…depends what your use pattern is, but I doubt you’d enjoy it.
The problem is the cached data will be fast, but the uncached will, well, be on a hard drive.
If you have enough cached space to keep your OS and your used data on it, it’s great, but if you have enough disk space to keep your OS and used data on it, why are you doing this in the first place?
If you don’t have enough cache drive to keep your commonly used data on it, then it’s going to absolutely perform worse than just buying another SSD.
So I guess if this is ‘I keep my whole steam library installed, but only play 3 games at a time’ kinda usecase, it’ll probably work fine.
For everything else, eh, I probably wouldn’t.
Edit: a good usecase for this is more the ‘I have 800TB of data, but 99% of it is historical and the daily working set of it is just a couple hundred gigs’ on a NAS type thing.
It is mostly professional/office use where this make sense. I’ve implemented this (well, a similar thing that does the same thing) for clients that want versioning and compliance.
I’ve worked with/for a lot of places that keep everything because disks are cheap enough that they’ve decided it’s better to have a copy of every git version than not have one and need it some day.
Or places that have compliance reasons to have to keep copies of every email, document, spreadsheet, picture and so on. You’ll almost never touch “old” data, but you have to hold on to it for a decade somewhere.
It’s basically cold storage that can immediately pull the data into a fast cache if/when someone needs the older data, but otherwise it just sits there forever on a slow drive.