How to decide what goes on SSD?

Discussion in 'iMac' started by Sid The Kid, Jul 14, 2017.

  1. Sid The Kid Suspended

    Jul 10, 2017

    Is there a possibility to manage what will go on the SSD manually on a Fusion Drive iMac?

    I know the system will learn to load faster the often used programs, but is there a way to remove stuff by yourself?

    Operating System automatically goes on SSD, Safari?.....

  2. Taz Mangus macrumors 68040

    Taz Mangus

    Mar 10, 2011
    The OS will determine what is stored on the SSD and when to move files to the spinner. The most frequently used items generally stay in the SSD. Think of the SSD as a file caching area. No way to override how the Fusion drive works.
  3. Sid The Kid thread starter Suspended

    Jul 10, 2017
    Okay so if you stop using Google Chrome for a month, the Fusion Drive will take it off from the SSD?
  4. Taz Mangus macrumors 68040

    Taz Mangus

    Mar 10, 2011
    I don't know, maybe. If you want control over what goes on the SSD and spinner, you can split the Fusion drive into 2 physical drives and use them separately.
  5. Sid The Kid thread starter Suspended

    Jul 10, 2017

    By the way, let's say you install Firefox, it means there's no possibility to download it on the HDD instead of SSD? I suppose it wouldn't make a difference if Firefox is often used and automatically goes on SSD.

    What else goes on HDD if Fusion Drive automatically put stuff when something is often used? I suppose .JPEG pictures... but app "Photos" on SSD?
  6. cruisin macrumors 6502a


    Apr 1, 2014
    Fusion means that the SSD is "hidden", and you only see the HDD. Everything is on the HDD, the SSD simply has a copy of the things you use often. Changes made on files stored on the SSD are copied over to the HDD in the background.

    The computer takes a list of all your files (including the separate files that make up your apps and the operating system) and keeps track of how often they are used. Then it tries to keep as many of your most used files on the SSD as possible.

    So only the parts of the operating system that you use often goes on the SSD (to save space). Similarly, only the parts of the app that are used often go on the SSD. Everything is handled automatically.

    If you delete everything on your computer (format everything), you can reserve a part of the SSD (or even the HDD) to use as you want. The computer will simply have a smaller SSD for the fusion drive. I don't think there is a way of doing this without deleting everything. It will likely be less efficient then if you simply kept the fusion drive as it originally was.
  7. ivanwi11iams Contributor


    Nov 30, 2014
    Kennesaw, GA
    ...there are YouTube videos showing how to do this.
    My drive is a 2TB fusion drive, i.e. 128GB SSD, seems to work fine. I wouldn't bother with splitting it, etc.

  8. ApfelKuchen macrumors 68030

    Aug 28, 2012
    Between the coasts
    Don't try to over-think this. Do you worry about managing what's loaded into RAM, or the caches of your CPU?

    The SSD in a Fusion Drive is basically managed the same way as RAM. The SSD contains data blocks, not entire files or apps - just as RAM does - only those parts of a file that are actually needed are pulled from the HDD. No need to load an entire app, which contains all sorts of stuff that you may never need, like localization (spoken languages) or features you never use. If a block has to be moved from HDD into RAM it is also moved into SSD, on the theory that, if the block is needed once, it will be needed again. As with RAM on a Mac, the block will stay on the SSD until something with a higher priority comes along - it's better to leave it in-place than to fetch it a second time from the slow HDD. The difference between RAM and SSD is that there is a lot more SSD than RAM, so the items that are moved to SSD will remain there far longer than they'd remain in RAM. So, if they're needed in RAM again, there's a good probability that they're coming from SSD rather than HDD.

    About the only type of data that doesn't benefit from this is streaming audio/video. However, as soon as you hit pause or rewind, it will benefit from Fusion.

    Let the computer manage this - there's no way anyone can possibly manage storage on the kind of micro level the computer can. The computer "knows" exactly what is needed, exactly what has been used. It's a mechanical, dispassionate, mathematical process that is not colored by our poor, human impressions of what, exactly, is happening inside our computers at any given microsecond.

    Do not "break" a Fusion Drive. Like RAM, SSD is an expensive resource, and the end-user does not have the tools to manage that resource as efficiently as the OS can. Period. If you believe otherwise... try managing your CPU for a while.
    --- Post Merged, Jul 15, 2017 ---
    When you install anything, it's going onto HDD. Only if it's needed in RAM will it make its way to the SSD.

    Consider it to be an experiment - the first time the data/code is needed, it's moved to SSD on the speculation that it will be needed again. Since we don't know the future, we don't know if it'll be needed again. If it is needed again sometime soon, we're rewarded for having it in SSD, and it will renew its "lease" on SSD space. Meantime, if a new "tenant" comes along, whoever is farthest behind on the "rent" will be evicted first. If no new tenants show up, maybe the guy on the bottom of the list manages to come up with another rent payment in time to stave-off eviction. And if someone is evicted today and shows up with the rent again two hours later, they're let back in, without prejudice.

    Fusion does not make its decisions based on "is this an app, or is this data?" Whether it's app or data, if it's needed by the CPU, it's loaded. If it's not needed, it stays where it is. You can safely assume that any part of an app or data set that is regularly needed will spend time in SSD. If it's seldom needed, the odds of being in SSD are lower, and if it's never used it will be on HDD (like all the bad photos you can't bring yourself to delete, or the songs from an album that you never want to hear again).

    This is a statistical process, based on the notion that whatever is used once will probably be used again. The more frequently that item is used, the higher the probability that it will forever remain in SSD. There will always be exceptions - but that's the nature of probability; nobody knows the future.
  9. Sid The Kid, Jul 15, 2017
    Last edited: Jul 15, 2017

    Sid The Kid thread starter Suspended

    Jul 10, 2017
    Thank you guys for your replies. Fusion Drive is sophisticated enough :0 Smart system. So the best thing to do is let the computer handle and manage everything by itself instead of messing with it.
  10. cynics macrumors G4

    Jan 8, 2012
    This is generally the case for most users.

    Without a specific reason you are better off leaving a fusion drive fused for several reasons.

    1. Like mentioned its moving blocks, not files. So on its own its capable of moving partial programs or even partial files to the SSD. For example if you use some editing suite but only use 1/2 of what the program offers, only that half will remain on the SSD. It can even do this to single files although unrealistic an example would be if you repeated watched 1/2 of a movie, only that half would stay on the SSD. These are things you can't manually do and what that would means for you is the corestorage is capable of keeping more of your frequently used stuff on the SSD.

    2. The OS will always remain on the SSD. But it can move OS apps off it like Mail, Messages, FaceTime, iMovie, whatever you don't use.

    3. It will leverage the SSD until nearly full (4gb left I believe) before even resorting to the HDD. So if you have a 2TB FD and have 100gb of data then all your data will be on the SSD anyway.

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