion of the Fusion drive. There is no way to control this. Parts of the OS would of course fall under that most used category and be kept on the flash storage.

    So using your RAW files as an example, if you are working with a group of raw files for a couple days, then they would be identified as frequently used blocks and moved to the flash section. But if you did not touch those same RAW files for a few days and worked with say another 100GB or so of RAW files, then those new RAW files would be moved over to the flash section and the old RAW files would be pushed back over to the hard drive. This is just a simplified explanation of how it works, but you get the idea. There is no way at all though to directly control what goes where.

    There is a very detailed article here that explains how it works.

    If I were you, I would just leave it as a Fusion setup for a month or so and see how you think it works out. You can always break up the Fusion later.
  12. robgendreau macrumors 68040

    Jul 13, 2008
    Sorta six of one....

    I do tons of Lr stuff as well, and I got the SSD only 512. But mostly because I needed more than 3TB anyway, and so would be moving stuff around, and because I've been burned lately with spinners. I can't say, of course, that the SSD is orders of magnitude more robust, it's just sorta personal preference.

    And remember that Lr uses a lot of caching, and by managing your previews you can help the OS use the SSD efficiently.

    I can't say that I have heard of any Lr RAW users being hurt by Fusion vs SSD performance, so it probably boils down to a coin flip.
  13. ApfelKuchen, Feb 10, 2016
    Last edited: Feb 11, 2016

    ApfelKuchen macrumors 68030

    Aug 28, 2012
    Between the coasts
    I'm reminded of Spock's comment about Khan's two-dimensional thinking...

    A cache, any cache, runs on an algorithm. A cache of this sort generally doesn't "think" in terms of file types, it only "cares" about utilization. The OS makes byte-shifting decisions with every tick of the clock - every read, every write is part of the equation. Unless you spend your time with your eyes glued to Activity Monitor and understand the function of every code module and cryptically-named data file on your system, your OS will "understand" utilization far better than you ever can.

    People came up with strategies for manually managing SSD in a mixed SSD/HDD environment - simple, broad-stroke techniques that wouldn't require excessive human intervention. Consistently, they expect these strategies to be reflected in Fusion and other SSD-caching schemes (and Fusion is hardly alone). Consider it a form of anthropomorphism. There is no reason for that over-simplified, "I think my stuff belongs here or there" thinking. Either the data/code is called, or it isn't. The end user thinks he knows where something belongs. The OS knows where it belongs.

    You're not likely paying attention to the specific location of your files on the HDD. Are all your RAWs neatly arrayed in contiguous sectors on the HDD? No. They're all over the friggin' place. If a file spans more than one block, you can't even count on the file's blocks being stored contiguously. The only thing that matters is that the OS accurately maps the physical locations of all that data.

    Most users are accustomed to partitioning large physical drives into multiple, smaller volumes. If the OS is incapable of addressing a memory store of that size, the disk must be broken into addressable chunks. And if necessity forced you to break the drive into those addressable chunks, there were logical reasons for placing data on one partition and apps/OS on another. But if the OS is capable of addressing the entire physical drive (or even multiple, spanned drives) as a single volume (as OS X can), then there's little logical reason to create multiple partitions - it's less complicated and more space-efficient to have everything on a single volume.

    Now, when you use Fusion, you're working with a CoreStorage volume - a single logical volume that spans multiple drives - in this case, one is an SSD, the other a HDD. (That's pretty much what RAID 5 does, too, but for different reasons.) If those multiple drives have differing performance characteristics, the OS is capable of optimizing storage allocation in order to maximize performance. That's all Fusion is.

    But every so often, that algorithm is going to get things wrong. The algorithms that manage RAM and CPU buffers sometimes get it wrong, too, not that we're likely to notice. What the coders are aiming for is statistical averages.

    The typical worst-case scenario for Fusion is that data/code that ought to be on SSD is not there, and gets read at HDD speeds instead. That's an infrequent occurrence. The person who proposes that he is able to do better by manually moving files from HDD to SSD, or by leaving all his data on HDD (guaranteeing that "worst case" is "every case") is simply fooling himself - like a gambler who counts only his winnings and never his losses.
  14. mpe thread starter macrumors 6502

    Sep 3, 2010
    I understand that the fusion moves around blocks rather than files. However, a file is ultimately just a sequence of blocks and files are often being read at once (unless doing some sort of memory mapping). So often the ssd will be caching complete files anyway. It could be different for writes. But as far as I know writes always go to SSD (at least up to a certain limit). I guess the logic for moving stuff around will be primarily based on frequency of block read access.

    However, there are claims that "that FD will always keep the operating system and other critical files pinned to the SSD". See this arstechnica article that referes to Apple docs.

    How they can potentially do that? If that's true there must be an API or something that allows tweaking CoreStorage to always keep certain blocks occupied by specific files on the SSD. Not unlike trim command that notifies SSD to free memory cells occupied by deleted files. The trim is also a block operation but issued as result of a change on the filesystem.

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