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Fusion doesn't work at the app or file level. It works at the block level. Think of a file or app that is made up of many blocks. Blocks that are used often will end up on the SSD part of the fusion drive. Blocks that are not used often will end up on the the hard drive. If you use vmware (fusion) or parallels and windows a lot... the blocks that you use often will end up on the SSD.

At least that is my understanding of how fusion is supposed to work.
 
The Parallels/VMWare filesystem is just a disk image stored on the native filesystem (unless you use the Boot Camp partition).

Supposedly, the trigger for SSD/HDD is 3 accesses in the last three user sessions. (reboots, sleep/wake)

4GB of the SSD is reserved for real-time accesses/caching. The rest is available. Fusion Drive will use 100% of the available space on your SSD.
 
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