I don't think so.
I put together a Fusion Drive as an experiment to see if was preferable to using a pure SSD vs. a high speed drive, and the trade offs, namely size vs. performance.
With my Fusion, the SSD was on the inside of the unit and I was using a Firewire external drive. The nice thing was that when the Firewire's drive activity light flashed, I knew it was being accessed. We have a program that generates a cacheing file, and periodically it changes. Here's what happened:
- When the program starts, it creates the cache file and then starts hitting it at about 1 second intervals.
- Initially, the Firewire, which was of course the HD part of the Fusion, would show no activity, which told me it was hitting the SSD.
- After a few seconds, the Fusion seemed to transfer the file to the Firewire drive, and then it would start flashing at the same intervals. This told me the Fusion had off loaded the file from the SSD to the hard drive.
- When the file changed, steps 1 through 3 would repeat.
The cache file wasn't big, but what was clear to me that the Fusion drive management was following Apple's proprietary algorithms, which, to the best of my knowledge, aren't known. That was done some time ago, right after Fusions and the ability to make them became known, so who knows if it even works that way.
If you want to control where the data goes, you might want to create a boot and system drive with the SSD then offload slower stuff that doesn't need to get hit that much to the hard drive and link them to the OS via symbolic links. This way you can control where stuff goes, what gets backed up and when, and if one of the units fails you'll know where and what data got lost. As-is, with a Fusion drive you can't tell at a given instant what's on which drive. I know what I'm describing is a two drive setup, but you may find it's generally faster than a Fusion drive as well.
Hope this helps.