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Let's say I have a Mac with 128GB of RAM and no SSD. Could I somehow configure OSX to use the RAM as an SSD, i.e. as part of a Fusion Drive?

Note: manually making a Fusion Drive is described here: http://jollyjinx.tumblr.com/post/34638496292/fusion-drive-on-older-macs-yes-since-apple-has

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    What possible benefit would you hope to obtain this way?! Assuming you could pull it off, you'd be trading RAM, the most valuable resource for avoiding massive amounts of I/O entirely, for Fusion Drive, an awkward compromise that makes some I/O a bit faster. – David Schwartz Apr 4 '13 at 23:18
  • Also RAM is volitile memory. Once the power is cut off, all of the data that was on it is gone. – cutrightjm Apr 5 '13 at 0:09
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You could use some of that RAM to create a RAM disk. There are commands that you can run from terminal or get some 3rd party GUI tool that allows you to do the same. However, there's a huge difference between SSDs and RAM disk. RAM is volatile, SSDs are not. If your computer suddenly loses power, crashes, hangs or reboots, everything that was in RAM and on your RAM disk will be lost. Obviously the same will not happen with SSDs.

Having said that, you might not realize it, but OSX already does something similar to what you are thinking of doing. All that free memory in your computer is not being wasted. OSX automatically uses it as a cache. Whatever your computer reads, gets cached in free RAM. The next time you need that file, it'll be available to you nearly instantly.

Take a look at this answer, it's much more eloquent. Isn't Inactive memory a waste of resources?

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I dont know if you can, but you probably would not want to. The reason being that RAM is volatile memory, while magnetic hard drives and SSDs are not. If for any reason you lost power, or your machine froze and was unresponsive, all the data in RAM would be lost.

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I was looking for the same setup and unfortunately it wont work. Fusion drive (as boot drive) has to be "working" at boot time with all the data on both drives. Reboot for any reason will erase the RAM disk, which will break the fusion drive and damage the data. Another issue would be creating the ram disk before the OS boot, but I didn't go that far.

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