On fusion, I guess you would open the file, save it, and it's on the ssd.
If the file was opened from the HD, it will probably be saved to the HD. FD may do housekeeping later to move it to the SSD, depending on its algorithm. However, we do not yet have a complete understanding of the FD algorithm, so we do not actually know what it will do.
Moving files around sounds like the go for control freaks with too much time on their hands.#
How much file moving is really needed? Put the OS, apps, and some key files on a non-FD SSD and it's done. In my case, "key files" would be the Lightroom catalog, Lightroom previews, and the ACR cache. Could I count on FD putting those key files on the SSD? I don't think it's being a control freak to want to be sure of that.
Tests show that even putting raw files on an SSD vs an HD does not make much difference for Lightroom. Other files, such as Word documents, will open just about as quickly from a HD or an SSD. I don't see the advantage of moving those types of files on and off the SSD based on usage.
We need more in-depth assessments of Fusion Drive. It is unclear, at least to me, exactly how it will work, and how well it will accomodate specific needs like mine.
Jeff